DIODE CIRCUIT DESIGNERS' HANDBOOK Diode Circuit Designers' Handbo
|
|
|
Top Searches for this datasheet
Microsemi-Watertown
DIODE CIRCUIT DESIGNERS' HANDBOOK
Diode Circuit Designers' Handbook written Microwave Design Engineer. Microsemi Corp. radically changed presentation this diode applications engineering material increase usefulness Microwave Circuit Designers. major part this Handbook devoted basic circuit applications this unique device.
July 1992, Microsemi Corporation, headquartered Santa Ana, California, purchased Unitrode Semiconductor Products Division (SPD), Watertown, Massachusetts, from Unitrode Corporation. This Microsemi division Microsemi Corp.-Watertown (MSC-WTR), committed same high standards quality products continuous customer service improvements that have been foundation Microsemi' thirty year evolution. Microsemi Corporation makes representation that interconnection circuits described herein will infringe existing future patent rights, descriptions contained herein imply granting license make, sell equipment constructed accordance therewith.
1998, Microsemi Corporation. rights reserved. This book, part parts thereof, must reproduced form without permission copyright owner. NOTE: information presented this HANDBOOK believed accurate reliable. However, responsibility assumed Microsemi Corporation use. Powermite registered trade mark Microsemi Corp.-Watertown. DOC. #98=WPD-RDJ007
Microsemi Pleasnt Street, Watertown, (617) Tel. FAX. (617) 924-1235
Microsemi Pleasnt Street, Watertown, (617) Tel. FAX. (617) 924-1235
Preface
This Diode Circuit Designers' Handbook written Microwave Design Engineer. major part this Handbook devoted basic circuit applications this unique device. each chapter, circuit function treated detail followed specific selected applications. example, Chapter common diode switch configurations presented, followed sections comparing those features diode switch designs unique high power microwave switches high power lower frequency (RF-band) switches. There many unique market applications, such Wireless Communications Market, where network applications system designs outpace component technology needed support them. Therefore, there sections that discuss unique circuit functional requirements appropriate these newer market applications. Wireless Telecommunications power control circuits discussed terms role diodes play providing distortion, Bit-Error-Rate (BER) performance Channel components, particularly next generation multimedia systems such UMTS. Additionally, characteristics high power Band switches treated detail well those switches designed Magnetic Resonance Imaging (MRI) systems. appendix distortion diode Switches Attenuators been included, because increased importance this parameter Channel performance Wireless Communications Systems. subject driver circuits diode switches Attenuator circuits always relevant practical component design, thus been included separate appendix. Diode Physics topics, such diode forward reverse bias operating characteristics equivalent circuits, stored charge lifetime, distortion non-linearity, thermal impedance, contained specific appendices supplementary reference material. hope that organization this material will found useful circuit system designers, whom this Handbook written. comments, additions, deletions would appreciated.
Doherty, Joos
bdoherty@microsemi.com rjoos@microsemi.com
Watertown,
Microsemi Pleasnt Street, Watertown, (617) Tel. FAX. (617) 924-1235
Microsemi Pleasnt Street, Watertown, (617) Tel. FAX. (617) 924-1235
DIODE CIRCUIT DESIGNERS' HANDBOOK
CONTENTS
CHAPTER CHAPTER CHAPTER THREE CHAPTER FOUR CHAPTER FIVE CHAPTER
DIODE GENERAL DESCRIPTION DIODE SWITCHES DIODE ATTENUATORS DIODE MODULATORS DIODE PHASE SHIFTERS DIODE CONTROL CIRCUITS WIRELESS COMMUNICATION SYSTEMS DIODE CONTROL CIRCUITS BAND INDUSTRIAL APPLICATIONS DIODES MAGNETIC RESONANCE DIODE PHYSICS COMPARISON DIODE RECTIFIER DIODES 101A DISTORTION DIODE SWITCHES DIGITAL COMMUNICATIONS LINKS 102A DIODE DRIVER CIRCUITS DIODE DISTORTION DIODE RADIATION DETECTORS MISCELLANEOUS FORMULAE DATA SURFACE MOUNT CRITERIA REFERENCES
CHAPTER SEVEN
CHAPTER EIGHT APPENDIX APPENDIX
APPENDIX
APPENDIX APPENDIX APPENDIX APPENDIX APPENDIX APPENDIX
Microsemi Pleasnt Street, Watertown, (617) Tel. FAX. (617) 924-1235
Microsemi Pleasnt Street, Watertown, (617) Tel. FAX. (617) 924-1235
CHAPTER
DIODE GENERAL DESCRIPTION
NOTES
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
DIODE GENERAL DESCRIPTION This chapter presents general overview diode operating characteristics form adequate basis subsequent chapters various diode functional circuits. Supplemental material Diode Physics included Appendices section Handbook. microwave diode semiconductor device that operates variable resistor Microwave frequencies. diode current controlled device contrast varactor diode which voltage controlled device. Varactors diodes design with thin epitaxial I-layers high reverse bias) little concern carrier lifetime Stored Charge).When forward bias control current diode varied continuously, used attenuating, leveling, amplitude modulating signal. When control current switched off, discrete steps, device used switching, pulse modulating, phase shifting signal. microwave diode's small physical size compared wavelength, high switching speed, package parasitic reactances, make ideal component miniature, broadband signal control circuits. addition, diode ability control large signal power while using much smaller levels control power. Microsemi diodes offer unique highly reliable package voidless construction, metallurically bonded structure, extremely rugged SOGO surface passivation. SOGO passivated devices driven into reverse voltage breakdown without reverse voltage characteristic collapsing. Microsemi diodes offer significant electrical thermal advantages compared diodes manufactured other suppliers. Microsemi diode generally constructed using chip that thicker I-region, larger cross sectional area longer carrier lifetime same basic electrical characteristics series resistance (RS), capacitance (CT). This results diodes that produce lower signal distortion frequencies power levels well devices that capable handling greater average peak power than those manufactured conventional techniques. addition, since there ribbons wires within Microsemi' package, large surge currents safely handled parasitic resistance inductance minimized.
Cross Section Basic Diode
Forward Bias Equivalent Circuit
Reverse Bias Equivalent Circuit
Figure Diode Corresponding Equivalent Circuits drawing diode chip shown Figure (a). performance characteristics diode depend mainly chip geometry processed semiconductor material intrinsic region, finished diode. When diode forward biased, holes electrons injected into
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
I-region. This charge does recombine instantaneously, finite lifetime I-region. diode reverse biased there stored charge I-region device behaves like Capacitance (CT) shunted parallel resistance (RP). These equivalent circuit parameters defined section below. voltage across diode zero, there remains some finite charge stored I-region, mobile. operated zero volts d-c, diode behaves somewhat lossy Capacitor. Some small Voltage (called "punch-through" Voltage) must applied Iregion sweep this remaining fixed charge. These ideas developed farther Appendix ELECTRICAL EQUIVALENT CIRCUITS PARAMETERS DIODE FORWARD BIAS EQUIVALENT CIRCUIT equivalent circuit forward biased diode, Figure (b), consists series combination series resistance (Rs) small Inductance (Ls). function Forward Bias Current (If) this function shown Figure 9552 Attenuator Diode. depends geometrical properties package such metal length diameter. small parasitic element that little effect Microsemi diode performance below
Figure 1.2. Typical Forward Biased Series Resistance Bias Current 9552 Diode forward biased diode Current Controlled Resistor, which useful distortion Attenuator Amplitude Modulator Applications. relationship described (Ohms) (Ohms)
where: I-region Width, Forward Bias Current, Minority Carrier Lifetime Electron Mobility, Hole Mobility This equation valid frequencies higher than transit time I-region: 1300/ microns). also assumes that signal does modulate stored charge (Appendix lower frequencies, diode rectifies signal (just pn-junction diode would). REVERSED BIAS EQUIVALENT CIRCUIT
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
Reverse Bias Equivalent Circuit consists diode Capacitance (CT), shunt loss element, (Rp), parasitic Inductance (Ls). defining equation
which valid frequencies above dielectric relaxation frequency I-region, where dielectric constant Silicon, =Diode Junction Area, Resistivity Silicon. decreases somewhat from Volts "Punch-Through" Voltage remains constant reverse bias Voltage (Vr) greater than "Punch-Through" Voltage. diode's reverse bias Capacitance Voltage behavior different than pn-junction diode, which exhibits continuously variable Capacitance Reverse Voltage Breakdown Voltage (VBR). reverse biased diode easier Impedance match than Varactor, because flat characteristic. shunt Loss (Gp) maximum Volts decreases fixed value reverse bias Voltage increased. upper cutoff frequency diode could defined that frequency which resonates with periodic average value
LARGE SIGNAL MICROWAVE DIODE OPERATION Under large Power control conditions Microwave bands above), following bias considerations apply: Forward Bias Condition: diode must forward biased (Low Loss State) that stored charge, much larger than induced charge that added removed from I-region cyclically current. This relationship shown inequality:
Reverse Bias Condition:
High Frequency versus Frequency diode, designed high frequency operation usually fabricated have capacitance because reactance diode condition must large compared line impedance. ratio PIN' area thickness adjusted obtain desired capacitance. resistivity doping level I-layer critical long greater than Ohm-cm operation GHz. transit time relaxation frequency requirements easily obtained. contrast operation frequencies places more constraints designer even more below MHz). relaxation frequency requires very high resistivity levels Ilayer. Microsemi uses 10,000 Ohm-cm Silicon obtain relaxation frequency. Long transit time requires very thick I-layers. Microsemi manufactures diodes with I-layer thickness Large values required control signal frequencies very critical attenuator applications where bias current increased without changing resistance value diode. Large values 0.1millisec) obtained careful process control good passivating surface I-layer.
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
Above GHz, period microwave signal much smaller than diode's minority carrier lifetime( this case, reverse bias condition (Isolation State) such that diode biased beyond punch through (Appendix large values current being switched, reverse bias voltage must large enough that voltage during forward excursion does induce flow current through diode. diode becomes warm when operating high power switch, reverse bias voltage should increased minimize this effect. diode's reverse breakdown voltage (VBR) must large enough that reverse excursion voltage does cause flow avalanche current under reverse bias conditions [1,2]. shown inFigure 1.3.
Figure Voltage Current Waveforms Superimposed Diode Characteristics FREQUENCY DIODE OPERATION Below transit time frequency I-region, diode behaves junction diode, rectifies voltage. frequencies somewhat higher than transit time frequency below Microwave Bands, sufficient reverse bias voltage should applied protect diode from burnout high power switch application(Figure 1.3). this frequency range, lifetime sufficiently large that induced stored charge controls power applied. completely safe, reverse bias should equal greater than peak value Voltage should equal greater than peak-to-peak value Voltage, that current flows during positive half cycle [3,4].
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
Figure Voltage Waveforms Superimposed Characteristics Diode BIAS-CIRCUIT CIRCUIT ISOLATION most applications, necessary provide some degree isolation between low-frequency bias circuit circuit. Otherwise, current flow into power supply's output impedance, causing effects that detrimental efficient operation power control circuit. bias supply isolated from circuits inserting low-pass filter structure between bias supply control circuit. many switch application (Chapter inductor, series with bias line, by-pass capacitor, shunt with power supply output impedance, will provide more isolation. higher values isolation needed, more complex lowpass filter structures necessary. Low-pass filters significantly increase switching time diode. switching time needed, low-pass filter must show very little loss frequencies (ie, filter's cutoff frequency least MHz). Shorter switching times require higher filter cut-off frequencies, which lead practical construction difficulties. Many commercially available bias tees adequate biasing high power switch prototype circuits because current rating low. DIODE SWITCHING SPEED CHARACTERISTICS Switching Speed (Ts) discussed detail specific switch configurations operating conditions Chapter from diode physics perspective Appendix switching applications, switching speed time required either fill remove charge from I-region. Switching speed depends both driver circuit's operating conditions specific switching states diode's equivalent circuit parameters. When diode forward biased current, current flow results charge, being stored I-region. This stored charge condition causes diode resistance state. forward bias current suddenly removed, positive negative charges diode will recombine time period called minority carrier lifetime. large reverse voltage applied
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
forward conducting diode, reverse current, flows. TFR, forward-to-reverse switching time, expressed terms lifetime
(sec.)
shape typical time curve, defining TFR, shown Figure 1.5.
Figure 1.5. Diode Reverse Bias Switching Speed speed with which charge removed from I-region during turn-off depends rise time amplitude switching-voltage pulse applied diode. using spiked waveforms (referred overdrive) reducing source impedance driver allow high reverse current flow, reduced substantially. time required I-region fill with charge primarily depends transit time I-region, (ie, I-region width) reverse voltage forward bias current that driver supply. This reverse-to-forward switching time, TRF, usually faster than turn-off time, TFR.
DIODE THERMAL IMPEDANCE diodes used control power circuits such switches, attenuators, modulators phase shifters. These diode applications discussed detail next four chapters. process controlling power naturally results some power being dissipated controlling device. amount power dissipated calculated various circuit diode circuit configurations appropriate chapters. diode dissipates power, junction temperature begins rise. diode's junction temperature depends amount power dissipated, ambient temperature Tamb, thermal impedance, between diode junction diode's ambient temperature. power rating
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
diode power dissipation that will raise junction temperature from ambient temperature (usually maximum allowable value, TJmax (150 oC). maximum power dissipation, determined from relationship: where maximum junction temperature Silicon diode ambient temperature, usually that diode's heat sink. calculated DCVDC where current, current, value diode's series resistance value forward bias (d-c) current chosen. Note, that maximum power that diode dissipate, maximum switched power! maximum switched power, depends diode's bias conditions related Characteristic Impedance Switch Circuit Voltage Current from Power Source. SHOULD DIODE Rugged, High Reliability High Voltage Capability 2000 Volts High Current Capability Amperes continuous High surge Current Capability Amperes pulse sine) Distortion -60dBc High Power Gain 10,000 Fast Switching speed Small Physical Size Various Thermal Packaging Available Relay Replacement mechanical, mercury, etc.
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
CHAPTER
DIODE SWITCHES
Microsemi Pleasant Street, Watertown, (617) Tel. Fax. (617) 924-1235
NOTES
Microsemi Corp.-WatertownM Pleasant Street., Watertown, 02472M Tel. (617) 926-0404M Fax. (617) 924-1235
CHAPTER DIODE SWITCHES
INTRODUCTION switch electrical component opening closing connection circuit changing connection circuit device [1]. "Ideal Switch" exhibits zero resistance current flow "ON" state infinite resistance current flow "OFF" state. practical switch design exhibits certain amount resistance "ON" state finite resistance "OFF" state. diodes switching element microwave circuits based difference between diode reverse forward bias characteristics [Chapter One]. lower microwave frequencies, GHz), diode (including package parasitics) appears very small impedance under forward bias very large impedance under reverse bias. difference performance between forward reverse bias states upon which switch operation relies. Most switch designs considered difference reflection, rather than dissipation, obtain switch performance. Very little power dissipated diode itself, thus permitting small devices control relatively large amounts microwave power. Thus, diode switches reactive networks, where losses second order effect. subsequent sections, will that switch circuits resemble filter circuits many ways.
FUNDAMENTAL PARAMETERS THAT DESCRIBE DIODE SWITCH PERFORMANCE
ISOLATION: Physically, Isolation measure microwave power through switch, that transferred load, both from Attenuation Loss Reflection Loss, when switch OFF. practical matter, Isolation measure effectively Diode Switch turned OFF. determined calculating difference between power measured switch output port with switch biased power measured switch output port with switch biased OFF. Isolation (dB) (Pout)on (dBm) (Pout )off (dBm) Equation
This equation avoids problem accounting Transmission Loss through physical structure Diode Switch (all switches have some finite Transmission Loss). Transmission Loss present whether switch OFF.
INSERTION LOSS: Insertion Loss (IL) Transmission Loss through physical structure diode switch. forward biased case (the state), large values bias current plus microwave current flow through switch structure, causing significant Ohmic Loss. reverse bias case (the Isolation state), only small values leakage current flow through switch, reverse bias loss small. switch mechanically thermally designed properly, Ohmic Losses Thermal Dissipation minimized Insertion Loss relatively 0.25 dB). Insertion Loss particularly critical parameter Communications System designer. Insertion Loss absorbs signal power, causing system' Noise Figure increase amount Insertion Loss.
Microsemi Corp.-WatertownM Pleasant Street., Watertown, 02472M Tel. (617) 926-0404M Fax. (617) 924-1235
DIODE POWER HANDLING LIMITATIONS System Requirement that usually determines choice particular Diode used power that switch must handle. Diode characteristically relatively wide I-region therefore withstand larger Voltages than Varactors microwave Schottky diodes. Chapter (Large Signal Diode Operation) forward reverse bias conditions, necessary insure safe high power switch operations were discussed. this Chapter, switch' Power Dissipation considered another limiting factor determining maximum power level that diode switch control without overheating. Power Dissipation depends (which function forward bias current) relative input power switch, well switch connection chosen. very important rating switching diode given manufacturers. Finally, maximum power that diode capable switching depends incident power, switch connection type, average Dissipated Power (Pd), Reverse Breakdown Voltage (VBR) rating. This parameter also supplied most manufacturers, with stipulation that Ohms that switch circuit series-connected. MICROWAVE SWITCH DESIGN CONFIGURATIONS this subsequent sections, circuit diagrams simple compound switches given, well additional performance information needed design switch. assume this development, that individual switch structure symmetrical linear port network that characteristic impedance (Zo) input power source, switch structure, load impedance, transmission lines connecting these components Ohms. more general case, where input equal output reader referred reference general text general network theory. SINGLE POLE SINGLE THROW SWITCHES SERIES SPST SWITCH
diode SPST used broadband designs. maximum isolation (ISO)obtainable depends diode' Capacitance (Ct). Insertion Loss (IL) Power Dissipation depend diode' forward biased Series Resistance (Rs). equations performance characteristics given below.
Figure Series SPST Switch Series SPST Switches: Power Dissipation (Pd) (2Zo Watts Zo)2
where maximum available power, Vg2/ (Watts).
Microsemi Corp.-WatertownM Pleasant Street., Watertown, 02472M Tel. (617) 926-0404M Fax. (617) 924-1235
These equations pertain only matched SPST switches. VSWR 1.0, multiply these equations factor designated "sigma", calculate
Peak Current (SPST) Amps
Peak Voltage (SPST) Volts
series SPST switch matched, multiply above equations factor "sigma".
SHUNT SPST SWITCH
Shunt SPST Switch (Figure 2.2) offers high isolation over broad frequency range (approximately singled diode switch). Insertion Loss because there switch elements series with transmission line. diode electrically thermally grounded side transmission line higher capability than SPST circuit. functions primarily depends design equations given below.
Figure Shunt SPST Switch
Shunt SPST Switches: Zo)2}
Power Dissipation Forward Bias): Rs)2 atts
Power Dissipation (Reverse Bias)
Watts
(where maximum available power) Peak Current (Shunt Switch) 8Pav Amps Peak Voltage (Shunt Switch) Volts
shunt switch circuit matched, multiply above equations "sigma" factor.
Microsemi Corp.-WatertownM Pleasant Street., Watertown, 02472M Tel. (617) 926-0404M Fax. (617) 924-1235
SINGLE POLE DOUBLE THROW SWITCHES
Figure Series SPDT Switch
Figure Shunt SPDP Switch
simplest example more general Single Pole Multi-throw Switch structure Single Pole Double Throw Switch, which signal power single input transmission line connected either output transmission lines. SPDT switch symmetrical, each switch branch performs like SPST equivalent; Isolation multi -throw switches increased This effect occurs because branch shunted branch termination, causing Voltage across diode less than would case equivalent SPST switch. Shunt SPDT Switch design Figure enhances electrical performance this switch inserting quarter-wavelength transmission lines between signal power source diodes. isolation this design approximately double (ie, that Shunt SPST Switch plus effect multithrow switch junction. However, bandwidth constrained less than octave. MULTI-THROW SWITCHES Multi-throw switches difficult realize using only shunt diodes. band-limited shunt multi-throw switch (less than octave) shown Figure 2.5, uses cascaded quarter-wavelength sections, each terminated shunt diode. This configuration gives branch high input impedance common (signal source) port prevent impedance "loading" that would otherwise occur.
Figure Band-Limited Shunt Multi-throw Switch
Microsemi Corp.-WatertownM Pleasant Street., Watertown, 02472M Tel. (617) 926-0404M Fax. (617) 924-1235
These configurations achieve very high isolation with additional shunt diodes transmission line sections. These designs would even more constrained bandwidth Insertion Loss increases sections added. microwave bands, isolation limited cross coupling between switch components, causing some direct signal feed-through between input output ports. COMPOUND SWITCHES Compound Switches differ from multi-throw switches that series-shunt switches used combinations improve overall switch performance. broad band Insertion Loss series switch combined with broad band Isolation shunt switch number combinations follow. SERIES-SHUNT COMPOUND SWITCHES
Figure Series-Shunt SPST Switch COMPOUND SWITCHES
Figure SP3T Switch simplest compound switches Series-Shunt Switch (Figure 2.6) Switch Figure 2.7). These circuits offer improved overall performance added circuit complexity degrades VSWR Insertion Loss. Since diodes simultaneously biased state other, there increase bias circuit complexity. summary overall performance parameters Series Shunt SPSTs
Microsemi Corp.-WatertownM Pleasant Street., Watertown, 02472M Tel. (617) 926-0404M Fax. (617) 924-1235
Series-Shunt Compound Switches shown comparison Table Performance parameter trade-off inevitable practical switch design.
TABLE SUMMARY FORMULAS SPST SWITCHES
TYPE
ISOLATION (dB)*
INSERTION LOSS (dB)
SERIES
SHUNT
SERIES-SHUNT
SPNT Switch, TUNED SWITCHES
simple tuned shunt SPDT switch shown Figure 2.4. presence quarter-wavelength transmission lines constrain overall bandwidth enhance switch' performance over that bandwidth. Similarly, many switch applications operate over limited frequency band. Distributed lines used improve switch performance following examples show.
Microsemi Corp.-WatertownM Pleasant Street., Watertown, 02472M Tel. (617) 926-0404M Fax. (617) 924-1235
Figure Tuned Series SPST Switch
Figure Tuned Shunt SPST Switch
Insertion Loss Isolation circuits Figures calculated from formulas Table total diode resistance, used these calculations twice that single diode SPST switch, unless bias current increased off-set this effect. maximum Isolation obtainable, using multiple diodes spaced quarter-wavelength, twice value obtainable with single diode switch. further increase Isolation obtained adding more quarter-wavelength sections these designs. Such tuned switches have band widths less than which quite adequate wireless radio applications (reference Chapter TUNED SERIES SPST SWITCHES Quarter-wavelength spacing reduces maximum voltage across each diode half that which would appear across single diode switch. Even series diode quarter-wavelength spacing, Isolation would increase because effective Capacitance half that single diode. this reduction Capacitance primary design objectives, diodes with increased Capacitance could used increase power handling capability switch
TUNED SHUNT SPST SWITCHES maximum isolation obtainable using Tuned Shunt SPST Switch twice value obtainable using only single diode switch. Figure shows Double-throw Tuned Shunt Switch. this circuit, Capacitive Reactance diode transformed quarter-wavelength line (into Inductive Reactance) resonates with Capacitive Reactance second diode. This effect lowers switch Insertion Loss about 50%, narrows operating bandwidth. with Tuned Series SPSTs, quarter-wave spacing higher power diodes with larger values Capacitance (Ct), effective bandwidth switch lowered considerably.
LUMPED CIRCUIT EQUIVALENT QUARTER-WAVELENGTH TRANSMISSION LINE Quarter-wavelength techniques, using distributed line elements, impractical frequencies below because their physical size. Quarter-wavelength lines simulated with lumped circuit elements network such that shown Figure 2.10. equations calculating equivalent values also shown.
Microsemi Corp.-WatertownM Pleasant Street., Watertown, 02472M Tel. (617) 926-0404M Fax. (617) 924-1235
Figure 2.10 Lumped Circuit Equivalent Quarter Wave Line
TRANSMIT RECEIVE SWITCHES Transmit-Receive Switches class Tuned Series-Shunt SPDT Switch, used designers Communications Transceivers alternately connect transceiver' antenna either Transmitter Receiver. Figure 2.11 shows typical quarter line switch lumped circuit equivalent.
Figure 2.11 Quarter-Wavelength Antenna Switches quarter -wavelength line Switch uses unique property quarter-wavelength impedance transformer [3]. Ordinarily, quarter-wavelength line used match network elements unequal impedance over narrow band. unequal impedances, then they will matched characteristic impedance transformer, related equation:
Microsemi Corp.-WatertownM Pleasant Street., Watertown, 02472M Tel. (617) 926-0404M Fax. (617) 924-1235
signal source matched load they connected quarter wave line characteristic impedance Ohms.
Switch uses this property protect Receiver. fixed (usually Ohms) either forward biased diode isolation state (nearly open circuit) reversed biased diode. nearly short circuit, input impedance (Z2) quarter wave line nearly open circuit. transmitter antenna disconnected from receiver. Similarly, when nearly open circuit (high Impedance), transmitter disconnected from antenna receiver connected antenna. quarter-wavelength switch relatively narrow band SPDT used many Wireless Telecommunication Transceiver designs. quarter-wavelength line constrains bandwidth 10%, which adequate most communications applications. When both diodes forward biased, transmitter connected antenna receiver protected terminating quarter-wavelength line. When reverse biased, transmitter port isolated high reactance quarter-wavelength line (terminated open circuit), Receiver port connected Antenna. biasing scheme very simple, requiring only Choke Coil Blocking Capacitors. Greater than isolation less than 0.25 insertion loss obtained with UM9401, which 0.75 maximum power, Pav, that this switch handle depends power rating diode, forward biased diode resistance, antenna mismatch (VSWR Pav, given equation:
antenna totally mismatched (perhaps connection broken), given
observe further, that current flowing both nearly same both diodes dissipate about same amount power.
BROADBAND ANTENNA SWITCHES
more than bandwidth required, more complex switch structures required. simplest broad band antenna switch construct uses series diodes Compound Switch configuration (similar Figure 2.7) shown here Figure 2.12.
Microsemi Corp.-WatertownM Pleasant Street., Watertown, 02472M Tel. (617) 926-0404M Fax. (617) 924-1235
Figure 2.12 Broadband Antenna Switch Figure 2.12 more broad band SPDT switch, biasing scheme more complex, requiring bias tees return coil, because alternately biased forward reverse now. When Transmitter (and Receiver OFF), forward biased reverse biased. reverse biased forward biased when Receiver transmitter OFF. Transmit Receive isolation state depends solely reverse bias Capacitance this becomes upper frequency limitation switch. Isolation increased using techniques discussed "Tuned Switches" section. replaced similar diodes series, Isolation increases without reducing bandwidth significantly. course, diodes will represent increase Insertion Loss unless bias current increased off-set increase Although diode parasitic reactances somewhat limit bandwidth over which Insertion Loss high Isolation achieved, operating bandwidth also limited bias network, which filter network that isolates bias current from circuit components. frequency response this bias network should measured with diodes removed from switch circuit. selected primarily based power handling capability. UM2101 series recommended Band UM4001 UM4901 VHF, UHF, L-Band applications, either axial leaded package insulated stud package) because their excellent thermal properties. circuit construction, UPP9401 recommended exposed high currents therefore should selected Capacitance distortion. 1N5767, UM7301B, UPP1002 (SMT) recommended example, UM9401 used 1N5767 used receiver isolation will greater than MHz, greater than HIGH POWER BROADBAND ANTENNA SWITCH example high power broad-band antenna switch, designed operate over band, shown Figure 2.13.
Microsemi Corp.-WatertownM Pleasant Street., Watertown, 02472M Tel. (617) 926-0404M Fax. (617) 924-1235
Figure 2.13 High Power Broadband Antenna Switch This switch control transmitter power with excellent distortion performance dBc). forward bias into Bias Terminal Ampere, power dissipation transmitter diode reverse bias Volts Bias Terminal that excessive current does flow state. Band MHz) switches should UM2010 series Band (0.3 MHz) switches should UM2310 series diodes.
MUPTIPLE POLE-MULTIPLE THROW SWITCHES SWITCHES) far, have only discussed single pole, single multiple throw switches. Switch Matrix generalization concept Switch, which inputs connected outputs means network interconnecting switches. Reference discusses this generalized case.
2.14 Double Pole Double Throw Switch simplest case Double Pole-Double Throw Switch Transfer Switch, which quite important circuit designers. DPDT Switch allows pair input terminals connected either pairs output terminals Figure 2.14. performance each pair connections analyzed SPST Switch. DPDT Switch will discussed detail Chapter when used Transfer Switch Amateur Radio Transmitter Antenna. application replace relays Power Amplifiers.
Microsemi Corp.-WatertownM Pleasant Street., Watertown, 02472M Tel. (617) 926-0404M Fax. (617) 924-1235
DEVICE
HIGH VOLTAGE >2000
HIGH AVERAGE POWER >100
HIGH PEAK POWER
HIGH POWER DUPLEXERS >100
ANTENNA SWITCHING >100
HIGH FREQUENCY 1GHz
FREQUENCY
ULTRA FREQUENCY
CURR
HUM2020 UM2100 UM2300 UM4000 HUM4020 UM4300 UM7000 UM7100 UM7200 UM7300 UM7500 UM9401 UM9415 UMM5050 UPP9401 UPP1004
Microsemi Pleasant Street, Watertown, (617) Tel. Fax. (617) 924-1235
CHAPTER
DIODE ATTENUATORS
NOTES
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
DIODE VARIABLE ATTENUATORS INTRODUCTION Attenuator network designed introduce known amount loss when functioning between resistive impedances: Zout defined terminal impedances which attenuator connected. MATCHED ATTENUATORS input attenuator matched output circuit matched attenuator loss entirely Transmission Loss Reflection Loss. source (input) load (output) reversed since resistive networks reciprocal. resulting matched attenuator design said symmetrical, exhibit network symmetry. Matched Attenuator Networks either balanced unbalanced (with respect ground), depending exact nature source impedance load impedance. Examples principle attenuator configurations their balanced, unbalanced, symmetrical forms, appear figures 3.1, 3.2, 3.3. These will referred later chapter diode attenuator designs obtained.
Figure Unbalanced Balanced Symmetrical
Figure Unbalanced Balanced Symmetrical
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
Figure Bridged Bridged
Design equations unbalanced symmetrical cases given below, because their usefulness later sections. Symbols used these design equations have following meaning:
terminal Impedances (resistive) which attenuator matched. (Symmetrical Case) ratio power absorbed attenuator from source, power delivered load. ratio attenuator input current, output current into load. (N)1/2 symmetrical case. attenuation (dB) log(N)
SYMMETRICAL SYMMETRICAL
BRIDGED
Design equations other cases given Reference REFLECTIVE ATTENUATORS: matched condition required, simpler networks designed reflective attenuators. These consist simple variable series shunt resistive element, that attenuates exhibiting necessary mismatch reflection transmission line. these instances, attenuation loss almost entirely Reflection Loss although some small amount Transmissiom Loss occur. Examples Reflective Attenuators occur later this chapter. ATTENUATOR DIODES basic attenuator configurations realized inserting Current Controlled Resistors (PIN Diodes) place variable resistances Figures 3.1, 3.2, 3.3. case Symmetrical Microwave Bridged Attenuator, Ohms, variable resistors, replaced diodes.
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
Variable attenuators, with diodes variable resistance elements, forward biased resistance characteristic (Figure 3.4) device over nearly complete forward bias range. extremely current range avoided because (see Appendix current values, diode' stored charge small diode rectify, causing attenuator' signal distortion increase.
Figure Typical Forward Biased Resistance Current, UM9552 DIODE ATTENUATOR CIRCUIT APPLICATIONS diode attenuator circuits used automatic gain control (AGC) circuits power leveling applications. They also used high power modulator circuits, which subject Chapter typical configuration shown Figure 3.5.
Figure Leveler Circuit diode attenuator simple reflective attenuator, such series shunt diode mounted across transmission line. Some attenuators more complex networks that maintain impedance match input power load attenuation varied across dynamic range. Other methods used implement Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
function, such varying gain transistor stage. diode circuit results lower frequency pulling lower signal distortion. Microsemi Corp. provides number diodes designed attenuator applications, such UM2100, UM7301B, UM4301B, UM9552, UM9301, which provide high dynamic range signal distortion frequencies from GHz. These devices available packages designed standard board construction packages suitable Surface Mount Technology. MICROWAVE MATCHED ATTENUATOR CIRCUITS design equations various matched attenuator circuits configurations have already been given. look practical implementation these designs microwave attenuators. QUADRATURE HYBRID ATTENUATORS Quadrature hybrids commercially available from GHz, with inherent bandwidths decade. Figures typical quadrature hybrid circuits with series shunt configured diodes. Quadrature Hybrids branch lines, attenuation function diode resistance shown Figure 3.8.
Figure Quadrature Hybrid Matched Attenuator (Series Mounted Diodes)
Figure Quadrature Hybrid Matched Attenuator (Shunt Mounted Diodes) Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
Figure Attenuation Quadrature Hybrid Attenuators
following equations summarize performance these quadrature hybrid attenuators:
Series Connected Diodes
Shunt Connected Diodes
Attenuation
Attenuation Zo)]},
quadrature hybrid configuration control twice power simple series shunt diode attenuators because incident power divided into paths hybrid. Reference shows that maximum power dissipated each diode only total incident power this occurs value attenuation. However, branch load resistors must able dissipate total incident power maximum attenuation. purpose branch load resistors make attenuator less sensitive differences between individual diodes increase attenuator power handling
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
Both types hybrid attenuators exhibit good dynamic range. series configured hybrid attenuator preferable attenuation levels greater than whereas shunt configured hybrid attenuator preferable attenuation ranges below
QUARTER-WAVE ATTENUATORS
Matched attenuators also configured using quarter-wavelength circuit techniques, using either lumped distributed circuit elements. quarter-wavelength matched attenuator with series connected diodes shown Figure with shunt connected diodes Figure 3.10. Performance equations given below circuit diagrams, attenuation characteristics plotted Figure 3.11 transmission system with characteristic impedance Ohms.
Figure Quarter-Wave Matched Attenuator (Series Connected Diodes)
Figure 3.10 Quarter-Wave Matched Attenuator (Shunt Connected Diodes) Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
following equations summarize performance these Quarter-Wave Attenuators: Quarter-Wave Attenuator performance equations: (Series Connected Diodes) Attenuation Shunt Connected Diodes Attenuation
Figure 3.11 Attenuation Quarter-Wave Attenuators Quarter-Wavelength Attenuators matched when both diodes biased same resistance. This usually occurs since both diodes connected series current supply, same forward bias current flows through both diodes. series connected configuration preferable higher values attenuation shunt connected configuration preferred lower attenuation levels.
BRIDGED ATTENUATORS
fundamental attenuator design configurations, together with design equations, were described initial section this chapter. most appropriate matched broadband attenuator applications, especially those bands from Band through Band, Bridged circuits. upper cutoff frequency these circuits often depends bias circuit isolation that obtained with practical circuit components. Feed through leakage higher values also affect highest value attenuation that particular design achieve. Bridged circuit shown Figure 3.12 circuit, Figure 3.14.
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
Figure 3.12 Bridged Attenuator Circuit attenuation Bridged circuit obtained from following equations[1,2]: Attenuation RS1),
These equations solved show that attenuation depends ratio RS1, whereas attenuator match conditions depends product RS2. relationship between forward biased resistance (RS1,2) diode forward bias current also needed determine sets values diode driver currents that needed maintain impedance match each value attenuation desired. Figure shows UM9552. design procedure Bridged circuit using UM9552' available [2]. attenuation curves Bridged Attenuator shown Figure 3.13.
Figure 3.13 Attenuation Bridged Attenuators
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
Figure 3.14 Attenuator Circuit attenuator circuit also equations that define dependence attenuation state values three diode resistances[1].
Attenuation (RS1 Z0)} where: (Ohms) RS12 (Ohms)
attenuator equations solved obtain performance curves shown Figure 3.15. that minimum value Ohms. simply means that attenuator symmetrical, power source load impedances same equal Ohms.
Figure 3.15 Attenuation attenuators
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
both Bridged Attenuator circuits, diodes biased different values resistance simultaneously these must track that attenuator remains matched different values over dynamic range attenuator. Suggested voltage controlled bias circuit shown Figure 3.16 Bridged attenuator Figure 3.17, attenuator.
Bridged Attenuator Bias Circuit Figure 3.16
Attenuator Bias Circuit
Figure 3.17
REFLECTIVE ATTENUATORS
contrast Matched Diode Attenuator Circuits, Reflective Attenuators designed using single series shunt diode switch configurations (Chapter this application, diodes only biased forward direction, utilizing current control resistance characteristic diode. Referring Figure 3.4, forward bias current continuously varied from high resistance resistance values. Attenuation obtained introducing impedance mis-match transmission line. This causes some power reflected back toward power source. This undesirable many systems applications because cause frequency pulling power instability. However Reflective Attenuators inexpensive design build. attenuation values obtained using these reflective attenuators calculated from following equations:
Series Connected Diode Attenuator: Attenuation
Z0),
Shunt Connected Diode Attenuator: Attenuation
These equations plotted Figure 3.18 series shunt attenuators with Ohms. These equations curves assume that Diode Impedance purely resistive. Above Band, Capacitive Inductive Reactances packaged diode chip must taken into account.
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
Figure 3.18 Attenuation Reflective Attenuators DISTORTION DIODE ATTENUATORS Distortion particularly critical parameter diode attenuator circuits defined, described, discussed Appendix reference [3].
APPLICATION High Power Frequency Ultra Frequency
RECOMMENDED DIODE TYPES UM2100, UM4000, UM4300, UM9552 UM4000, UM6000, UM7000 UM2100, UM4000, UM4300, UM9552 UM2100, UM9552
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
CHAPTER
DIODE MODULATORS
NOTES
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
DIODE MODULATORS
INTRODUCTION
Chapter said that Microwave diode semiconductor device that operates variable resistor, whose value defined bias state frequency (compared Carrier Wave) variable bias. Chapter Diode described Switching Element, whose control current switched control signal. Chapter Diode described Attenuating Element, whose control current varied continuously (but perhaps also discrete steps) produce various levels attenuation signal. main difference between applications manner which bias conditions defined diode circuit. both applications, only signal band signals) present circuit. Chapter diode described Modulator Element. Modulator applications much more complex analyze that discrete signal frequencies present diode simultaneously. These consist Carrier Wave (usually single frequency Microwave Bands) much slower varying, lower frequency signal sub-band range). lower frequency signal current represents relatively slowly varying "bias current" that modulates I-region impedance that diode exhibits Carrier Wave current, causing amplitude Carrier Wave change. detailed analysis specific modulator design depends relative maximum amplitudes signals, location signals frequency spectrum, waveform frequency modulating signal [1]. modulator designs, described this chapter, Continuous Amplitude Modulation Pulsed Amplitude Modulation. They readily implemented with diodes. These modulator networks assumed broadband with restrictions impedance termination various sideband frequencies. reader referred general literature other design constraints. Microwave modulation techniques discussed Chapter distinct from Digital Modulation Techniques that prepare information signal transmission through Channel [2].
MODULATION BASIC CONCEPTS
Modulation process whereby certain characteristics Carrier Wave varied modified accordance with message information signal which Analog Digital format. Modulation also called Up-Conversion since information signal "up-converted" from Signal Band (usually some segment band, depending waveform Signal)) Carrier Wave Band (usually Microwave Bands) efficient transmission through Channel. Ordinarily, there least separation frequency between Signal Band Carrier Wave Band ease designing Filters needed provide isolation between circuit components operating multi-band network.
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
MICROWAVE AMPLITUDE MODULATION
Carrier Wave Continuously Amplitude Modulated Analog Signal Source, Modulated Wave always present modulated output. This Diode Modulator Circuit actually Diode Attenuator circuit which diode "forward biased" signal wave while Carrier Wave also present diode. forward biased Resistance diode (relatively) slowly continuously varied information signal waveform producing Continuous Amplitude-Modulated wave, shown Figure 4.1. Note that carrier frequency retains sinusoidal wave-form while amplitude envelope varies modulation frequency. carrier wave peak amplitude "A", while modulation wave peak amplitude "B". modulation index given measure depth modulation. Carrier Wave said modulated.
Figure Continuous Amplitude Modulated Wave frequency spectrum Continuous Amplitude Modulated wave shown Figure 4.2, which shows three distinct frequencies: Carrier (FC), lower sideband FS), upper sideband FS). sidebands separated from carrier frequency magnitude frequency modulation signal (Fs). Figure frequency domain representation waveform Figure because only amplitude each sinusoidal wave appropriate location frequency spectrum shown. Both sidebands exist because modulation network broadband they therefore terminated Characteristic Impedance Ohms. Balanced Amplitude-Modulation used suppress Carrier Wave. This achieved using hybrids, each Carrier Frequency Modulation Signal Frequency, diodes, balanced network [4]. sidebands then filtered obtain Single Sideband Output Waveform (SSB-AM), which greatly increases transmitter efficiency.
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
Figure Frequency Spectrum Continuous Amplitude-Modulated Wave
MICROWAVE POWER MODULATORS diodes preferred active elements Microwave Power Modulators. switching speed must fast enough diode respond modulating signal, without introducing non-linear modulation effects. diode' minority carrier lifetime should long enough provide level Intermodulation Distortion. diode Modulator applications circuit configurations that similar diode attenuator circuits. Since modulation signal into bias port, bias circuitry must sufficiently broadband that modulation signal distorted. Isolation between modulation insertion port Carrier input port should least circuitry should sufficiently broadband terminate carrier both sidebands Ohms. pulsed continuous (linear analog) modulators, quadrature hybrid circuit shown Figure satisfies bandwidth Isolation requirements. Such quadrature hybrids available from about compact form.
Figure Quadrature Hybrid Matched Modulator Dynamic Ranges achievable certain Continuous designs since Diode' lifetime characteristic improves modulation linearity over signal amplitude range. unique characteristic large signal Diode Continuous modulators that Diode device parameters adjusted that modulation efficiency linearity optimized.
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
Carrier Wave pulse modulated, signal output present between pulses. This Diode Pulse Modulator Diode Switch circuit that rapidly biased Insertion Loss state (the high Isolation State according alternating polarities pulsed information signal. pulse modulation mode, Carrier Wave transmitted during state. Usually, output signal Pulse Information Source sufficiently weak that must amplified modulation driver (Amplifier) circuit that Diode driven without distortion pulsed output waveform. DEMODULATION Demodulation described here complete view Modulator integral part Channel [2]. Baseband Signal Processing prepares Modulation Signal Up-Conversion Channel' Carrier Band. Ultimately, Modulated Carrier received Demodulated additional Baseband Signal Processing. success with which original Modulation Waveform retrieved this process depends linearity (both amplitude phase) modulation process free space characteristics Channel. Demodulation Detection inverse process Modulation. Receiver, Amplitude Modulated Waveform inputted Demodulator Modulation Signal Down-Converted baseband (d-c MHz). Ideally, Demodulated Wave should faithful replica original Modulation Wave that inputted Transmitter' Modulation Circuit. re-labeled version Figure shown below indicate that basically, Demodulator circuit Modulator circuit with inputs output reversed (Figure 4.4).
Figure Quadrature Hybrid Marched Demodulator
APPLICATION High Power Frequency Ultra Frequency
RECOMMENDED DIODE TYPES UM2100, UM4000, UM4300, UM9552 UM4000, UM6000, UM7000 UM2100, UM4000, UM4300, UM9552 UM2100, UM9552
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
CHAPTER
DIODE PHASE SHIFTERS
NOTES
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
MICROWAVE PHASE SHIFTERS
INTRODUCTION Chapter Four mentioned that "all three characteristics Carrier Wave: Amplitude, Frequency, Phase, modulated". Since only Amplitude Modulation diodes described, more complete discussion Carrier Wave form required. present chapter Phase Shifters, phase Sinusoidal Wave form must described that concept Phase Shift through Microwave Circuit more than intuitive heuristic significance reader.
TIME VARYING SINUSOIDAL WAVEFORMS Input signals Microwave Circuits usually described physically terms single sinusoidal wave forms, more complex wave forms, described composite summation number sinusoidal wave forms. most frequent form electronic-circuit applications, sinusoidal voltage function time general form:
where: amplitude, frequency, phase angle with respect some arbitrary phase angle reference, shown Figure 5.1.
Figure Sinusoidal Function
Figure 5.1, frequency related inverse period wave, phase angle wave with respect phase reference. phase reference chosen arbitrarily define initial value Phase Angle input circuit. Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
PHASE ANGLE Phase Angle measure progression periodic wave time space from chosen instant position. Phase periodic wave, v(t), frequency which corresponds period fractional part period through which advanced relative arbitrary origin phase reference. Circuit components have some physical size Voltage wave form passes through these components with certain finite velocity time transit. total phase angle, will increase "shift" amount related this transit time through these components. This residual phase shift associated with specific component specific wave form frequency. Small circuit-components operating relatively frequencies (i.e., Band) will exhibit very small residual Microwave Bands, however, these same components will exhibit significantly larger residual phase shifts. MICROWAVE DIODE PHASE SHIFTERS Microwave Phase Shifters utilize these properties wave propagation through circuit component structures. They designed produce phase shift effects required operation certain classes Antenna Systems RADAR Communications applications. Reference treats subject Semiconductor Phase Shifters Antenna arrays great detail. treatment here deals specifically with diode Phase Shifter circuit design.
diode considered lumped variable-impedance microwave circuit element. Microwave signals passing through diodes experience some finite phase shift. lumped element representation permits design compact phase shifter circuits frequency bands. However, higher microwave frequency bands, losses increase power handling capability decreases. Figure shows example diode phase shifter with driver-amplifier connections. Note that there driver circuit each phase shifter bit.
Figure Diode Phase Shifter With Driver-Amplifier Connections diodes utilized series shunt connected switches phase shifter designs. switched elements either lengths transmission line reactive elements. criteria choosing diodes Phase Shifters similar those used other switching applications. addition, there possibility introducing phase distortion, particularly high power levels reverse bias voltages. Microsemi diodes characterized thick I-regions long carrier lifetime these characteristics that result phase distortion.
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
REAL TIME DELAY PHASE SHIFTER major application electronically controllable, rapid-acting microwave phase shifters antenna systems Phased Array Radar. these systems, phase shifter placed series with each radiator array antennas. radiating-phase-front direction controlled varying time delay from source common signal each radiating element array. steering array independent radiation frequency. circuit used produce this time delay real-time-delay phase shifter nondispersive phase shifter. essentially switched section transmission line. time delay through circuit must independent phase-shift state. phase shifter nondispersive because overall phase shift directly proportional frequency. Phase-Frequency Characteristics dispersive nondispersive phase shifters shown Figure discussed next section.
Figure Phase-frequency Characteristics Phase Shifters
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
TYPES REAL TIME DELAY PHASE SHIFTERS simplest design real-time-delay phase shifter uses circulator coupler provide matched input output switched elements, shown Figure 5.4.
Figure Elementary Real-Time-Delay Phase Shifters
both phase shifters, SPST diode switches used either short circuits open circuits over finite bandwidth. Dependence section line time delay causes bandwidth limitation this class phase shifters, even ideal couplers circulators identical SPST switches used. narrow band performance delay lines produces dispersion that results nonconstant time delay. narrow band systems, this design viable approach. POWER LIMITATIONS choice diodes limits maximum power that carried these phase shifters. diodes have sufficient stored charge prevent them from producing wave form distortion. Their capability limited thermal dissipation absorbed power voltage breakdown. choose diode specific design, specify maximum allowable current voltage applicable forward- reverse-biased diode. Using these values, calculate maximum power that phase shifter design carry. hybridcoupled phase shifter Figure 5.4, calculate from structure SPST diode switch that each switch carry Amperes forward biased state Vmax Volts reverse biased state. then state that Pmax max2
where characteristic impedance transmission line which SPST built (which must matched generator) Pmax maximum incident power that carried this line when switch maximum current state. DISPERSIVE PHASE SHIFTERS distinguishing feature this type phase shifter production frequency-independent phase change. Figure illustrates hybrid-coupler type dispersive phase shifter, which uses diodes shunt
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
Susceptance across transmission lines. entire Phase Shifter would contain number these "unit Cells" that each contribute specific Phase Shift Bit, value example, "Cell" could designed 22.5 degree Bit, another "Cell", degree
Figure Hybrid-coupler Phase Shifter "Cell"
Figure Possible Loading Elements Hybrid-coupler Figure 5.5. diode alone switchable Capacitance terminating transmission line. Forward reverse bias values chosen such that, forward bias case, when reverse biased, where determine Phase Shifter value desired [2,3]. total phase change difference between forward reverse biased phase changes: which desire frequency independent. Thus impose condition:
which condition equal time delay bias stated. This characteristic that distinguishes dispersive from nondispersive phase shifter. dispersive phase shifter, using diode branch, shown Figure (a). suitable values approximately degrees. dispersive phase shifter, using diodes branch, shown Figure (b). This configuration produce phase changes excess degrees.
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
GENERAL DISCUSSION DIODE PHASE SHIFTER CIRCUITS schematic diagram 4-bit phase shifter[2] which gives steps increments 22.5 degrees, shown Figure 5.7.
Figure Schematic Transmission 4-bit Phase Shifter phase bits take three basic circuit topologies. Switched -line phase (Figure 5.8) Hybrid-coupled phase (Figure 5.9) Periodically loaded-line phase (Figure 5.10) choice circuit topology phase depends factors such number diodes required, power level, fabrication ease, cost. Minimum insertion loss condition implies equal loss each state. SWITCHED-LINE PHASE SHIFTER
Figure Switched-Line Phase Shifter switched-line phase shifter shown Figure 5.8. This circuit consists SPST switches lengths transmission line each bit. Four diodes required minimum. transmission line lengths Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
arbitrary, circuit used phase values from degrees. also used time delay network. Isolation switch branch must greater than avoid phase errors. Minimum loss occurs each SPDT switch when energy absorbed branch equal loss pass branch. diode resistance equal both states, minimum loss occurs when Characteristic Impedance T-junction equal average Capacitive Reactance diodes. Analysis this type shows that circuit optimized minimum loss properly choosing T-junction, peak-power capacity twice that other type phase bit. line loss neglected, Insertion Loss same sizes. HYBRID COUPLED- PHASE SHIFTER
Figure Hybrid-Coupled-Bit Phase Shifter Hybrid-coupled-bit phase shifter, Figure 5.9, hybrid junction with balanced phase bits attached coupled branches. Analysis this type shows that voltage (VBR) required diodes depends size which diode used, assumed that equal power incident cascaded bits. highest degree reduced smaller bits. Similarly, insertion loss also function size. loss degree then loss smaller bits hybrid-coupled-bit phase shifter least loss three types uses only diodes bit. analysis hybrid-coupled phase shows that peak-power capability optimum impedance level equal loss both switch states given where Equation shows that obtain high-power handling capacity, diodes must have high VBR, relatively high Capacitance, operate transmission lines with impedance levels. breakdown voltage required diodes depends size which diode used. This requirement highest 180o reduced factor,sin( /2), smaller bits. dissipation loss 1800 loss smaller bits sin( /2). Hybrid Coupled Phase Shifter least loss three types being considered here, uses least number diodes. Shorter bits obtained from degree using transformed-switch technique[2], which consists placing impedance transformer one-eighth wavelength before input port degree bit. impedance transformation ratio transformer varied produce various phase-bit sizes.
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
LOADED-LINE PHASE SHIFTER
Figure 5.10 Loaded-Line Phase Shifter loaded-line circuit uses switched loading Susceptances spaced quarter-wavelength apart along transmission line (Figure 5.10). Adjacent loading Susceptances equal switched into either Capacitive Inductive state. Impedance-matched transmission both states maintained choosing impedance level transmission-line section between diodes magnitude loading Susceptance, characteristic Impedance connecting transmission line, related required phase shift, section Phase Shifter optimum impedance equations (5.6 (5.7
diodes either directly mounted stub mounted across transmission line. Average power handling this configuration limited diode practical level which characteristic impedance reduced. peak power capacity function phase step size. equal insertion loss each state phase step, peak power capacity loaded line phase shifter half hybrid-coupled-bit design. insertion loss small phase steps cascaded achieve degree phase shift times loss degree hybrid-coupled-bit circuit. achieve high power capability, loaded-line phase shifter uses many diodes small phase increments. CONCLUSIONS: comparison with loaded line phase shifter, hybrid coupled design handle twice average peak power when using same diodes. both hybrid loaded line designs, power handling capability maximum size related maximum current peak Voltage that diodes withstand. characteristic impedance circuits variable that used adjust current voltage stress within device ratings. This means that reduced below Ohms reduce voltage stress favor higher currents. maximum current rating Microsemi diodes depends power dissipation rating while maximum voltage stress dependent I-region thickness.
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
DEVICE
HIGH VOLTAGE >2000
HIGH AVERAGE POWER >100
HIGH PEAK POWER
HIGH POWER DUPLEXERS >100
ANTENNA SWITCHING >100
HIGH FREQUENCY 1GHz
FREQUENCY
ULTRA FREQUENCY
CURR
HUM2020 UM2100 UM2300 UM4000 HUM4020 UM4300 UM7000 UM7100 UM7200 UM7300 UM7500 UM9401 UM9415 UMM5050 UPP9401 UPP1004
CHAPTER
DIODE CONTROL CIRCUITS WIRELESS COMMUNICATIONS SYSTEMS
NOTES
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
DIODE CONTROL CIRCUITS WIRELESS COMMUNICATIONS SYSTEMS INTRODUCTION Chapter discusses Control Circuits that appropriate Wireless Communications Systems, from Band (2-30 MHz) through GHz. Many control circuits discussed Chapter (PIN Diode Switches) Chapter (PIN Diode Attenuators) suitable specific wireless system applications. Some most important circuit applications, using diodes, discussed below. DIODE ANTENNA TRANSMIT/RECEIVE SWITCHES Transmit Receive Antenna Switches commonly used connect Transceiver' Antenna either Transmitter port Receiver port. Physically, this circuit Single Pole Double Throw Switch (SPDT) that either position, must have very loss "ON" State (less than high Isolation (typically, "off" State. Several Switch circuits appeared Chapter repeated here Figures 6.2.
Figure QUARTER WAVE ANTENNA SWITCH Figure narrow band SPST switch configured switch Antenna Port between either Transmitter Receiver Ports. Quarter-wavelength line narrow band limitation, systems that have been allocated only signal bandwidth, this very practical solution. When both forward biased, Transmitter connect Antenna, Receiver protected isolation network terminating quarter wavelength line. When reversed biased, Transmitter Port isolated quarter wavelength line Receiver Port connected Antenna. biasing scheme very simple, requiring only Choke Coil Blocking Capacitors. quarter wavelength line simulated low-pass network conserve board space.
Figure BROADBAND ANTENNA SWITCH
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
Figure broad band SPDT switch, biasing scheme more complex. requires bias tee' return coil because alternately biased forward reverse. example, when Transmitter "ON" (and Receiver "OFF"), forward biased reversed biased. Transmit Receive Isolation State depends solely Reverse Bias Capacitance bias conditions exchanged when Receiver "ON". Isolation increased using diodes series instead "D2". MICROSEMI UPP1001 Powermite® series distortion surface mount packaged diodes were designed distortion Wireless Communications handset applications where battery power management extremely critical issue with circuit designers. They excellent handset, mobile, base station applications where very Insertion Loss required. UPP1001, mounted network, with forward bias current less than UPP1000' have nominal Capacitance (Ct) 1.25 which adequate bandwidth applications (similar Figure 6.1) without inductive tuning. this series broader applications (similar Figure 6.2), either devices used series single device placed branch tuned switch design.
DIODE ANTENNA DUPLEXING SWITCH DUPLEX (communications) pertains simultaneous two-way independent transmission both directions [1]. HALF-DUPLEX (communications) pertains alternate, time, independent transmission. this sense, Duplexing Switch, shown both block diagram form Figure schematic form Figure (b), half-duplexing switch.
Figure Duplexing Switch Cellular Telephone: Block diagram Schematic
block diagram emphasizes basic function blocks duplexing switch. receive transmit signals connect Antenna port power combiner Y-Junction, which provides matched, loss connection Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
either signal path. individual switches SPST switches, discussed Chapter diode shunts resonant circuit that determines pass band either receive transmit switch. single dual polarity switch control suffices both switches, since polarity inverter provides opposite polarity receive switch.
receive channel below transmit channel, depending specific system application. Figure represents cellular phone, receive switching circuit tuned transmit switching circuit tuned band. antenna channel filter network passes both transmit receive bands.
DIODE ANTENNA DIVERSITY SWITCH CELLULAR TELEPHONES This antenna switch cellular telephones provides antenna function well antenna space diversity function cellular telephones installed vehicles. provided with switched d-c- connection port charge telephone' battery pack, referred "docking switch". battery pack charging function shown because concern here with switching circuits. functional diagram antenna diversity switch shown Figure 6.4.
Figure
Diode Antenna Diversity Switch Cellular Telephones
This composite switch contains both SPDT SPST switch (Chapter would suitable either United States European cellular systems. switch function performed SPDT switch. added feature that SPST switch positioned signal strength circuit provide connectivity antenna that provides stronger receive signal receive port. typical vehicle mounted application, this composite switch would designed switch transmitter power with compression point dBm. Isolation between transmit receive ports would about Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
Both isolation compression point specified with transmitter connected main antenna (M-ANT port). maximum switching speed SPST SPDT VSWR input output ports less than possible positions switches. SPDT switch could rendered either series diode switch (Figure 2.7) shunt diode switch (Figure 2.8), depending system design parameters such bandwidth, power handling, circuit construction technology. UPP9401 would excellent choice surface mount SPDT SPST switches.
DIODE ELLIPTICAL DIRECTIONAL ANTENNA ARRAYS WIRELESS DATA MODEMS Several firms adopting elliptical directional antenna array provide space diversity their mobile wireless data modems. serving base station would have such antenna array well. array signal processor, with sufficient number antenna elements, reduce volatility link between mobile unit base station. design concepts were originally described base stations [1,2].
simplified version base station array concept implemented lower diminish effects multi-path fading co-channel interference mobile data modem also. multi-element array, consisting either antenna elements used Improve "front-to-back" ratio (directivity) array Improve "self-interference" problem dual arrays Improve antenna gain elements yields antenna gain available power (Pav) from transmitter output stage depends system' overall linearity specification (Appendix diode switches used switch elements antenna array, because power output stage-antenna switch interface optimized achieve most linear solution [3]. effective radiated power from data modem GHz, various combinations antenna elements available power gain will result following solutions: Antenna Elements Antenna Gain Power Output Linearity* more linear less linear
quantitative statement linearity depends their system parameters than those considered here. With fewer antenna elements, transmitter output gain increased obtain required output power, thus decreasing transmitter-switch overall linearity. elliptical antenna array specially configured switch matrix. switch matrix, where either depending particular data modem antenna design. array center connectivity between input feed branch antenna elements provided divider/combiner network. radial combiner geometry, given [3], which circular symmetry, consists array branch transformers Ohms Ohms), that connect antenna elements central junction antenna feed port. elliptical antenna array would require more complex network branch transformers since elliptical symmetry simple circular symmetry. However, focus here diode switch design Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
bias conditions. analysis shunt mounted diode switches branch arms radial combiner generally applicable other combiner geometries well.
Figure
Shunt Mounted diode Switching Network
shunt mounted diode switch placed about quarter wavelength from central junction that appears virtual short circuit point connection open circuit central junction). Each branch antenna switch connected central junction with transmission line transformer Ohms Ohms). exact position switch along transformer determined modeling switch sub-network transformer combination until high reflection coefficient degrees phase angle observed transformer. This modification switch position ensures that open circuit seen center radial switch antenna ports that turned off. Figure shows location diode compensating network. Shunt switches work having diode connected between transmission ground plane. When diode impedance state, short circuit created incident wave traveling along transmission line totally reflected. diode high impedance state, appears circuit all, allowing incident wave pass unimpeded. practice, diode finite dimensions performance "ON" "OFF" states affected Resistance Capacitance diode junction parasitic reactances diode package. These effects compensated incorporating diode' equivalent circuit into sub-network design model that determines design compensating network. Diode Automatic Gain Control (AGC) Loops
Automatic Gain Control (AGC) important power control function mobile communications systems. base station establishes connection between mobile units cellular phones within cell area that Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
controls. also maintains that link until transmitted signal either mobile units sufficiently weak that specified Quality Service (QoS) cannot maintained. weaker station then handed adjacent call' base station controller. circuit mobile unit element establishment maintenance call. base station monitors signal strength each mobile unit under control. transmits control signal mobile unit' circuit (via link maintenance channel) when transmitter output power must adjusted maintain call continuity. Thus, mobile unit' function fundamental part base station call link control function. block diagram generic circuit shown Figure (Chapter Figure typical block diagram function cellular radio.
Figure
Typical Loop Cellular Radio
base station control signal detected detector circuit decoupled circuit. This signal amplified level attenuator adjust input power pre-driver. mobile unit approaching base station, will signal larger amount attenuation mobile unit' level provide smaller amount transmitter power needed maintain call. Attenuation decreased more transmitter output power needed call link maintenance. Obviously, base station senses that mobile unit cannot increase transmitter power further, base station call hand controller initiates procedure call handoff adjacent cell' base station. level attenuator circuit chosen from numerous designs presented Chapter usual choices Bridged Diode Attenuator (Figure 3.12) Diode Attenuator (Figure 3.14). UM9301 would excellent choice this level above. OPTIMIZATION TRANSMITTER LINEARITY important parameters optimizing overall performance transmitter Switch combination discussed reference [4], which appears Appendix
APPLICATION WIRELESS
RECOMMENDED DIODE TYPES UPP1001-1004, UPP9401
Microsemi Corp.-Watertown580 Pleasant St., Watertown, 02472Tel. (617) 926-0404Fax. (617) 9241235
CHAPTER
DIODE CONTROL CIRCUITS BAND INDUSTRIAL APPLICATIONS
NOTES
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
DIODE POWER CONTROL CIRCUITS INDUSTRIAL APPLICATIONS
INTRODUCTION series thick I-region diodes have been developed Microsemi specifically Industrial Power Applications. UM2100 Series developed Band Band), UM2300 Series developed 3000 Band Band). unique characteristics these diodes will discussed terms their performance number selected industrial applications. BROADCAST BAND ANTENNA SWITCHES Fundamental work very thick I-region, long lifetime Diodes reported 1980' former David Sarnoff Laboratories [2,3]. These diodes were designed capable replacing mechanical drive, rotary transmit receive switches then being used multi-band antenna switches exterior communications systems. bands interest were band portion band), band band band (VLF band). Subsequently, Microsemi developed series thick, long lifetime diodes provide devices these similar high power, lower frequency applications. UM2100 Series (nominal lifetime microseconds) switch high power Band newer UM2300 Series (nominal lifetime micro-seconds) switch high power through Band (200 3000 KHz). seven-position single pole, useful Band transmitter switching applications, shown Figure 7.1. specifications seven- position switch shown Table 7.1.
Figure Band Switch Configuration
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
Table Specifications Position Band Switch
Power Handling Capability Voltage Current Characteristic Impedance Number Positions (Channels) State Isolation State Insertion Loss Forward Bias Current State) Reverse Bias Voltage (OFF State) Distortion Products
1000 (Max) 450.0 VRms (Max) (Max) 50.0 Ohms 25.0 (Min) (Max) 300.0 -400.0 -60.0
equivalent circuit switch branch, convenient calculating reverse biased switch performance parameters, shown Figure 7.2.
Figure Diode Equivalent Circuit Figure similar Figure Chapter except that series Inductance omitted because Reactance negligible Band. equations calculating Insertion Loss Isolation Single Pole, Single Throw Switch given Chapter Band, UM2110 diode recommended this application. diode forward biased such that Ohm, then less than Isolation depends diode' Capacitance (Ct) under reverse bias. MHz, Isolation greater than MHz, Isolation about note that state Insertion Loss constant with frequency state Isolation varies with frequency since function state Capacitive Reactance diode. Referring Figure 7.1, these Insertion Loss Isolation calculations valid from seven input ports (individually) common output port. When ports state, their input port input port Isolation increased above that single branch) because state Capacitance diodes dual branch half that single diode. Note that input ports switched simultaneously. Appendix contains discussion "Non-linear Effects Semiconductor Devices" which interrelationship between device non-linearity rectification impressed signal developed. main
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
difference between pn-junction diode diode that pn-junction non-linear resistance that rectifies input signal. diode, over specific operating conditions, presents linear resistance input signal, thus, does rectify signal. These operating conditions discussed Chapter sections titled: Large Signal Diode Operation Frequency diode Operation. been experimentally observed [1,2,3] that incremental increase signal power while signal frequency continually lowered, cause on-set rectification diode. This effect observed when induced stored charge I-region longer least five times larger than induced stored charge. diode rectifies signal because presents non-linear impedance power source load under these operating conditions. Under normal operating conditions, diode switches need relatively small amount bias current control (switch) large values current. This unique feature diode. Several hundred milli-amps current control several hundred Amperes current. PN-junction diodes (Varactors) used switch high values power, current demands switch' power supply enormous. hundred Amps current must switched pn-junction diode, hundred Amps bias current must available from bias current supply. High current levels greatly complicate implementation isolation circuits, especially frequencies. BAND TRANSMISSION LINE TUNER Industrial manufacturing equipment, such sputterers, length transmission line connect high power (several transmitter power amplifier load, some distance away from source. Invariably, load presents impedance mismatch connecting transmission line. usual solution introduce matching network between source cable. This matching network consists number Capacitors varying values that switched depending nature mismatch. typical impedance matching network shown Figure 7.3. diode switches gradually replacing relay switches this application.
Figure Band Transmission Line Matching Network Sputtering systems delivering line voltage being produced semiconductor industry currently systems design phase. Microsemi manufacturers several series High Voltage diodes these systems. HIGH POWER TRANSFER SWITCH BAND TRANSMITTERS number manufacturers Band Power Amplifiers Amateur Radio market have introduced solidstate switching using diodes[4]. comparing various switching devices available switch antenna, such vacuum relays, reed switches, solid state devices, ARRL Handbook (reference states Perhaps most modern elegant approach switch antenna between transmitter receiver diodes. There keying-speed constraints when diodes used, proper devices
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
selected, spectral purity output signal will affected diode switch). most important parameter this regard (minority) carrier lifetime". Reference referring Transmit/Receive Switches described Chapter Figure 2.11 shows narrow band Quarter Wavelength Antenna switch configuration Figure 2.12 shows broadband Antenna switch, both adequate several hundred Watt applications. Figure 2.13 shows High Power Broad band Switch that control Kilo-Watt transmitter power with excellent distortion characteristics (IM3 less than dBc). Another antenna switch configuration, shown Figure 7.4, Double Pole Double Throw (DPDT) Switch Transfer Switch, which used calibrate transmitter-antenna interface.
Figure 7.4. Equivalent Circuit Antenna Transfer Switch transfer switch allows either "feed through" line Power Amplifier switched Antenna. connector shown center "feed through" line terminated either short circuit (feed through state) load, transmitter calibration.
Figure 7.5. Double Pole-Double Throw Transfer Switch
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
general, Transfer Switch (Figure 7.5) four port device that valid operating states that defined "truth table" (below). SPDT Switches only connect single input either loads. SPDT Switches cannot perform function inserting sections transmission line between source load (antenna). Note that Figure exact representation Figure 7.4, since lower path Figure contains amplifier there independent Port available.
TRUTH TABLE Control Input State
Truth Table shows bias control conditions that either valid operating states Transfer Switch. control state power flows from Port Port branches RF4. case Figure 7.4, this control state would provide "feed through" line Port terminated short circuit. Control state "transfers" inserted path RF4.
Figure DPDT Switch Configured With SPDT Switches
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
DPDT Transfer Switch Figure configured using SPDT Switches interconnected shown Figure such that output #1ports both SPDTs joined common connector terminal output ports connected amplifier. When bias control (and bias control OFF), through line inserted between source antenna. When bias states reversed, amplifier appears between source antenna. biasing sequence same that indicated "truth table" above. performance characteristics this Transfer Switch depend choice SPDT design. Various SPDT Switch design tradeoffs were discussed Chapter choices consist Series Diode SPDT, Shunt Diode SPDT, Compound (series shunt connected diodes) SPDT switches. Each design bandwidth power handling abilities. Port Transfer Switch bridge network hence extremely wide bandwidth capability. Transfer Switch shown Figure combines wide bandwidth characteristic with high power handling capability compound switch. Several hundred Watts bandwidth switching capability achievable with this design.
Figure 7.7. Broadband High Power Transfer Switch Configuration
RECOMMENDED DIODE TYPES DIODES HUM2020, UM2100, UM2300, UM4000, UM4300 MODULE UMM5050
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
CHAPTER
DIODE CONTROL CIRCUITS MAGNETIC RESONANCE IMAGING SYSTEMS
NOTES
Microsemi Pleasant St., Watertown, (617) (617) 924-1235 Tel.
MAGNETIC RESONANCE IMAGING SYSTEMS
INTRODUCTION Magnetic Resonance Imaging (MRI) Systems manufacturers substantial number very sophisticated Microwave Components Signal Processing parts their systems. basic system used medical diagnosis consists very large, powerful magnet to10 Telsa) surrounding chamber that large enough patient down inside also uses high power, frequency tunable source that rapidly switched off. This produces large field perpendicular magnetic field. This field focused body coil. source both coils must tunable both frequency impedance "match impedance" patient's body. HIGH FIELD RECEIVER COILS Special-purpose coils designed optimize signal-to noise-ratio (SNR) from given region body. State-of-the-art coil systems include four more coils with four separate receivers. This method often referred phased array system although signals added such that signal phase information included [1]. Normally, signal range MHz. During typical clinical image measurements, entire frequency spectrum interest order KHz, which extremely narrow band, considering that center frequency about MHz. This allows single-frequency matching techniques coils because their inherent bandwidth always exceeds image bandwidth. This extremely important consideration when specifying diodes coil switching elements. quality images depends Signal-to-Noise Ratio (SNR) acquired signal from patient. utmost importance obtaining clear images interior human body.
COIL DESIGN PRINCIPLES RELATED SWITCHING DIODES
Figure Simple Circular Loop With Switched
Microsemi Pleasant St., Watertown, (617) (617) 924-1235 Tel.
Figure simple model circular loop with single Capacitive gap. shunted series combination coil (LS) diode. individual reactances about Ohms operating frequency. simplicity, bias circuitry output line across diode shown. value chosen such that Inductive Reactance coil (LS) Capacitive Reactance parallel (phase) resonance when diode forward biased. This parallel resonance causes large impedance zero Conductance) appear across gap, causing loop current decrease zero (open circuit state). Multiple Diode Switch Configurations also used System designs [1]. practical coil would have more gaps [1]. second needed apply synchronization pulse frequency distribution {[sin time initial test pulse image response pulse. Capacitive gaps permit flow current through Loop. diode bias network inhibits flow current through diode, although diode must withstand line voltage when back biased.
CHARACTERISTICS COIL SWITCHING DIODES Exclusion Magnetic Materials
diode must contain magnetic materials, either associated with die, attach metalization system, package assembly. existence magnetic materials diode structure distorts static magnetic fields associated with various coils will interfere with system calibration accuracy clinical measurements. Maximize Signal Noise Ratio
When coil switches (i.e., reverse biased), receivers listening image return pulse. receivers' degraded impedance switch. This effect specified Reverse Bias Leakage Current (IR) diode's Reverse Bias Breakdown Voltage (VBR). alternatively specified equivalent parallel resistance (RP) reverse biased diode [Chapter One]. gradual increase increase reverse bias leakage current result poor passivation diode's I-region. Microsemi diodes passivated with unique proprietary glass passivation process avoid this problem.
Impedance Matching Coils Patient's Body Impedance. most usual frequencies used commercial System design MHz. Image search tune bandwidths KHz, KHz, KHz. absolute values diode parasitic impedances less important than their potential variation from lot. such narrow band applications, these parasitic impedances compensated initial design switch.
Microsemi Pleasant St., Watertown, (617) (617) 924-1235 Tel.
DIODE BIAS CONDITIONS COIL SWITCH APPLICATIONS Since Systems designed operate Bands, bias conditions described Chapter Band High Power Switches apply, with proviso that diodes embedded parallel resonant circuit across loop gap. FORWARD BIAS CONDITION Since forward bias resistance (RS) diode decreases bias current increases (Figure 3.4), Microsemi recommends maximum bias current level compatible with bias circuitry available, with minimum value. forward biased diode Figure adds loss switch circuit when parallel resonance, which decreases open circuit impedance across gap.
REVERSE BIAS CONDITION conservative design would require that diode chosen switch design should least equal peal-to-peak Line Voltage that reverse bias applied diode state least equal peak value Line Voltage.
SWITCH DRIVER CONSIDERATIONS During High Power Reverse Bias condition, Voltage should exceed diode's VBR. Voltage swing exceeds VBR, driver must have sufficient reverse bias current capability achieve desired Switching Speed (Ts), must also provide excess reverse current required during high power pulses. Under this reverse biased leakage condition, diode heat appreciably, causing increase leakage current. leakage current large enough, thermal runaway will cause diode destroyed.
POWER HANDLING practical SPDT switch body coil would required handle power. Power Handling capability specified device manufacturer depends
Maximum Power That Dissipated Diode Max)
Thermal Impedance From Thermal Ground Temperature Which Thermal Ground Maintained (TA)
Microsemi Pleasant St., Watertown, (617) (617) 924-1235 Tel.
Current designs diodes mounted threaded stud packages, either directly electrical ground (C-style) insulated from thermal ground D-style).
MICROSEMI DIODES SOLD U.S. EQUIPMENT MANUFACTURERS
4006 4010 4306 4010 4006 4010 9415 2106 2110 HUM2010 HUM2015 HUM2020
1000 1000 1000 1000 1000 1500 2000
These devices available glass leaded package (Style Stud package (Style insulated Stud package (Style 2100 series available MELF package. HUM2020 series available axial leaded studed versions. stud thread size 4-40 6-32 isolated stud version.
Microsemi Pleasant St., Watertown, (617) (617) 924-1235 Tel.
APPENDIX
DIODE PHYSICS
NOTES
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
APPENDIX
INTRODUCTION
Appendix addresses variety diode physics topics that interest circuit designers wish probe more thoroughly into such issues diode controls large values current with relatively small values current (sometimes referred "current gain"), what minority carrier lifetime related stored charge (Qs), might turn-off time (TRF) diode longer than "turn-on" time (TFR), large signal operation diode different microwave bands than band? Chapter contains just enough preliminary information about diodes that Chapters through understood. Chapters through written that circuit designer focus specific control circuit function being described. felt that introduction some these physics topics Chapters through would distract many readers from main focus particular chapter. Appendix meant substitute text semiconductor device physics. treatment above topics varies thoroughness, references literature given those wish delve more deeply into particular issue. Some physics topics discussed other appendices also. Appendix compares diode circuit characteristics those pn-junction devices. Appendix discusses topic signal distortion diodes.
SOME CHARACTERISTICS DIODE I-REGION
Since presence relatively wide intrinsic layer diode responsible unique properties, worthwhile discussing I-layer first then characteristics, such lifetime stored charge. fact, equivalent circuit parameters, switching times, distortion characteristics diode dependent properties I-layer well. Ideally, desirable I-layer intrinsic (the Silicon crystal structure completely free chemical impurities crystal growth imperfections). With intrinsic I-layer, loss diode under reverse bias would minimized Capacitance would vary least with reverse bias frequency. reality, truly intrinsic layer achievable within diode structure because feasible maintain intrinsic resistivity I-layer throughout processing steps diode. Thus concept layers separated intrinsic layer somewhat artificial. Microsemi P-I-N diodes have Ilayer resistivities 8,0000 12,000 Ohm-cm, either slightly p-type slightly n-type These much lower loss devices than competitive devices with much lower values I-layer resistivity. ideal diode impurities I-layer. Thus, zero bias, I-layer already depleted carriers. Since there mobile carriers I-layer support current flow, diode open circuit signals Capacitance does vary with reverse voltage frequency. practical diode some space charge I-region, presence impurities. Some reverse bias required deplete I-region mobile charge. value reverse bias which space charge depleted referred swept-out "punch through" voltage. important bias diode beyond swept-out voltage most applications, since current flow these mobile carriers constitute unwanted source signal loss noise.
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
desired electrical parameters from "intrinsic" silicon difficulty passivating such high resistivity silicon. impurity level 10,000 Ohm-cm P-type silicon impurity atom approximately billon silicon atoms boron (P-type)! This what makes passivation process difficult.
MINORITY CARRIER LIFETIME
volume bulk lifetime semiconductor defined average time interval between generation recombination minority carriers homogeneous semiconductor. Carriers constantly being generated thermally bulk Silicon [2]. generation rate function ambient temperature Silicon. Carriers also introduced into bulk Silicon connecting Voltage source across diode junctions. There recombination mechanisms that determine effective lifetime semiconductor device.
RECOMBINATION PROCESSES
BULK RECOMBINATION fundamental effect that limits lifetime bulk Silicon thermal-equilibrium condition, ni2, where free carrier density bulk Silicon ambient temperature. basic recombination process band-to-band recombination where electron-hole pair recombines. Some bulk recombination also occurs recombination centers traps [2]. SURFACE RECOMBINATION surfaces Silicon, nearly entire recombination process presence surface states traps surface. Surface recombination velocity determines minority carrier lifetime near Silicon surfaces. Effective device lifetime determined relationship Jeffective /Jbulk Jsurface
CHIP GEOMETRY
diode chip structure shown Figure A.1. When diode wafer processed, bulk Ilayer have minority carrier lifetime somewhere range perhaps This wide range possible values lifetime related particular details Silicon wafer processing steps. lifetime diode obtained from wafer will much less than that wafer.
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
Figure Diode Chip Structure
geometry (ie, volume) I-region determines bulk lifetime major factor affecting effective lifetime finished device [3]. I-region cylindrical shape (Figure A.1). areas outer surface also degrade (decrease) minority carrier lifetime. Because periodicity I-region Silicon stops these surfaces, rate which electrons holes recombine higher these surfaces than center I-region. Figure shows contours effective lifetime Silicon function mesa (I-region) diameter.
Figure Effective Lifetime Mesa Diameter
STORED CHARGE
When diode forward biased, electrons holes injected into I-region, where they have finite lifetime before they recombine. charge density intrinsic region geometry determine resistance device. Lifetime determines approximate frequency limit useful operation.
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
diode' resistance proportional stored charge. charge related diode current dQ/dt where diode forward current, charge stored diode, effective recombination lifetime diode biased with only constant current, stored charge constant equal frequencies below 1/2B signal modulates stored charge same bias diode behaves ordinary diode.
FORWARD BIAS SERIES RESISTANCE (RS)
forward biased diode behaves current controlled resistor that presents linear resistance flow current through diode. This property diode that enables device used power control element linear attenuators modulators. forward bias equivalent circuit diode, Figure A.3, consists forward bias resistance (Rs) series with fixed series inductor (Ls). varied over range tenths 100,000 Ohms direct frequency control current.
Figure Forward Bias Equivalent Circuit ideal diode equation, with external voltage subtracted from junction voltage, O[e-q (V-IRs) /nkt "ideal diode equation"
where diode' space charge diffusion limited space charge recombination limited. Typical rectifiers have typical diodes have 2.0. forward voltage characteristics rectifiers have been extensively treated Herlet, Benda, Spenke [4,5]. Both articles provide closed form solution similar "ideal diode equation"; however computer programs have been written utilize their solutions. frequency resistance diode nkT/qIdc Ohms where well constructed diode. room temperature 52/Idc(mA) Ohms
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
total resistance diode made other series resistance contained chip (Ohmic contacts, diffusion layers, substrate resistance) resistance package leads. frequencies above dielectric cutoff frequency diode I-layer' resistance nearly linear [Appendices resistance I-layer given W2/(2: where ambipolar diffusion constant, I-layer width, lifetime I-layer does contain region area explicitly only I-layer width. Since parameter that I-layer geometry dependent, I-layer resistance implicitly depends area well. CAPACITANCE
reverse bias equivalent circuit, Figure A.4, consists shunt combination I-layer capacitance (Ct) parallel resistance (Rp) series with inductance (Ls). reactance resistance depleted I-layer represented contains stray capacitance effects diode' package structure well junction capacitance (Cj).
Figure Reverse Bias Equivalent Circuit structure section separated I-layer, sections accumulate charge. thin depleted layer with charge occurs. This layer acts parallel plate capacitor (Cj). where varies with reverse bias voltage. well designed frequency diode will have flat Capacitance versus Reverse Voltage curve even frequency. Figure shows such diode with flat curve below MHz.
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
Figure Diode Capacitance Reverse Bias Voltage Frequency
VIII
REVERSE BIAS PARALLEL RESISTANCE
reverse bias equivalent circuit, Figure shows shunt loss element (Rp) associated with diode' total capacitance (Ct). This industry' conventional representation loss element reverse biased capacitor. zero bias shunt loss represented whose value infinity (ie, open-circuit). series representation would serve just well shunt representation, might lend some intuitive insight (zero loss would represented series element whose value zero!). Reference authoritative text Varactor Applications. Varactors only useful devices when reverse biased. main premise reference ease with Varactor circuits analyzed series representation Varactor loss used. ideal structure (Section above) would have shunt loss, because there mobile carriers I-layer support conductive current. practical diode structures, highly dependent wafer processing steps passivation cylindrical surface diode chip prior packaging. well passivated diode would have excess 50,000 Ohms when back biased beyond punch through voltage.
REVERSE BIAS EQUIVALENT CIRCUIT
Figure gives reader intuitive feel connection between diode chip model reverse bias equivalent circuit. also shows both high frequency (microwave bands) frequency band) paths through model [8].
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
Figure Reverse Bias Equivalent Circuit Microsemi' unique passivation process produces very leakage very stable reverse characteristics which device driven hard into reverse breakdown. pulse form currents large approximately 3000 volts safely handled. Figure typical passivated high voltage PIN.
Figure TYPICAL PASSIVATED HIGH VOLTAGE
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
SWITCHING
forward reverse characteristics have been discussed separately. What happens when diode switched[9,10]? REVERSE BIAS FORWARD BIAS diode, with sufficient reverse voltage fully deplete I-layer, will depleted mobile carriers. This depletion region will collapse almost instantly injection carriers begins with finite time fill I-layer with charge. FORWARD BIAS REVERSE BIAS turn-off time reverse turn-on time. When large reverse bias applied forward bias diode large current flows limited only impedance voltage source circuitry bias feed diode. [The choke pass filter supply bias current should have very impedance.] edges closest contacts deplete charges quickly; however, charges near center have diffuse external contacts. more more charges removed, reverse voltage across I-layer begins increase aiding removal charges. using spiked voltage drivers reducing source impedance, switching time diode substantially reduced. Figure shows charges I-layer their removal function time.
Figure Charge removal from chip versus time
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
APPENDIX
COMPARISON DIODE RECTIFIER DIODE CHARACTEISTICS
NOTES
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
Application Note 101A Comparison Diodes Rectifier Diodes
W.E. Doherty, Microwave Engineering Manager Introduction
Many circuit designers unclear about diode works circuit differs fundamentally from circuit performance rectifier diode. This Application Note compares physical properties electrical behavior classes Silicon semiconductor diodes: those that rectify signal (Silicon pn-junctions Silicon Schottky junctions) those that rectify signal (Silicon diodes). begin with discussion various electrical properties Silicon rectifier diodes because reader most likely familiar with them already. then continue with discussion unique electrical properties Silicon diodes make comparison applications both classes diodes.
March 1998
Rectifier Diodes
Rectification generally defined process converting alternating current unidirectional current. rectifier device conducts current substantially direction only. ideal rectifier diode would open circuit direction short circuit other direction. also would dissipate power during rectification process. junctions Schottky junctions rectify current. current flow junction comprised minority carriers (holes electrons), whereas current flow Schottky junction consists only majority carriers (electrons). reader familiar with these types junctions various mechanisms current flow across semiconductor junctions referred reference [2].
Figure Half-wave Rectification Waveform
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
rectifier presents non-linear resistance (Figure current source. rectifier diode' characteristic shows exponential forward bias curve given equation Saturation Current Ideality Factor, reverse bias "blocking state, values reverse voltage less than reverse breakdown voltage (VBR), removes negative half cycles input signal. equation reverse bias characteristic quite complex [2]. Empirically, reverse bias current increases gradually with reverse voltage until occurs. breakdown current consists both avalanche Zener components depends mainly surface conditions along periphery junction. full sine wave signal shown negative vertical axis Figure peak voltage sine wave less than VBR. output current flows forward direction only consists forward positive half cycles input current wave shown horizontal axis Figure1. negative half cycles blocked from output rectifier' high reverse bias resistance. current output from rectifier unidirectional must filtered useful bias source.
Rectification Non-linear Effects
Equation defining relationship rectifier diodes. relationship between exponential therefore highly non-linear. Heuristically, observe that device linear output signal faithful replica input signal. input signal' amplitude increase (gain) decrease (attenuation) shape waveform does change. shape waveform changes, device non-linear effect input signal. case Figure only positive half cycles appear output rectifier. rectifier performed non-linear transformation input signal output some added harmonics input frequency that input signal have. subject non-linearity signal distortion discussed detail reference [3]. Non-linear devices highly useful frequency conversion processes. Frequency up-converters (modulators) frequency down-converters (mixers) depend non-linear devices their performance characteristics. rectifier diode converter. However, when non-linear devices used switch attenuate power, they will seriously distort input signal unless biased properly.
Diode Current Controlled Linear Resistor
diode pn-junction with doping profile tailored such that intrinsic layer (I-region) sandwiched between p-layer n-layer. diode junction junction series (separated length I-region). Microwave diodes manufactured using epitaxial process. 0layer nominally Ohmcm layer, grown heavily doped type substrate. These diodes provide adequate switching performance above GHz. Microsemi Corp' diodes based high voltage rectifier technology. these structures, only high resistivity layer contributes series resistance (Rs).
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
Under forward bias conditions, I-region resistance given equation (2).
where:
ambipolar mobility minority carriers effective lifetime minority carriers effective width I-region forward bias current
Figure Diode Series Resistance Forward Bias Current 9401 This relationship between shown Figure 9401. When diode forward biased, current controlled resistor that presents linear resistance flow current. Above begins approach parasitic resistance layer, Ohmic contact layers. range linear operation occurs above lower cut-off frequency given equation (3).
power that controlled depends frequency operation peak current [3]. diodes control relatively large amount current with small amount bias current frequency operation satisfies equation (3). bias current condition linear operation given equation (4).
where peak value current waveform. Optimum bias conditions high power operation band discussed detail references [3,5]. lower bound equation (4),
Microsemi Pleasant St., Watertown, (617) Tel. Fax. (617) 924-1235
Non-linear Effects Diodes
diodes designed manufactured enhance linearity forward biased resistance forward bias current characteristic equation Figure operating frequency approaches cutoff frequency defined equation forward bias current inadequate control current, diode will begin rectify non-linear distortion effects will evident. This effect seen experimentally discussed reference [4]. induced charge I-region) nearly equal induced charge, forward bias resistance only controlled bias, modulated also current. Equations define conditions linear operation diodes.
Comparison Rectifier Diode Applications
far, diode discussion focused forward bias characteristic that implies that diode being used power attenuator circuit. Rectifiers entirely unsuitable this application signal distortion issue. differences electrical performance either rectifier diode most apparent switch application. Within constraints equations (4), diode switch large values current with small amount bias current. band, bias control Ampere current minority carrier lifetime adequate (2us). rectifier diode used toswitch Ampere current, bias current must least Ampere also!
Conclusions
diodes perform unique function switch attenuator designers that pn-junction Schottky junction devices cannot perform. switch application, small levels bias control control large amounts line current distort input signal waveform. Their unique forward bias characteristic provides nearly distortion free signal attenuation when biased over proper current ranges. Longer lifetime diodes becoming available which will provide devices needed switch attenuate large levels power from band
Other recent searches
uPA1754 - uPA1754 uPA1754 Datasheet
TGA1319A-EPU - TGA1319A-EPU TGA1319A-EPU Datasheet
STPS20S100C - STPS20S100C STPS20S100C Datasheet
LT111 - LT111 LT111 Datasheet
LT311 - LT311 LT311 Datasheet
LM111 - LM111 LM111 Datasheet
LM311 - LM311 LM311 Datasheet
DUB03 - DUB03 DUB03 Datasheet
PUB03 - PUB03 PUB03 Datasheet
AN9799 - AN9799 AN9799 Datasheet
AK4953A - AK4953A AK4953A Datasheet
Privacy Policy | Disclaimer
© 2012 Datasheet Archive
