APN1012 Introduction Voltage Controlled Oscillators (VCOs) have c
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Designs Wireless Handset CATV Set-Top Applications
APN1012 Introduction
Voltage Controlled Oscillators (VCOs) have come forefront designs together with first circuits. before PLL, oscillators were mostly free running only rare cases were varactors used modulation temperature compensation. Nowadays, rarely free running oscillators, instead they have become varactor controlled oscillators. This because most applications require band coverage, which realized through circuit requiring sources power. reference source frequency often VCXO TCXO, while other frequency controlled phase detector. Usually, both VCXO/TCXO voltage controlled oscillators. difference between reference oscillator tuned that former usually very high-Q resonator, which allows very stable oscillation, while latter lower-Q resonator, allowing relatively high tuning range. reference oscillators, varactors used fine tuning temperature compensation (TCXO). tunable oscillators, varactors used change (tune) frequency. some VCOs, varactors used also modulation, example DECT system where modulation used generate constant-envelope GMSK signal. Although small part design, often major headache designers. goal, this application note, show Alpha's products services help overcome concerns help make your design among best products market.
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APN1012
VCOs Digital Wireless Phones
Consider hypothetical wireless handset phone. Today, handset dual-band (cellular/PCS) multimode system employing many functions. There many ways realize these functions, making virtually impossible specify frequency tuning range designs. However, there certain common features that outlined Figure typical receiver, dual conversion superheterodyne solutions usually employed. They convert either (cellular) (PCS) down frequency range, which between 90-400 MHz.
Further, this signal either down-converted demodulated into digital signal using lower frequency VCO. transmitter path either directly modulated uses dual conversion scheme requiring least VCOs. When dual-band requirements needed, more VCOs required satisfy specific frequency plans. This often technically economically restrictive solution. Many designers solve this overVCOed problem using both smart frequency planning multi-band VCOs, shown Figure
Control Control
Switch Coupler Detector Control Control
Ranging: 400-1900
Ranging: 100-400
Figure VCOs Digital Wireless Phone
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APN1012
Fundamental Noise Colpitts
characteristic feature Colpitts that uses capacitive divider feedback consisting inductive branch including parallel resonator series capacitor parallel resonator includes inductive element (that discrete inductor lower frequencies length micro-strip line capacitive branch, consisting varactor series capacitor(s). entire inductive branch should have inductive impedance frequency oscillation, otherwise there will oscillation. This means that resonant frequency should higher than oscillation frequency. Note that resonator current circulates through varactor, series capacitor inductor largest current tank circuit. Because this, losses introduced this current path crucial ones with respect phase noise.
Without delving deeply into phase noise theory, note that phase noise inversely proportional power bypassed through feedback loop, loaded tank circuit. Thus, more power lost transistor base, higher noise. clear that varactor loss plays crucial role phase noise property VCO. phase noise issue, varactor series resistance should carefully considered. There additional concern because phase noise only function varactor loss. varactor capacitance voltage characteristic crucial impact phase noise well. With higher capacitance ratio, varactor's coupling resonator reduced resulting lower resonator current. Therefore, hyperabrupt varactor having higher series resistance often better choice than lower capacitance ratio abrupt varactor having lower series resistance.
(0603) 0.35 (0402) SMV1493 2SC5007 ("34") 2SC5008 ("44")
0.75
0.75
0.75
0.75
(Trim)
VTUNE
Voltage Hyperabrupt Varactor Figure Noise High Performance Colpitts
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APN1012
Differential Integration with
Designs based solutions, with built-in VCOs, often employ differential configuration. possible differential configuration shown Figure this case, tank circuit formed resonator Here again, resonator current plays decisive role phase noise definition. Thus, phase noise strongly dependent resonator loss. Capacitors help establish correct phase shift value feedback loop moving oscillations
closer resonant frequency. This principal difference between Colpitts differential VCO. Colpitts case, resonant frequency always higher than oscillation frequency; differential resonant oscillation frequencies coincide. Thus loaded circuit becomes significantly higher, feedback loop losses increased higher resonant currents. When this happens, differential more vulnerable resonator loss than Colpitts usually shows 5-10 higher noise compared equivalent Colpitts case.
VVAR
SMV1493-079
Voltage Hyperabrupt Varactor Figure Differential Integration with
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APN1012
Dual-Band Switchable Schematic
improve design economics multi-VCO requirement employ band switching VCO. frequency switching required isn't very large (say within 20%) usually realized within same tank circuit, switching "on" "off" additional capacitor inductor. However, required switching more than 30%, becomes very difficult satisfy both wideband noise requirements single design. possible solution separate tank resonator circuits switched with diodes. this case, feedback needs optimized both band requirements same time. Thus, trick used connecting capacitor
parallel with when lower-band resonator selected. This provides significant improvement phase noise since then optimized best performance high band, lower band. Another important feature this switching scheme that diodes resonator current path. Because this, phase noise sensitive diode resistance. This fortunate, since means less forward current needed. addition, noise diode bias current (common noisy digital environment today's phones) would cause significant modulation noise.
SMV1139-011 VSW_Low
Current Switching Diodes
SMP1320-011
VTUNE
NE68519
SMP1320-011
SMV1408-011
Voltage Hyperabrupt Varactor
VSW_High
Figure Dual-Band Switchable Schematic
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Designs Wireless Handset CATV Set-Top Applications
APN1012
Dual-Band Switchable
mentioned before, relatively small (less than 20%) frequency switching achieved inside same tank circuit connecting disconnecting capacitors (and sometimes inductors). diode performs tricky task adds more capacitance parallel with existing parallel capacitance resonator, also adds more capacitance parallel with existing series
capacitor. This technique used overcome problem increased resonator when connecting additional parallel capacitance, decreasing with higher series capacitance. allows keep phase noise near optimum both bands. Another diode output matching circuit tunes buffer frequency doubler mode when working band.
VVAR
VCTL2 SMV1493-079 SMP1322-079
VCTL1
PINs Using Both Resonator Tank Output Matching Circuits Figure Dual-Band Switchable
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APN1012
VCOs Set-Top Cable Down-Converter
typical set-top down-converter dual-conversion receiver employing up-conversion down-conversion techniques overcome image problems wideband environment 50-1000 MHz. dual up-/downconversion scheme, problem image channel input filtering virtually does exist because there signal image channel. image channel always higher than highest frequency cable signal.
VCOs required dual down conversion. first wideband tuned from 1100-2000 with control voltage from 1-20 other narrow band VCO, which CDR, coaxial dielectric resonator, 1144 MHz. digital system second signal further demodulated, requiring additional VCO. specific action wideband wideband tuning requirement. consider some possible solutions wideband VCO.
54-860 Mixer 1100 Mixer 45.75/44
Upstream filter
1154-1960 Control
1144/1145 Control
Upstream
Low-Distortion Diode Attenuator
Wideband Tuning 1100-2000 Range
Narrow-Band 1145
Figure VCOs Set-Top Cable Down-Converter
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APN1012
Wideband Colpitts Schematic
unique action wideband Colpitts tank circuit design, which uses inductor with varactor connected series parallel capacitor contrast noise Colpitts described Figure feedback capacitors optimized best power
flatness over entire frequency band. Back-to-back varactors often used minimize parasitic mounting capacitance (between mounting pads adjoining components). This circuit usually designed minimize parasitic parallel capacitance that caused component pads transmission lines close inductive path.
NE68519 VTUNE mils
SMV1265-011 SMV1265-011
1.62
Output
High Ratio Hyperabrupt Varactors Figure Wideband Colpitts Schematic
carefully designed layout with minimum parasitic capacitances show large frequency coverage, example 980-2120 performance indicates.
varactor selection crucial part design. Alpha's SMV1265-011 varactor specifically designed this wideband application.
Useful Tuning Range: 980-2120
Fexp
Frequency (GHz)
POUT (dBm)
Varactor Voltage
Fmodel POUT_exp POUT_model
Figure Wideband Colpitts Performance
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APN1012
Wideband Balanced Schematic
even wider tuning range achieved with balanced configuration. reason wider tuning performance that phase response this VCO's active element flatter over range tuning
compared Colpitts VCO. This allows tank circuit more control over oscillation frequency. best results achieved with back-to-back connected SMV1265 varactors, where there 820-2120 coverage.
1000
VCC1
NE68119
NE68119
High Ratio Hyperabrupt Varactors
SMV1265 1000
SMV1265
VVAR1
Figure Wideband Balanced Schematic
Measured
Measured
Frequency (GHz)
Simulations Useful Tuning Range: 820-2120
Power (dBm)
Measured Simulated
Varactor Voltage
Varactor Voltage
Frequency Tuning
Power Response
Figure Wideband Balanced Performance
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APN1012
Varactor Fundamentals
consider some fundamental properties varactors. varactor specially designed junction diode, whose capacitance changes significantly reverse bias mode. There three important parameters characterizing varactors. first capacitance ratio reverse voltages; this value characterizes tuning ability varactor capacitance most important parameters. second value capacitance given voltage. third series resistance varactor. structure basic varactor, called abrupt junction varactor, shown Figure Generally, built structure, using epitaxial N-growth substrate with constant doping level Nregion. lower doped N-region active area where electron concentration changes depending reverse voltage applied between anode cathode varactor. There certain limitations level doping N-region, which usually defined required capacitance ratio varactor. Because this,
conductance N-area major contributor varactor's series resistance. Note that reverse voltage increased, series resistance (due N-area) will decrease along with capacitance. hyperabrupt junction varactor more complicated doping profile. Because much higher doping border, electron concentration changes much more abruptly compared abrupt junction. result, capacitance hyperabrupt diode zero bias much higher than abrupt diode. Therefore, capacitance change reverse bias becomes significantly higher hyperabrupt diodes. trade-off this better capacitance ratio increased series resistance. reason that doping level N-area been reduced keep average doping level over N-region same abrupt diode level. There many ways bring series resistance hyperabrupt diode level possible. Modern state-of-the-art hyperabrupt diodes noise VCOs have series resistance almost discrete ceramic capacitors.
Abrupt Junction
Hyperabrupt Junction
Doping Level
Doping Level
Depth from Anode Electron Concentration Electron Concentration
Depth from Anode
VSAT
VSAT
Figure Varactor Fundamentals
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APN1012
Varactor Packaging
Most high-volume discrete applications require varactors cost, small surface mount plastic packages. Alpha Industries provides large variety both plastic
ceramic packages. recent most advanced miniature plastic package, SC-79, shown Figure small 0402 discrete components.
(1.58
(0.62
SC-79 Figure Varactor Packaging
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APN1012
Relative Capacitance Change Temperature
Figure shows typical relative capacitance variations temperature different reverse voltages. indicates total capacitance change 5-6% range -40°C +80°C. comparison, temperature compensated
ceramic capacitor residual variation shown typical ±100 device. This possible total capacitance change over When comparing overall effect temperature varactors ceramic capacitors, coupling devices resonator circuit should considered.
Percentage Variation
VVAR VVAR -2.5 VVAR
Deviation Range Typical Temperature Compensated Discrete Ceramic Multilayer Capacitor
Consider Varactor Coupling!!
Temperature (°C)
Figure Relative Capacitance Change Temperature Hyperabrupt Varactors
coupling coefficient derived from known typical) values tuning frequency varactor capacitance variation. Note that total temperature drift this case about 0.5%, compared maximum
drift caused temperature compensated discrete ceramic capacitors. Even those numbers extremely small when compared temperature drifts caused transistor.
CSER2 CRES LRES CVAR CDIV2 CDIV1
Typical Wireless Case: -0.04 Using SMV1235-011 Varactor: Total Temperature Drifts Varactor +85°C Becomes:
0.08 0.24 CVAR
0.54%
Compare Discrete Capacitor!
Figure Varactor Temperature Effect Oscillation Frequency
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APN1012
Varactor SPICE Model
model varactor most commercial simulators, recommend available PN-junction diode SPICE model. specify barrier junction capacitance parameters CGO, instead default parameters. addition, value parallel with junction
capacitor, which package capacitance. ideal abrupt junction varactors, parameters constant defined from physical theory. However, actual abrupt hyperabrupt varactors, these values constant. these cases, same equation, parameters, best compliance with measured capacitance voltage response.
SPICE Model SMV1142-011
Figure Typical Varactor SPICE Model
Because formalization, parameters describing junction capacitance hyperabrupt varactors significantly different from default values used SPICE simulators ideal silicon PN-junction. example, typical hyperabrupt varactor SMV1235 fitted with opposed 0.5, which follows silicon
diode theory. Note that some SPICE simulators offer fixed default values which can't changed. this case, diode model used, however, direct nonlinear capacitance used defined given formula.
SMV1235 16.13/(1-Vv/8)^4
Capacitance (pF)
SMV1235 Approximation
16.13
VVAR
Varactor Voltage Figure Curve Fitting Typical Hyperabrupt Varactor
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APN1012
Super-hyperabrupt Varactor Modeling
overcome limitations "standard" PN-junction SPICE model hyperabrupt super-hyperabrupt devices, such SMV1265, interleaving technique used. this technique, entire capacitance reverse voltage range broken into several subranges. These
subranges small enough formula provide good approximation only within given subrange, also certain extensions beyond extension margin defined from previously estimated signal amplitude. Such interleaving ensures that formula would work well only terms bias, large-signal analysis well.
SMV1265
Approximation Measured
Capacitance (pF)
Interleaving Splines
Varactor Voltage (VVAR 2.500009 6.50009 11.0009 1.8) (VVAR 2.500009 6.50009 11.0009 1.85 1.85) (VVAR 2.500009 6.50009 11.0009 0.56 0.56) (VVAR 22.5 22.5 2.500009 6.50009 11.0009 20)*10
(VVAR 1.85 1.76 1.65 1.61 1.5) Figure Piece-Wise Curve Fitting High Ratio Varactors
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APN1012
Modeling Concept
purpose modeling analysis, design simulated amplifier with parallel feedback. This analysis involves measuring loop gain using specific idealized directional coupler called "OSCTEST"
Libra (For Harmonica users there application note showing implement OSCTEST function using S-parameters file. Refer your Harmonica vendor more information).
Feedback Model Colpitts Loop Gain Observation Plain Amplifier
Simplified Colpitts
Tank Circuit Figure Modeling Concept
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APN1012
major goal large-signal open loop analysis observe magnitude (defined phase open loop voltage gain identify particular features designed VCO. First, need establish optimum conditions oscillations given tuning range. Second, need find whether there possibilities parasitic
oscillations both lower higher frequency ranges. there parasitic oscillations, some preventive measures should taken. Third, need find ways make both loop power (PIN) high possible facilitate phase noise performance. Finally, other features need addressed, among them load pulling pushing.
Oscillation Happens Gain Loop Phase Shift
(Ku) Oscillation Point -100 -100 -150 -200 -200 Transitor -250 Resonator Different Varactor Voltages VVAR VVAR
(Ku)
(Ku)
Frequency (GHz)
Parallel Resonance: When nears oscillation point, tank circuit losses increase; noise increase power decrease follows
Frequency
Figure Typical Loop Gain Results Colpitts
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APN1012
Wideband Colpitts Model
OSCTEST component interrupts oscillator feedback, allowing designer analyze ordinary two-port circuit (amplifier). observe loop response, define open loop voltage gain
more details, please refer application notes listed References section. varactor model defined PN-junction diode SPICE model large signal harmonic balance analysis. transistor described Gumel Poon SPICE model with parameters provided vendor.
Transistor Subcircuit
Varactor Model
Seamless Loop Opener
Figure Wideband Colpitts Model
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APN1012
Differential Fundamentals
differential utilizes paired transistors common-emitter common-base configurations. phase balance condition sustaining oscillations requires significantly lower phase shift comparison
Colpitts design (ideally degrees degrees). This makes possible resonator tuned exact resonant frequency. However, feedback losses higher because higher resonating currents will cause increased ohmic losses.
Transistors Loop Give Advantage Higher Loop Gain
Common-collector Common-base Phase Shift Ideally
Resonator Works it's Parallel Resonance, Giving Best Phase Slope Performance
Figure Concept Differential
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APN1012
Balanced Fundamentals
Fundamental properties balanced more clearly understood using simplified circuit diagram shown Figure transistors common collector configuration. This characterized high input impedance, looking from transmission line referenced Capacitor simulates transmission lines grounding effect mounting pads. Coupling current ICPL circulates between transistor bases drive them with 180° phase shift.
emitter current forms feedback loop, carrying amplified energy surplus that needed sustain resonant current IRES coupling current ICPL through emitter-base path. Unlike Colpitts VCO, this circuit does require frequency dependent feedback match internal transistor high frequency phase shifts. When properly compensated wideband performance with inter-base inductor, this circuit will more broadband than Colpitts VCO.
Collector Currents Shifted 180° Phase. That's Call "Balanced"
ICPL
Rcol LSER
IRES
LPAR
Low-pass Matching Serves Improve High-frequency Performance
Rcol DVAR
Figure Balanced Fundamentals
References
"Varactor SPICE Models Applications." Applications Note APN1004, Alpha Industries, Inc., 1998. Colpitts Wideband (0.95 GHz-2.15 GHz) SetTop Tuner Applications." Applications Note APN1006, Alpha Industries, Inc., 1998. Balanced Wideband Set-Top Tuner Applications." Applications Note APN1005, Alpha Industries, Inc., 1998. "Switchable Dual-Band 170/420 Hand-Set Cellular Applications." Applications Note APN1007, Alpha Industries, Inc., 1998.
Wideband General Purpose Attenuator." Applications Note APN1003, Alpha Industries, Inc., 1999. "Wideband Set-Top Applications." Microwave Journal, April 1999. "Circuit Models Plastic Packaged Microwave Diodes." Applications Note APN1001, Alpha Industries, Inc. "Design with Diodes." Applications Note APN1002, Alpha Industries, Inc. availability above materials, visit Alpha Industries site www.alphaind.com.
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