Anthony Harris ABSTRACT designing high levels emitter ballasting,
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POWER TRANSISTOR BALLASTING IMPROVES PERFORMANCE SURVEILLANCE RADARS
Anthony Harris
ABSTRACT designing high levels emitter ballasting, power transistors achieve good thermal stability without reducing collector efficiency. This important interrogators transponders used with generation secondary surveillance radars that employ Mode-Select (Mode-S). major requirements interrogators transponders surveillance radars delivering sufficient levels power, while maintaining efficiency gain that allows final amplifier efficient small possible. Historically, power transistors capable short pulses higher power levels have been derated output power reducing collector order substain longer pulse widths. This results solid-state transmitters that often criticized tube advocates being marginal, claiming that systems must designed around devices rather than vice versa. However advances processing device manufacturing have produced family power transistors that reliably satisfy requirements called Modes-S type Traffic Control Radar Beacon Systems (ATCRBSs). Mode-S techniques method addressing interrogations particular aircraft without ambiguity confusion such exists congested traffic corridors, present surveillance radar systems. appreciate case Mode-S essential that some level understanding exist principles secondary surveillance radar operates alongside primary radar. OPERATION SURVEILLANCE RADARS. Figure Primary Secondary Surveillance Radar
INTERROGATION REPLY
PRIMARY RADAR
COMBINED DISPLAY
July 2000
AN1225 APPLICATION NOTE Secondary surveillance radars operate conjunction with primary surveillance radar provide traffic control information, shown Figure However, rather than timing received echo with respect transmitted pulse, case primary radar, secondary radar transmits pulse train that triggers on-board transceiver reply. Therefore there parts system: interrogator transponder. interrogator asks each target identify itself. aircraft transponder replies with flight number, destination altitude (range bearing having already been ascertained primary radar). Presently, Secondary Surveillance Radars systems interrogate targets based spatial selection enhanced ISLS (Interrogation Side-Lobe Suppression) mono-pulse techniques. antenna normally mounted above that primary. too, course, must directional antenna because high gain antennas produce side lobes. Interrogation side-lobe suppression employed prevent interrogation these lobes. side lobes primary radar present such problem, range proportional fourth root power opposed square root case SSR. There three different antennas transmitting interrogation: directional, main beam omnidirectional (Figure pulses transmitted, main beam omni, aircraft that receives those pulses compares respective power levels two, discern whether main beam whether reply warranted. notched antenna, rather than omni transmitting effective beam width controlled varying transmitted power level with respect that However situation still occur that aircraft close proximity triggered that their replies overlap time thus cannot decoded. This known "garbling". Confusion also arises from "fruit". This term given replies received aircraft that triggered different traffic control center. Mode-S system addresses this problem specifically enables each aircraft observed distinctly. They able time interrogations that replies overlap. "Fruit" still occur, since Mode-S transponder replies with address code system aware that "fruit" ignore erroneous responses. Figure Interrogation Side-Lobe Suppression
Main Beam
AIRCRAFT
Notched AIRCRAFT
Transmit Receive Receive
REPLY IGNORE
Interrogation side-lobe suppression allows transponders discern whether they main beam antenna.
AN1225 APPLICATION NOTE ADDING MODE-S SYSTEMS. Mode-S maintains absolute compatibility with present radar surveillance systems. interrogation frequency always, 1030 reply frequency 1090 with interrogation signal being transmitted phase shift keying reply signal pulse position modulation. When aircraft first enters airspace surveyed particular control site address unknown. Unless transferred verbally from site another address obtained "all-call". Figure ATCRBS Interrogation Scheme
µsec
Interrogation
µsec
µsec
µsec
µsec
ISLS Control
µsec
µsec
ATCRBS Only Call.
order maintain compatibility, Mode-S systems must capable interrogating ATCRBSs type transponders.
Three types "all-call" employed: ATCRBS Mode-S all-call ATCRBS only all-call Mode-S only all-call. Case number usual Traffic Control Radar Beacon System (ATCRBS) interrogation. pulses µsec. Mode-S transponders will reply this sequence. However, lengthened µsec., shown Figure both Mode-S ATCRBS type transponders will reply. third case Mode-S only all-call, transmitted main beam surpasses ATCRBS transponders. other words, fools them into "thinking" that they being interrogated side lobe. further pulse transmitted control channel same time sync-phase reversal such that masks from targets main beam; thereby, producing side-lobe suppression Mode-S transponders. Once site address Mode-S type transponder addressed individually. However, still transmitted main beam prevent random triggering ATCRBS type transponders, shown Figure (Pulse also transmitted control channel although latter somewhat superfluous). data block Mode-S interrogation (Figure either 16.25 µsec. 30.25 µsec. long. phase-sync reversal occurs 1.25 µsec. into block. Data transmitted µsec. after sync reversal.A phase reversal take place every 0.25 µsec. phase reversal occurs, then logic zero
AN1225 APPLICATION NOTE appears. result, information occupies 0.25 µsec. µsec. guard band maintained each data block ensure that trailing edge pulse does interfere with processing. Therefore, with 16.25 µsec. pulse, data bits occur with 30.25 µsec. pulse data bits will result. first bits both interrogations replies used address. Mode-S also capability transmit Extended Length Message (ELM). This consists maximum pulses with µsec. pulse spacing. second format this transmits pulses with µsec. pulse spacing. Figure Mode-S Interrogation Scheme
µsec
1.25 µsec µsec
µsec
µsec
Main Beam
Sync Phase Reversal
Data Block
ISLS Control
Phase shift keying used data transfer Mode-S interrogation.
Mode-S transponders employ pulse position modulation. data block either µsec. µsec. long. information occupies µsec. time. pulse width used µsec. logic SPACE SPACE before, there data blocks consisting bits information. DESIGNING INTERROGATOR TRANSMITTER. With this background mind, consider what specifications such system places upon transmitter. There are, fact, three types transmitters required. transponder interrogator. second transmitter interrogator necessary because transmitted same This transmitter used pulses transmitted control channel. While power levels required somewhat higher than those primary channel, pulse conditions very light. STMicroelectronics currently supplies single-ended devices this application that deliver excess Watts typically. transistor designs such applications very mature, this transmitter design need considered further. real challenge lies with transmitter requirements primary channel interrogator transponder. Consider typical interrogator form shown Figure Assume output oscillator/modulator Watts required output channel amplified
AN1225 APPLICATION NOTE achieve modularity amplifier split into sections; driver High Power Amplifier (HPA). This allows uniform corporate-structure design, providing required output power level with most reliability smallest number power transistors. Obviously, design such amplifier requires knowledge performance that expected from transistors selected. This turn will determine "fan-out" ratio thus total number devices required. most important transistor specification that output power reliably delivered, consistent with high efficiency gain. This will allow final amplifier efficient small possible. Figure Mode-S Interrogation Block Diagram
From Ant. Level Cont/Mon Driver Oscillator Modulator (Sum) From
Driver (Control)
Level Cont/Mon
From
Figure Junction Temperature Pulse Width
AM1011-300
Pout 350W (Pdiss. 483W)
Junction Temperature Rise (°C)
70-75°C Rise Mode-S (E.L.M)
Pulse Width (µsec)
Infrared scan shows spot temperature rise with pulse width AM1011-300 power transistor.
AN1225 APPLICATION NOTE Extended Length Message (ELM), discussed earlier, formats: pulses with µsec. pulse spacing pulses with µsec. pulse spacing. these pulse conditions that present biggest problem power transistor manufacturers since must very rugged thermally stable. Typically, transistors capable short pulses higher power levels must derated output power reducing collector voltage meet these conditions. This method provides transistor with called collector ballasting. level this ballasting inversely proportional collector potential square root function) determined amount undepleted epitaxial material. other words, undepleted acts resistor series with collector. biggest drawback this approach loss collector efficiency. STMicroelectronics been developed that uses very high level emitter ballasting. This allows attain thermal stability without reduction efficiency. fact, efficiency actually enhanced. emitter ballasting does, course, reduce gain increasing emitter-base time constant. However, using with inherent excess GHz, gain achieved 1030 MHz. This very high level emitter ballasting cannot obtained using normal processing techniques. overlay geometry used with poly silicon site ballasting. addition this, there emitter finger ballasting performed also with poly silicon. using poly silicon ballast emitter fingers well, possible attain high level ballasting required maintain current density achieve desired levels reliability. Figure Mode-S Interrogator Typical Line-Up
300W
AM1011-300
235W
AM1011-300
2.3W
AM80912-005
AM80912-085 AM80912-015
AM1011-300
Freq. 1030 Burst Burst Period Overall duty cycle
Pout Pulses µsec µsec 17.6 msec 5.82
AM1011-300
Infrared thermal scans show this extremely stable over severe pulse conditions. These scans were taken from device under various pulse lengths addition Mode-S Extended Length Message -ELM- format). Both formats were tested. found that first condition pulses µsec. pulse spacing (64% duty) most severe. illustrated Figure 70-75 temperature rise obtained with conditions. Further, seen that device safely operated with much longer pulses. Also note that equivalent single pulse around 200-250 µsec. This equivalence only dependent thermal time constant purely function thermal resistance.
AN1225 APPLICATION NOTE test results this device, known AM1011-300, indicates that very suitable long pulse, single frequency applications. AM1011-300 both input output matched. typically delivers Watts with gain. lowest efficiency that recorded (45% typical) highest temperature rise, spot, 70-75°C, measured 30°C flange. designing interrogator amplifier with these transistors, minimum specification 300-325 Watts with gain used. This means input drive Watts minimum required. output stage used transistors, Corporate Structure Amplifier (CSA) design would have form shown Figure This provides combining loss eight-way splitter/combiner final isolator output. driver stage easily designed around line-up AM80912-005 driving AM80912-015 followed AM80912-085. This would then produce minimum output transmitter. system, however, there further losses associated with switching networks band-stop filters. Interrogator designs. order both Traffic Control Radar Beacon Systems (ATCRBSs) Mode-S systems function with minimum error necessary ensure that rise time transmitted pulse less than nsec., measured between voltage amplitude points) fall time less than nsec. Also, comply with spectrum confinement requirements these rise fall times must greater than nest. problem that circuit designers face trying reduce rise time increase fall time. transponder applications this achieved collector modulating transistors. However, interrogator (where power levels concerned) number output transistors much higher, such method becomes impractical. Here necessary reduce rise time approximately nsec., allowing fall time nsec. then employing spectrum confinement filter that both rise fall times will increased order meet system specifications. problem then becomes, attain rise time nsec. There many circuit dependent factors that affect rise time. closer device saturation faster rise time will Also emitter-base return choke collector feed, associated with video circuitry, must kept minimum. collector, necessary least three decoupling capacitors. decoupling best affected with ceramic chip capacitor around there main reservoir capacitor that supplies current duration this pulse. third capacitor (commonly axial lead chip) supplies current duration rise time, percentage When mounting this capacitor imperative that leads short possible reduce parasitic inductance, like inductance this network will serve increase rise time. also impedes switching introduce pulse ripple. value main reservoir capacitor will depend upon power level, efficiency allowed voltage, related pulse droop. must ensure that this capacitor series resistance series inductance. compromise that larger capacitance larger physical size. important, therefore, when evaluating transistors such applications consider dependency power level applied voltage such that figure maximum allowed voltage droop obtained. final droop will function both voltage thermal droop. order meet often restrictive space constraints, advantageous employ device with minimal thermal droop. Thermal droop course, related junction temperature device's performance over temperature. Efficiency again problem droop. keeping high efficiency, power dissipated reduced well junction temperature. Further, less power required from power supply there less voltage droop. response complete video network must carefully considered ensure that allows transistor exhibit smooth transition both rising falling edges that does introduce instabilities. Instabilities often caused poor grounding decoupling point. This allows
AN1225 APPLICATION NOTE signals both video frequencies penetrate power supply lines couple back input. This will then manifest itself oscillation pulse, severe cases, regeneration. Transponder designs. Transponders pose different problems. They must compact capable operation over severe temperature ranges. Because size critical, power transistors selected this application must offer very high gain that overall transistor chain will small. Since pulse conditions this part beacon system much reduced comparison that interrogator, high levels ballasting such those used AM1011-300 required. pulse burst length only µsec. only duty cycle this duration. transponder with output Watts required, this obtained with output devices. Allowing loss splitter/combiner, possible achieve this with line-up AM80912-030 driving AM0912-150 driving AM1011-500s parallel (Fig.8). Collector modulating these devices will decrease size necessary power supply; discussed earlier, will enable meet rise fall time specifications. CONCLUSION advantages solid-state transmitters with regards reliability graceful degradation well known. However, counter claim that laid solid-state that devices, while capable high peak power, only useful short pulse applications their thermal characteristics. Newly developed Mode-S systems allows great amount data transfer congested traffic control situations. Although this places rigorous demands transistors advances processing techniques, device manufacturing have produced family transistors than reliably satisfy this application. Figure Mode-S Transponder
MSC81118 MSC82003
AM80912-030 AM0912-150
AM1011-500
AM1011-500
AN1225 APPLICATION NOTE
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2000 STMicroelectronics Printed Italy rights reserved
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