CMOS SWITCHES From February 2004 High Frequency Electronics Copyr

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CMOS SWITCHES From February 2004 High Frequency Electronics Copyr

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High Frequency Design
CMOS SWITCHES
From February 2004 High Frequency Electronics Copyright 2004 Summit Technical Media,
CMOS Switches Offer High Performance Power, Wideband Applications
Theresa Corrigan Goggin Analog Devices, Inc. performance switches among building blocks required modern wireless communication systems. Switches that provide insertion loss, high isolation between ports, distortion current consumption much sought after high frequency applications such phase shifters, switchable filters, transmitters receivers radar systems-ranging from large installations anti-collision radar cars-and communication systems from base stations cell phones. Traditionally, only processes were available developing good wideband/RF switches. Gallium arsenide (GaAs) FETs, diodes electromechanical relays have dominated market, standard CMOS making mark. GaAs been popular because resistance, capacitance high linearity high frequencies. CMOS process geometries continue shrink, performance CMOS switches increased point that they have broken barrier, able compete with GaAs switches. Designed maximize bandwidth while maintaining cost, CMOS switches offer alternative expensive GaAs switches cost, power applications. This article explains specifications associated with wideband switches, traditional methods used wideband switching, benefits CMOS switch technology. also shows CMOS switches outperform GaAs switches power, high fre-
performance CMOS switches increased point that they have broken 1-GHz barrier, able compete with GaAs switches
quency applications. This should convince that CMOS wideband switches ideal solutions applications such cable modems, scanners, xDSL modems, next-generation cordless phones, applications aviation industry that require operating frequency beyond.
Wideband Switch Basics
Wideband switches designed meet demands devices transmitting frequencies higher. selection devices these types applications completely dependent they perform over frequency. There important ways describe performance switch: insertion loss closed state isolation open state. Insertion loss attenuation between input output ports switch when switch insertion loss critical systems that require overall noise figure. Because switch first components signal path, minimum receivable signal very important. Figure shows typical plot insertion loss versus frequency ADG919. This plot shows that insertion loss less than MHz, GHz, GHz. This comparable many GaAs switches with typical values between GHz. isolation defined attenuation between input output ports switch when switch off. Figure shows typical plot isolation versus frequency. High isolation demanded most wideband switching, generally critical specification determining part suitable
High Frequency Electronics
High Frequency Design
CMOS SWITCHES
Figure Insertion loss frequency ADG919.
Figure isolation frequency ADG919.
particular application. This plot shows that switch isolation better than about MHz, GHz, about GHz. This isolation specification outperforms many GaAs switches about dB/decade. Typical GaAs switch values between Channel-to-channel isolation assures minimum crosstalk between channels. other important switch specifications related power levels that switch handle. first compression point, P1dB, which input power level which switch insertion loss increases over low-level value. P1dB measure much power switch handle before will distort compress signal, therefore measure power handling capability. Next, when closely spaced tones passed through switch, nonlinearity switch causes false tones generated. input third order intercept point, IIP3, measure power these false tones directly related amount distortion caused switch. Figure shows P1dB compression point versus frequency. this case, P1dB dBm. IIP3 this switch dBm, making well-suited medium power, high frequency applications, including switching cellular handsets that employ other enhanced functionality.
Traditional Switching Solutions: Diodes
diode fundamentally current-controlled resistor radio microwave frequencies. diodes have commonly been used switching signals, they highly linear when conducting exhibit very good distortion characteristics. diodes fabricated placing high resistivity intrinsic region between P-type N-type silicon regions. resistance value diode determined only forward-biased current, making useful switching functions. When diode forward-biased, electrons holes injected from regions into region. electrons holes recombine immediately; instead finite charge stored, resulting lowering resistivity region allowing conduction. Typical values resistance obtained different bias currents order kohm Accordingly, first drawback when using diodes that they require large amounts power resistivity insertion loss. This huge disadvantage when they used portable equipment such PDAs handheld meters, since battery life directly proportional power consumption. also difficult achieve more than isolation higher frequencies when using single-series shunt-connected diode. obtain higher levels isolation, more diodes must connected series-shunt combination. This undesirable effect increasing insertion loss. typical transmit/receive (TX/RX) diode switch schematic shown Figure consists diodes, some blocking capacitors, inductor feed forward bias control signal, some discrete components create quarter-wave line. extra ports need switched, more series-connected diodes required, which results increased insertion loss. effect increased insertion loss this application twofold. side, every fraction decibel lost
Figure P1dB frequency ADG901.
High Frequency Electronics
High Frequency Design
CMOS SWITCHES
Figure typical TX/RX switch using diodes.
between power amplifier antenna means that signal needs more amplification, thus reducing battery life. side, increase insertion loss will serve reduce receive signal strength, degrading overall signal-to-noise ratio (SNR) receiver sensitivity. Another limitation diodes that they require external driver control switching speed TX/RX switch interface levels that used.
Figure schematic.
typical transistor based TX/RX switch
GaAs Switches
recent years, GaAs switches have dominated wideband switch market their power consumption (compared diodes). GaAs switches made arrays transistors that voltage-controlled resistors. GaAs transistor three-terminal device; gate voltage (Vg) controls resistance between other terminals. increase isolation between ports, they connected series shunt devices ground. Unlike diodes, placement multiple transistors series actually aids power-handling linearity, with little effect insertion loss. typical TX/RX switch schematic shown Figure Here, transistors connected seriesshunt configuration obtain optimum insertion loss isolation performance. drawback this configuration that complementary switching MN1/MN4 MN2/MN3 requires that both high logic levels available simultaneously, increasing complexity control circuitry thus cost. MMIC GaAs switches available from wide variety companies. N-channel depletion-mode basic switching device. characteristics displayed these devices follows: Switch Negative Switch (Pinchoff) Pinchoff defined voltage which channel becomes high impedance. This usually occurs depending process used. levels input
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switch modulate thereby causing distortion products resistance switch varies. high control voltages will reduce this effect, only cost generating voltages approximately control switches. GaAs processes don't offer complementary devices, takes more current create inverter logic functions that easily available CMOS technology. This inherent liability GaAs switches makes them hard integrate into most modern electronic systems that positive voltage power supplies. GaAs switch manufacturers around this issue adding blocking capacitors series with pins switch. This effectively floats relative ground, which allows switches controlled with positive control voltages. drawback, however, that blocking capacitors limit bandwidth that switches handle, thus restricting their usage wideband systems. capacitors themselves relatively cheap, additional printed circuit board area manufacturing costs significantly overall switch cost. Also, component placed series with switch will insertion loss have direct impact system SNR. increase inductance path length will degrade performance devices, thus giving system designer additional issues problems overcome. mentioned earlier, control signals GaAs switch need generated from external source. There number methods this; easiest most common being addition external CMOS driver This extra only draws power, also adds board area assembly cost. More recently, GaAs switch manufacturers have introduced MultiChip Module (MCM), which packages CMOS/BiCMOS driver GaAs switch into large BGA-type package. These MCMs work quite
well, have major drawback high overall solution cost, made individual costs both die, increased assembly cost larger package interconnect.
CMOS Rescue
Moore's predicted, standard CMOS process geometries have continued decrease. markets that opened CMOS area RF/Microwave ICs. With reduced transistor lengths, CMOS achieved resistance, capacitance good linearity beyond. This case switches also, CMOS switches with GHz, bandwidth available. Like GaAs switches, CMOS switches NMOS FETs, which essentially voltage-controlled resistors. characteristics displayed these devices follows: Switch Switch defined threshold voltage above which conducting channel formed between source drain terminals. FETs have interlocking finger layout reduce parasitic capacitance between source drain, thereby increasing isolation high frequency. stated previously, CMOS switches suitable many low-power applications. Their power handling capability reduced frequency reduced reasons: first, inherent NMOS structure shown Figure consists regions N-type material Ptype substrate, which leads parasitic diodes being formed between regions. When signal, biased VDC, placed source transistor, such that transistor turned parasitic diodes forward-biased some portion negative half cycle input waveform. Once input sine wave goes below approximately -0.6 diode will begin turn This will cause input signal compressed. frequency, input signal below -0.6 level longer periods time, thus having greater impact compression point (P1dB). second mechanism partial turn-on shunt NMOS device when supposed off. mentioned previously, NMOS transistor state with With signal source shunt device, there will time negative half cycle waveform where thereby partially turning shunt device. This will cause input waveform compress shunting some energy ground. Both these mechanisms overcome applying small bias (~0.5 input signal when switch being used frequency (<30 MHz) high power
Figure Physical NMOS structure.
(>10 dBm). Unlike GaAs switches, blocking capacitors required with CMOS switches.
Benefits CMOS
There number benefits using CMOS design-wide bandwidth switches. main advantages high performance specifications highlighted following sections. Cost: most important advantage today's costdriven market that CMOS processes significantly less expensive than GaAs processes, leading reduced cost. CMOS switches positive voltage control, require blocking capacitors. GaAs transistors, other hand, negative voltage controlled devices. This precludes CMOS drivers, requires blocking capacitors used inputs, causes increase overall solution cost. CMOS switches both less expensive easier than their GaAs counterparts. Single-pin Control Interface: CMOS switches have single-pin control interface that enables maximum circuit layout efficiency, benefiting many applications such mobile wireless systems. This possible because CMOS allows integration driver/switch control circuitry onto same switches, effectively reducing number control pins. CMOS technology provides control interface that simple single-pin control contrast complementary control signals implemented many GaAs switches. example, SPDT (Single Pole Double Throw) switch, GaAs part needs control lines whereas CMOS device requires only one. Easy System Integration: on-chip driver CMOS switch technology interface with both CMOS logic levels, allowing parts easily integrate with other CMOS/BiCMOS such microcontrollers. Control inputs that both CMOS LVTTL compatible provide very simple interface many applications. There need addition blocking capacitors inputs CMOS switches, eliminating concerns over reduced bandwidth impact
February 2004
High Frequency Design
CMOS SWITCHES
Figure Return loss frequency (off/on switch) ADG918.
Figure Switch Timing ADG901.
reduced system performance-reasons which were described previous section. Reduced Package Size: easy integration driver/switch control circuitry added benefit small package size. overall size CMOS smaller, allowing CMOS devices assembled smaller packages with lower counts than MCMs being offered GaAs manufacturers. standard SPDT switch count range from modest 8-pin package CMOS solution pins GaAs solution. This space saving even more obvious SP4T devices. CMOS devices available tiny 16-lead LFCSP (Lead Frame Chip Scale Package) sets. GaAs SP4T products require negative positive/negative voltage supplies, eight control lines. They packaged 24-lead 10.65 15.6 wide body SOIC (Small Outline) 28-pin 12.57 12.57 PLCC (Plastic Leaded Chip Carrier) packages. Lower Power Consumption: extremely power consumption CMOS switches makes them ideally suited portable applications. Available CMOS switches operate from single 1.65 2.75 supply, have typical current consumption less than µA-significantly lower than that specified equivalent GaAs solution. Other CMOS Switching Performance highlights: Matching also very good CMOS switches. Figure shows typical plot return loss versus frequency on/off switch. This plot shows that values switch switch. This amount reflected power relative incident power port. large return loss indicates good matching. CMOS switches have additional flexibility offering choice reflective ohm) absorptive ohm) versions, allowing switch matched application. example, ADG918 absorptive matched) SPDT switch (2:1 Mux) having terminated shunt legs, ADG919
High Frequency Electronics
reflective SPDT switch that direct shunt ground. absorptive switch would preferred applications where impedance matching most critical. Similar isolation performance CMOS switches described earlier, return loss specified CMOS processes outperforms many GaAs switches about dB/decade. Switching time also excellent, with CMOS switches featuring typical switching time ns-up times faster than many GaAs parts. Figure shows switching time typical ADG901. timing specifications this part extreme temperature conditions maximum, compared with microseconds many GaAs switches.
Conclusion
CMOS switches present simpler overall solution than GaAs devices high frequency switching requirements, with added benefit lower cost. ADG9xx family wideband CMOS switches have frequency greater than GHz, exhibit very insertion loss GHz, have greater than isolation GHz. Such characteristics make these parts ideal many applications from beyond. switches offer major CMOS challenge GaAs switches best solution power, high performance, high frequency switching applications.
Author Information
Theresa Corrigan Goggin engineers Analog Devices' facility Limerick, Ireland. They reached e-mail these addresses: theresa.corrigan@ analog.com ray.goggin@analog.com.
Note
Moore's predicted that number transisters square inch integrated circuits would double every year after invention integrated circuit that this trend would continue foreseeable future.

CMOS SWITCHES From February 2004 High Frequency Electronics Copyr

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