Continuous Aperture Phased MIMO: Basic Theory and Applications
Active Phased Array Antenna Development for …...the aperture, transmit and receive losses are...
Transcript of Active Phased Array Antenna Development for …...the aperture, transmit and receive losses are...
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600 JOHNSHOPKINSAPLTECHNICALDIGEST,VOLUME22,NUMBER4(2001)
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ActivePhasedArrayAntennaDevelopmentforModernShipboardRadarSystems
Ashok K. Agrawal, Bruce A. Kopp, Mark H. Luesse, and Kenneth W. O’Haver
urrentandfutureNavyradarrequirementsaredrivenbyrapidlyevolvingthreats,includingboth cruisemissiles and tactical ballisticmissiles.To address these threats,arrayantennaswillhavetooperateoverwiderbandwidthswithenhancedsensitivity,higher radiated power levels, improved stability, and improved electronic protectiontoaddressreducedtargetradarcross-sections.Inaddition,thereisagrowingneedforreduced array signatures and a practical need to control costs, including acquisition,operational,andsupportcosts.Activephasedarrayantennashaveemergedasafunda-mental technology foraddressing theseevolvingNavy radar systemneeds.APL’sAirDefenseSystemsDepartmenthas longbeenat the forefrontof phased array antennadevelopment for shipboard radar systems, and the Department is contributing to thedevelopmentofactivearrayantennasforthenewgenerationofNavyradarsystemscur-rentlyunderdevelopment.Thisarticleprovidesanoverviewoftheemergingactivearrayantennatechnology.
INTRODUCTIONShipboardradarsystemstypicallymustprovidesur-
veillance of thousands of angular locations and trackhundredsoftargetsandguidedmissiles,allwithinrela-tivelyshortreactiontimes.Theserequirementscanbemetonlywithphased array antennas that allowelec-tronic repositioning of radar beams to widely diverseangularlocationswithinmicroseconds.Overa40-yearspan,APL’sAirDefenseSystemsDepartmenthaspar-ticipatedinthedevelopmentofphasedarrayantennasforNavyradarsystems.1
Inadditiontoenhancedsensitivity,improvedsystemstability will be required to detect low-flying cruisemissilesinseaorlandclutter.Widerbandwidthswillbe
requiredtoperformdiscriminationandtargetidentifi-cationfunctions.Atthesametimethatradardemandsare increasing, there is a practical need to reduceacquisition and operation and support (O&S) costs,improvereliability,andreducemanningrequirements.Activephasedarrayantennasareemergingasafunda-mental technology foraddressing thisevolvingNavyradar system need, and APL’s Air Defense SystemsDepartmentisplayingamajorroleinthesedevelop-mentefforts.
Although the concepts of phased array antennasare fairly straightforward, the factors that determinethedesignareextensiveandevensomewhatcomplex.
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Designfactorssuchasaperturesizing,patternsynthesis,andbeamswitchingspeedshavechangedlittle.Ontheotherhand,solid-statecomponenttechnologydevelop-ment has exploded over the last decade, exhibiting acontinuing and significant impact on the design andperformance of phased array antennas. The heart ofanactivephasedarrayantennaisthetransmit/receive(T/R) module. A T/R module at each radiating ele-mentprovidespoweramplificationduringtransmitandlow-noiseamplificationduringreceive,aswellasphaseshift control for beamsteering.The emergenceof gal-liumarsenide(GaAs)monolithicmicrowaveintegratedcircuit (MMIC) technology has enabled thedevelop-ment of T/R modules with the required performance,excellent reliability, and acceptable cost in quantity
military radar systems. A radio-frequency (RF) blockdiagram of a typical passive phased array antenna isshowninFig.1a.Acentralizedtransmitter,whichgen-erally consists of high-power microwave tubes (e.g.,travelingwavetubes)orcross-fieldamplifiers,providesthe power to the radiating elements through a high-powerbeamformernetwork.High-powerferriteordiodephaseshiftersarecontrolledateachradiatingelementtoelectronicallysteerthebeamtothedesiredangle.Inreceivemode,theoutputsoftheradiatingelementsandphase shiftersarecombinedusinga low-powerbeam-formingnetwork.Typically,threesimultaneousreceivebeams are provided to support monopulse tracking.Low-noise amplifiers (LNAs) are used to amplify thesignal at the output of the beamformers. One of the
Figure 1. (a) RF block diagram of a passive phased array antenna. (b) Beamformer archi-tecture of an active phased array antenna.
production.Anactivephasedarrayradarcan
provideordersofmagnitudeperfor-mance improvement over its pre-decessorpassivephasedarrayradar,whileatthesametimeimprovingreliabilityandreducingtotalown-ership costs. Virtually all high-performanceradarsunderdevelop-menttodayemployanactivearrayantenna.Navyradardevelopmentprograms forwhichactivephasedarraysareakeyenablingtechnol-ogyincludetheAN/SPY-3multi-functionradar,volumesearchradarfor long-range surveillance, andadvancedradarsforNavyTheaterWide Ballistic Missile Defense.Other active array radars includethe Theater High-Altitude AreaDefense,NationalMissileDefense,and High Power Discriminatorradars and the F-22 and JointStrike Fighter fire control radars.Active array technologies havebeen used in commercial com-munications applications, includ-ingIridiumandGlobalstarsystems;however, these systems have notso far proven to be economicallyviable.
ACTIVE PHASED ARRAY ANTENNA OVERVIEW
Tofullyappreciatewhatactivearray technologyhas tooffer, it isuseful to first review the conven-tional, or passive, array approachcurrentlydeployedinseveralfielded
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best examples of passive phased array radars is theAN/SPY-1 radar (Fig. 2), which has been in servicefor more than 20 years and is the Navy’s highest-performingfieldedradar.
Passive array systems have several inherent perfor-mancelimitationsandinefficiencies.Forexample,thetransmitbeamformertypicallyhassignificantlosses,andthetransmittermustgeneratealargeamountofpowertoovercometheselosses.Inessence,asignificantpor-tion of the RF power generated by the transmitter isdissipated as heat before being radiated. High-powercentralizedtransmittersusuallyemploymicrowavetube–based technologies, operate at lower duty factors,and have limited waveform flexibility. High receivebeamformerlosses,particularlywhenlowsidelobesarerequired,significantlydegradereceivesensitivity.Also,transmitnoisefromacentralizedsourceisoftenalimi-tationinclutter-drivenradarapplications.Finally,high-powertube-basedtransmittersandtheirattendanthigh-voltagepowersupplieshavelowerreliabilityandhighermaintenance and replacement costs than solid-statetechnology.Thislastissueisparticularlyimportantforshipboardapplicationsthatinvolverelativelylongmis-sionsandastrongdesiretoavoidat-seamaintenance.
Evolving threats are driving the need for order-of-magnitudeimprovementsinradarperformance.Activearraytechnologyoffersthecapabilityofachievingtherequiredperformanceimprovementswhileatthesametimeofferingimprovementsinreliability,maintainabil-ity, availability, and life-cycle costs. In active arrays,bothtransmitandreceive functionsaremovedto theaperturebyplacingaT/Rmoduleateachradiatingele-ment(Fig.1b).TheT/Rmodulesprovidepowerampli-fication during transmit and low-noise amplificationduringreceive,aswellasamplitudeandphasecontrolfor beamsteering and sidelobe reduction. Because thisconfigurationplacesthepoweramplifiersandLNAsattheaperture,transmitandreceivelossesaresignificantly
reduced, resulting in increased radar sensitivity for agivenamountofgeneratedmicrowavepower.
Keyradarsystem–leveladvantagesofactivephasedarrays over passive phased arrays are summarized asfollows:
• Increased sensitivity. Lower transmit and receivebeamformerlosses,coupledwiththeabilityofsolid-state T/R modules to operate at higher duty cyclesthanconventionaltube-basedtransmitters,generallyenables order-of-magnitude improvements in radarsensitivity.
• Improved target detection in clutter. Inanactivearray,keysourcesoftransmitnoiseandinstabilities(e.g., T/R modules and power supplies) are distrib-utedattheaperture.Consequently,theirnoisecon-tributionsdonotaddcoherentlyinthesamefashionasthetransmittedsignal,andtheircontributionstopulse–pulse variations undergo an averaging effect.Theresultisasignificantimprovementintheabilityofanactivearrayradartodetectsmallmovingtar-getsinseaorlandclutter.
• Improved waveform and pattern flexibility. Themultiplefunctionsofdetection,tracking,targetiden-tification, illumination, kill assessment, andmissilecommunications can be better optimized by thewaveformflexibilitythatthesolid-stateactivearraytechnologyfacilitates.Also,becausebothamplitudeandphasecontrolareprovidedbytheT/Rmodulesat the element level, radiation patterns are morereadily optimized for the radar mode of operation,includingtheuseofnullsynthesistechniques.
• Improved wideband operation. Thesolid-statetech-nologyemployedbyactivearrayscansupportinher-ently wideband microwave frequency operation. Inaddition, active array architectures are conduciveto the implementation of practical true time delaydevices,whichsupportwide-bandwidth,highrangeresolutionwaveformsandtargetimagingcapability.
• Reliable operation. Solid-state technologyandtheassociated low-voltage power supplies have inher-ently good reliability. In addition, the distributednatureoftheT/Rmodulesandpowersuppliesallowsthearraytobedesignedsothatoperationalperfor-mancerequirementscanbemetwhenafewpercentof the modules fail. Active arrays can be designedto be serviced at long periodic intervals, avoidingthe need for at-sea maintenance. The increasedreliability of active arrays is projected to result insignificantly lower O&S costs over the lifetime ofthearray.
ACTIVE PHASED ARRAY SUBSYSTEMS
Thefollowingkeyparametersaretypicallyspecifiedforanactivearray:
SPY-1/Dantenna
Figure 2. SPY-1/D phased array antenna on DDG 51.
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• Operatingfrequencyandbandwidth• Effectiveisotropicradiatedpower• Scancoverage• Beamwidths• Sidelobelevels(allmonopulsereceivechannels)• Trackingaccuracy• Waveformparameters(duty,pulsewidth)• Systemnoisefigure• Third-orderintercept• Stabilityandphasenoise• Harmonicandspuriousoutput• Reliability,maintainability,andavailability• Manufacturingandlife-cyclecosts• Primepowerrequirementsandcooling• Shipboardenvironmentalrequirements
Theoperatingfrequency,waveformparameters,andarrayperformancerequirementswillvarydependingonthe particular applications served by the radar. Theeffectiveisotropicradiatedpowerandbeamwidthdeter-mine the number of elements and the required T/Rmodule output power. The sidelobe levels determinethe amplitude and phase characteristics of the T/Rmodules,aperture,andtransmitandreceivebeamform-ers.Thephasenoise,stability,reliability,andmaintain-abilityrequirementsallinfluencethearrayarchitectureas well as the characteristics of the T/R modules andpower supplies. The antenna cost and weight dictatetheselectionoftechnologiesfordifferentcomponents
Radarcomputer
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Figure 3. Block diagram of an active phased array radar system.
and, in conjunction with the environmental require-ments, the array structure. The antenna cost, weight,andperformance typically forma critical design tradespace.
A block diagram of an active phased array radarsystemisshowninFig.3.Anactivephasedarraycon-sists of a transmit and receive antenna aperture thatincludestheradiatingelements,radome,andstructure;T/Rmodulesandassociatedcontrolcircuitry;RFbeam-formers; DC power distribution; and a beamsteeringcontroller.Keyfeaturesandfundamentaldesigntradesoftheseactivearraysubsystemsareaddressedinthefol-lowingparagraphs.
Transmit/Receive ModulesThe fundamental building block and key enabling
technology for active array radar antennas is the T/Rmodule. Depending on the application, active arrayscancontainhundredsor,moretypically, thousandsofT/R modules. These T/R modules have an importantroleindeterminingarrayperformance;theydrivemanyaspectsofthepackagingdesignandcanaccountforasmuchas50%of thecostof theactivearrayantenna.GaAsMMICtechnologyiskeytorealizingtherequiredmicrowavecircuitdensityinthesmallfootprintavail-able at each antenna element. Semiconductor batch-processingproductionofGaAsMMICsiskeytoachiev-ingtheactivearrayperformanceadvantagesatdesired
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arrayacquisitioncosts.AtypicalT/RmoduleisshowninFig.4.2
Figure 5 shows a block diagram of a typical T/Rmodule. Each module contains a transmit path and areceive path. The transmit path consists of a phaseshifter,avariablegainamplifierorattenuator(VGA),a driver amplifier, and a power amplifier. The poweramplifiersectionmayconsistofseveralpoweramplifiers,typicallytwoorfour,wheretheoutputpoweroftheseamplifiers is combined to obtain the required outputpowerfortheradiatingelement.Acirculatorisusedtoprovide theduplexing functionand isolationbetweentransmitandreceivepathsandtopreventloadpullingofthepoweramplifiercausedbyvariationsintheantennaelement’s input impedance changes during beamscan-ning.3Thereceivepathconsistsofalimiter,anLNA,aphase shifter,andavariableamplifierorattenuator.Thismodulearchitectureprovidesanoptimumperfor-mancecompromisewithrespecttomodulenoisefigure,third-orderintercept,anddynamicrange.4
Voltage regulation and digital control circuitry arealso often included in the T/R module. Local energystoragecanbeusedtomaintaintransmitpulsecurrentlevels and satisfy rise-time requirements. Also, seriesregulatorscanbeusedforsomeorallinternalvoltagesto reducepower supply rippleandnoise toacceptable
levelstomeetincreasinglyseverespectralpurityrequire-ments.5Low-resistanceHEXFETswitchesaretypicallyusedtocontrolthebiascurrentstothevariousampli-fiers. Digital signals to control the phase shifter andattenuatoraretypicallyfedseriallyintotheT/Rmoduletoreducepackagingcomplexity.Thisserialdatastreamis converted to parallel data with a shift register andclock signal. Memory may also be contained in themoduletoreducethetimerequiredtoswitchbetweenpredeterminedbeampositions.
T/Rmodulerequirementsarederivedfromthephasedarray antenna requirements andcanvary significantlydependingontheapplication.Thefollowingkeyparam-etersaretypicallyspecifiedforaT/Rmodule:
• Operatingfrequencyandbandwidth• Outputpower• Power-addedefficiency• Spuriousandharmonicoutput• Dutycycleandpulsecharacteristics• Receivenoisefigure• Receivegainandthird-orderintercept• Numberofamplitudeandphasebits• Amplitudeandphaseroot-mean-square(rms)errors• Meantimebetweenfailure(MTBF)• Cost
The frequency, bandwidth, and output power aredriven by the system application. A typical nominaloutput power for an X-band module, achievable withcommerciallyavailablepowerMMICs,isaround10W.Higher power levels are typical at lower frequencies.Power-added efficiency is an important parameter forminimizingtheprimepowerrequirementsandcoolingloadoftheactivearray.Dependingonthetechnologyused,transmit-power-addedefficienciesontheorderof20to25%aretypicallyachieved.LNAMMICnoisefig-uresinthemicrowaveregimetypicallyrangefrom1to2dB. Including lossesandothereffects,modulenoisefiguresinthe3-to4-dBrangearegenerallyachievable.Modules used in low sidelobe applications require ahighernumberofphasecontrolbits and lowerampli-tudeandphase rmserrors.Third-order intercept isanimportant parameter in mitigating interference fromsurface Navy radars where the radars, and thus themodules, are often operating in the vicinity of high-powerradarsonnearbyships.BecauseoftherelativelylongoperatingmissionsofsurfaceNavyradars,modulereliability is a critical factor in achieving low O&Scostsandminimalmaintenanceactions.AT/RmoduleshouldtypicallyhaveanMTBFinthehundredsofthou-sandsofhours.
Because of the large number of T/R modules inan active array, module production costs are criticalto active array affordability. Module production costscan vary depending on performance, design complex-ity,productionquantities,andotherfactors.Although
Limiter and LNA Phase shifter VGA
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Figure 4. Photograph of a typical T/R module.
Figure 5. Block diagram of a typical T/R module.
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themodulecostbreakdowncanvarydependingontheapplication, the typical cost breakdown of an X-bandmodule(Table1) is representativeofcurrentstate-of-the-artX-bandT/Rmodules.ThecostofaT/Rmoduleconsists of the cost of the semiconductors (MMICs),packaging,othercomponents,assembly,andtest.TheMMICsaretypicallythemostsignificantcostelement.AmoredetailedT/Rmodulecostdiscussionisprovidedin Refs. 6 and 7. The MMIC cost will increase withhighermoduleoutputpower.
Semiconductorcostisdeterminedbywaferprocess-ing cost, wafer diameter, MMIC area, and yield. ThewaferprocessingcostforGaAsishighlydependentonthevolumeofwafersproducedbyafoundry.Foundriestypicallycanproducemorethan20,000wafersperyear.However,productionratesofatleast10,000wafersperyear are desirable to maintain low foundry overheadcosts.TypicalX-bandT/Rmoduleproductionratesdonot require a sufficient number of wafers to providehighfoundryloading.One4-in.waferhasenoughareato supply the GaAs needed for more than 50 typicalX-band modules. The production of 100,000 X-bandmodulesperyearwouldthusrequirelessthan20004-in.wafersperyearorlessthan10006-in.wafersperyear.Commercialvolumeusingsimilarpersonnelandfacili-tiesisrequiredtoprovidethelow-overheadstructureforcost-effectiveproductionofX-bandT/Rmodules.SomeGaAs producers have successfully achieved this prod-uctmixthroughhigh-volumesales tosupportwirelesshandsetproducts.
Radiating Elements and Antenna ApertureThecriticaldesigntaskfortheradiatingelementis
designingonethatradiatesefficiently,withgoodimped-ancematch,overtheoperatingfrequencybandandthescanvolumeof thearray.Cost is a significantconsid-eration because of the large number of radiating ele-ments typicallypresent.Because theperformanceofaradiating element is affected by mutual coupling withotherradiatingelements,theradiatingelementmustbedesigned for the radiatingenvironment rather thanasanisolatedelement.Thedesignprocessisusuallyiter-ative and consists of simulation using numerical elec-tromagneticmodelingtools,fabricationofawaveguidesimulatortoverifyperformanceataselectedscanangle,
andfabricationofasmalltestarray(generallyconsist-ingofupto100elements)toverifyfullperformance.Awell-designedelementwillprovideanelementpatternontheorderofcos1.25sovertheintendedscanvolume,wheresisthescananglefromarraybroadside.
Thelistofthedifferentkindsofelementsthathavebeenusedinarraysystemsincludesdipoles,microstrippatches, microstrip and waveguide slots, waveguides,horns,andflarednotches(Fig.6).8Thechoiceofele-ment depends on several factors, such as power han-dling, polarization, bandwidth, environmental condi-tions,feedingarrangement,andmanufacturingcost.
Dipolesandpatchesgenerallyhavenarrowbandwidth.The element bandwidth is defined in terms of loss ingainwithrespecttothecenterfrequency.Aflarednotchelement9 (Figs. 6a and 6b), where an open circuitedorthogonalcentralconductorexcitesthenotchesintheouter conductors, is typically used for wideband arrays.Bandwidths up to 6:1 have been demonstrated.10 For amicrostrip slot antenna, a cavity must be used behindeach slot to restrict radiation to the front hemisphere.AmicrostripslotradiatorisshowninFigs.6cand6d.
For high-power radar arrays, variants of the rect-angular or cylindrical waveguide radiating elements(Fig.6e)aregenerallyused.Waveguidearrays, thoughheavy,tendtohavelowlossandgracefulscandegrada-tion.Ridgedwaveguidescanbeusedforwide-bandwidthapplications.Single,double,andquad-ridgedwaveguidesare shown in Fig. 6e. Quad-ridged waveguides extendthese features to circularly polarized phased arrays.Often,thewaveguideelementisdielectricallyloadedtomatchitsimpedancetofreespace.Awide-angleimped-ancematching(WAIM)sheetcanbeusedtoproducesusceptancevariationwiththescananglethatpartiallycancelsthearrayscanmismatch.WAIMsheetsarelesspracticalforshipboardenvironmentsbecauseiceforma-tion at the aperture is not permitted, and some kindofheatingarrangementisrequired.Forexample,intheSPY-1antenna,iceisinhibitedbyplacingaluminawin-dowsontheindividualwaveguideradiatorsandheatingthesewindowsthroughconductionheating.
Microstrip patch elements (Fig. 6f) can be fab-ricated with low-cost lithographic techniques. Thetwo most common feed techniques are an in-linemicrostrip feed and a coaxial probe feed. Patch ele-ments are narrowband. For electromagnetically cou-pled patches in a phased array, the bandwidth canbe increased to more than 15% by choosing patchdimensions,substratethickness,anddielectricmateri-als.Usingdouble-stackedpatches,essentiallyprovid-ing a double-tuned element, can increase the band-widthfurther.
Radio-Frequency Beamformer ArchitecturesThe RF beamformer plays a critical role in deter-
miningtheradiationpatterns,particularlythesidelobe
Table 1. Typical T/R module cost breakdown.
PercentageofT/RCostelement modulecostMMICs 45%Package/substrates 25%Digital/analogcircuitry 15%Assembly 10%Test 5%
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levels, of an array radar. On transmit, the RF beam-former distributes the input signals to the individualT/Rmodules.Typically,this isdonesothateachT/Rmodule receivesan identical inputpower level.EqualamplificationineachT/Rmodulethenproducesauni-formtransmitaperturedistributionthatmaximizesthetransmitantennagain.
In the receive mode, amplitude tapering acrosstheapertureistypicallyappliedtoreducethereceive
sidelobe levels. Active array radars generally requirelowreceivesidelobestominimizesusceptibilitytojam-ming.Theamplitudeandphaseerror levels thatcanbemaintainedat theaperturedetermine theachiev-ablesidelobes.TheprimarysourcesoferrorincludetheT/RmodulesandtheRFbeamformers.Phaseshiftcon-trolintheT/Rmoduleprovidesamechanismforcali-bratingthemoduleandbeamformerphaseerrors.Toobtain low residual and quantization phase errors to
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Figure 6. Radiating elements for phased-array antennas.
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supportlowsidelobeperformance,asmanyas7bitsareusedinthedigitalphaseshifterMMICwithintheT/Rmodule.
Passivearrayradarshavesimilarrequirementsforlowsidelobes. The amplitude control can be applied onlyin thereceivebeamformers inpassivearrays,while theamplitudetapercanbeappliedeitherinthebeamformersorintheT/Rmodulesinactivearrays,asdescribednext.
To obtain sufficient tracking accuracy, radars typ-ically employ monopulse tracking techniques thatrequire separate receive channels, or RF beamformerarrangements, for the receive sum,deltaazimuth,deltaelevation,anddelta-deltachannels.Eachchannelhasanoptimumamplitudedistributionforlowsidelobeperfor-mance.Also,becausethechannelsareemployedsimulta-neously,asinglephaseshifterintheT/Rmodulecannotsimultaneously correct phase errors in all monopulsechannels,andoftensomecompromiseismadeinthedif-ference channel sidelobes relative to the sum channelsidelobes.Therearemanytrade-offsindevelopingbeam-formerarchitecturesforactivearrayradars.Twocommonexamplesarediscussedinthefollowingparagraphs.
Figure 1b shows a simplified beamformer architec-ture of a monopulse active phased array antenna. In
isdesiredif theradar istodetectsmallreturns.Thesetwoarchitectureshavebeenanalyzed,11andtheresultsshowthatforalargeactivephasedarrayantenna,thedifference in the noise figure for the two is approxi-mately0.5dB.Thechoiceofcommonorseparatebeam-formers is a function of beamformer complexity andantennanoisefigure.
DC Power DistributionBelow-deck AC-to-DC converters convert a ship’s
AC power into DC power that is supplied to thearrays.Thevoltageintoanactivearraytypicallyrangesfrom 200 to 500 VDC and, as such, is stepped downtovoltagelevelsrequiredbytheT/Rmodules(around10VDCorless)byDC-to-DCconverters.Therequire-ments for a DC-to-DC converter include voltage andcurrentrequirementsoftheT/Rmodules,outputvoltagedroopandripple,randomnoise,efficiency,dynamicstepresponse,andenoughinputandoutputenergystoragecapacitorsthatthemaximumdroopduetoloadchangeduringthelongesttransmitperiodmeetsrequirements.
DC power can be distributed in an array usingeither a distributed or a centralized system. In a dis-tributed system, a single DC-to-DC converter (power
Figure 7. Beamformer architecture with separate transmit and receive beamformers.
thereceivemode,theradiatingele-ments’ outputs are first combinedusing column beamformers. Theoutputs of the column beamform-ers are then combined using hor-izontal beamformers to form thesum,azimuthdelta,elevationdelta,anddelta-deltabeams.Thereceiveaperture weighting is applied inthe T/R modules by using theattenuatorsorvariable-gainampli-fiers. Because the receive ampli-tude weighting is applied in theT/Rmodules,thetransmitsumandreceivesumbeamformershaveuni-formdistributionandaresharedforthetransmitandreceivefunctions.Theorderofcolumnandrowcom-biningcanbeinterchanged.
Theamplitudetaperforanactivearray can also be applied in thebeamformers rather than in theT/R modules, as shown in Fig. 7.Becausetransmitandreceiveampli-tude tapers are different for thisarchitecture,separatebeamformersarerequiredforthesumreceiveandtransmitbeams.Althoughthearraywithseparatetransmitandreceivebeamformers is more complex, ithas a slightly lower receive noisefigure.Alowerantennanoisefigure
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supply) feeds a small group of T/R modules (typicallytwotoeight)andDC-to-DCconvertersaredistributedthroughout the array, as shown in Fig. 8a. The DC-to-DC converters can be mounted on the same base-plateastheT/Rmodules.Inthecentralizedpowerdis-tribution system,agroupofDC-to-DCconvertersarecombinedtofeedalargesectionoftheantenna.Redun-dancy is provided in each group of power supplies toincreasereliability.ThecentralizedpowerdistributionisshowninFig.8b.
In both cases, the high-voltage DC is delivered totheconvertersusingalow-current,high-voltagebus.Inthecentralizedsystem,thelow-powerDCvoltagesaredistributedthroughoutthearray,requiringlow-voltage,high-currentbuses.Theconvertersconverthigh-volt-ageDCto lowvoltages requiredby theT/Rmodules,andvoltageregulationisprovidedtogeneratevoltageswith very small ripple. Both approaches provide forimprovedclutterperformancebecauseof theuncorre-latedrandomnoise.12However,theDC-to-DCconvert-ersforthecentralizedsystemcanbesignificantlylargerthanthoseofthedistributedsystemand,tocompensatefortheimpactofasmallernumberofDC-to-DCcon-verters on clutter performance, higher voltage regula-tionwouldberequired.
The DC-to-DC converters can be either averagepower or peak power switching frequency converters.Theswitchingfrequencydeterminesthesizeofthecon-verter, as the converter size decreases with increasingfrequency.Theaveragepowerconverters require stor-age capacitors to maintain the desired voltage droop.As the pulse width increases, the storage capacitancerequirementincreases,puttingaphysicallimitonthelengthofthepulse.TheotherapproachistouseapeakcapacitormultiplyingDC-to-DCconverter.Thiscon-verterhastheuniqueadvantagethatthepulseenergycanbestoredontheprimaryhigh-voltagesideoftheconverter,therebyresultinginasubstantiallysmaller
energystoragecapacitor.Bothconvertersarecompa-rable, and the choice of converter depends on pulsewidth,cost,andvolume.
Beamsteering ControllerTransmitandreceivebeamsinanactivephasedarray
antennaaresteeredbychangingthe insertionphaseofthe phase shifters contained in the T/R modules. Anantenna beamsteering controller (BSC) generates thephaseshiftcommandsforalloftheT/Rmodules.Gen-erationof thephase shiftcommandscaneitherbedis-tributedthroughoutthearrayorperformedinacentral-izedlocation.ThesetwoarchitecturesarereferredtoasadistributedBSCandacentralizedBSC,respectively.
In the distributed BSC architecture (Fig. 9a), acentral controller generates simple commands suchasscanangle, frequency, and timing.13TheT/Rmodulecontrolelectronicscontainanapplication-specificinte-gratedcircuit(ASIC),electricallyerasableprogramma-ble read-onlymemory(EEPROM),fieldeffect transis-tor(FET)switches,etc.ThephasesettingsforeachT/Rmodule are calculatedby theASIC, given the simpleinputcommandsandthemodule location.EEPROMsmay contain linearization amplitude and phase tablesfor eachT/Rmodule.Thesedata aremodule specific,storedinEEPROMsonthebasisofmodulefactorytestresults,andcanbeerasedandreloadedwithnewdataatanytime.ThedatatransferratefromASICtoT/Rmodulescanbeoftheorderof20Mbps.
Inadditiontosendingthescanangleandfrequency,the central controller sends a command to set theantenna ineither transmitor receivemodeby settingswitchesappropriatelyintheT/Rmodules.Alocalcrys-tal oscillator generates the clock at each local groupof modules. The clock speed determines the time ittakesforcommandstoreachallT/Rmodules.Distrib-utingtheclockatthelocallevelminimizesthenoise,becausetheclocksarenotsynchronized.Becausemany
Figure 8. (a) Distributed and (b) centralized power distribution architecture.
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Figure 9. (a) Distributed and (b) centralized beamsteering controller architecture.
differentbeam-direction/gain-phasecombinationscanbestoredinmemoryinadvance,switchingcanreadilybeaccomplishedamongvariousbeamswithout recal-culationwithinasingledwell.
In the centralized BSC architecture (Fig. 9b), mostofthecomputationsareperformedinacentrallocationand the data required by each T/R module are sentdirectly to theT/RmodulesorgroupsofT/Rmodules,suchas lowest replaceableunits (LRUs),usingparallelbuses.Thecentralbeamsteeringcontrollermaycontaina number of digital signal processor (DSP) cards; eachcardisassignedresponsibilityforaspecificgroupofT/Rmodules and stores all calibration values (linearizationtables)forthatgroupofmodules.EachDSPcardperformsasetofbeamsteeringcalculationsforeachT/Rmoduleinitsgroupwithinaminimumpulserepetitioninterval(PRI).ThePRIandthenumberofmodulesdeterminethetotalcomputationrequirement.Severalparallelprocessorsmaybeneededtomeettherequirements.Thedatathatcontain thephaseandgainbitsandthatare sentoverthecontrollinesmustbemanipulatedforfinaldeliverytotheT/Rmodule.ForthecentralizedBSCarchitecture,thisfunctioncanbeimplementedinfield-programmablegatearraysthatcaneasilybereprogrammedtomeettherequirementsofanewT/Rmodule.
TheadvantagesofthecentralizedBSCincludecen-tralprocessorsthatcanbepurchasedcommerciallyandreconfiguredfordifferentradarsystems.Thecentralcon-trollocationrequiresasubstantialdataflowbetweenthecentral control unit and the T/R modules. For a largeactive phased array, the data rates can range between100and500Mbps,requiringsimilarclockspeeds.Fiber-opticdatalinksmaybeappropriateatthesespeeds.
EitherthedistributedorthecentralizedBSCarchi-tecturecanbeadopted fora largeactivephasedarraysystem. Preliminary estimates show that the cost ofthesetwoarchitecturesisverysimilar.
Mechanical PackagingThe predominant packaging considerations associ-
atedwiththemechanicaldesignofactivephasedarray
antennasincludedesignforeaseofmaintenance,ther-mal management, packaging the DC power distribu-tionsystem,RFbeamformers,radiatingaperturedesign/interface, and structural design.Theoverridingdriverinwhatpackagingoptionsareavailabletothedesigneris the antenna operating frequency. As the frequencyincreases, element spacing decreases, requiring tighterspacingofthesupportingelectronics.Fortunatelyforthedesigner,higher-densityarraystendtohavelowerT/Rmoduleoutputpower requirements;hence, theworst-casethermaldesignproblemstypicallydonotcorrelatewiththeworst-casepackagingdensities.
Tooptimizeanarraydesignforeaseofmaintenance,mostoftheactiveelectronicsareconfiguredasLRUs,which can include T/R modules, DC-to-DC powerconverters, and various control/processor assemblies(Fig.10).ReliabilityandsystemimpactintheeventoffailuredeterminewhetheranassemblyisdesignedasanLRU. System architecture, LRU reliability, and LRUcostdefinewhatisincludedwithinagivenLRU.Faultisolation down to at least the LRU level is providedtominimizeservicetimerequiredduringmaintenanceactions. Structural components, the coolant distribu-tion system, RF beamformers, DC power distribution,and cabling are typically considered of sufficient reli-abilitytobenonrepairableatsea.
Thermaldesign is critical formaintaining junctiontemperaturesoftheelectronicsatdesiredlevelstosup-port reliability requirements and maintain control oftemperature-induced module-to-module phase errors.TheT/Rmodulesaccountfor70–80%oftheheatgen-eratedwithinanarray.Becauseofever-increasingpowerdensity heat dissipation in modern shipboard activephasedarrays,liquidcoolingisnormallyrequired.Thepredominant cooling techniques in use today employconductionawayfromtheT/Rmoduleintoliquid-filledcoldplatesordirectliquidflow-throughcoolingonindi-vidualLRUs.Ifthespacingallowselectronicstobecon-tainedononesideoftheLRU,theLRUscanbedirectlyattachedtothecoldplatesconductingacross the largesurface area opposite the components. Alternatively,
RCCBSC
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LRUs can be edge cooled. Figure 10 shows a typicaledge-cooledLRU.Ascomponentpowerdensitiescon-tinuetoincrease,itmaybecomenecessarytomounttheT/Rmodulesdirectlytoliquid-filledcoldplates.
Because T/R modules exhibit phase changes withtemperature variations, tight thermal control of themodulesmustbemaintainedacrossanarray.PhaseerrorscanbeactivelycompensatedforiftheMMICtempera-tureisknown.However,becauseofthedifficultyinmea-suringtheactualgatetemperaturesduringvariousopera-tionalmodesanddutycycles,thishasnotbeenshowntobepractical.Currenttechniquesfocusonmaintaininguniformcoolanttemperaturesacrossthearrayandcal-ibrating each element during one or more operatingmodes.
Maintenancerequirementsusuallydictatethatcom-ponents not require removal to replace an LRU. Inaddition, maintenance must be performed from thebackside(deckhouseinterior)ofthearray,andthere-fore LRUs are inserted and extracted from the backof the array. It is imperative that RF beamformers,powerdistributionsystems,controlsignaldistributionsystems, and their associated cabling be installed inamanner that allows ready access to theLRUs, andtheseelementsmustbedesignedtofitbetweentheT/RmoduleLRUs.ThisrequiresblindmateconnectionstotheLRUsfromthecontrol,power,andRFdistribution(andtoliquidconnectionsonliquid-cooledLRUs).
Two packaging configurations are shown in Fig.11. In Fig. 11a, the T/R modules are mounted on athermallyconductivebaseplateattachedtotheliquid-cooledmountingstructure.Thisdesignissimilartothetechniqueemployedinairtransportablerackequipmentandallowscomponentstobemountedtobothsidesofthebaseplate.This techniqueoffers a simple, easy-to-maintainpackagingdesignthatcanaccommodatetightelement spacing. In addition, the coldplates becomepartoftheantennastructure.Becauseoftheincreasedthermalpath length, thepowerdissipationcapacity issomewhatpowerlimited.
T/Rmodule
DC-to-DCconverter
Controlcircuits Baseplate
Top view Side view
Figure 10. T/R module LRUs: components on both sides of a baseplate.
In the configuration shown inFig. 11b, the T/R modules aremounted to one side of an LRUbaseplate. The single-sided LRUsare mounted on a large vertical,fixed coldplate using wedge locksto press the baseplate against thecoldplate. This configuration pro-vides a large contact area to thecoldplate and allows easy accesstotheLRUs;however,ittendstobe better suited to larger elementspacing,whichallowsLRUattach-mentbyinsertionononeside.Theprimarydifficultyinthisapproachcomes from theLRU-to-coldplate
interface. To readily accommodate sliding insertionand extraction, the interface needs to be free offillermaterial.Thisrequirestighttolerancecontrolofbothsurfacesandmakesachievingrepeatablethermalresistance from module to module difficult. Alterna-tively, a phase-change–type interface material couldbeemployedthatwouldrequireheatingtheinterfacepriortoextraction.
(a)
(b)
Figure 11. Antenna packaging assembly with (a) horizontal man-ifolds (edge-cooled LRUs) and (b) fixed vertical coldplates (one-sided LRUs mounted directly on both sides of the coldplates).
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LRUs either must contain the radiating elementsormustblindmate to the radiatingelementson thearray face. For low-sidelobe radar arrays, the latteris typically the case when the radiating elementsare machined out of a faceplate to maintain tightelement location tolerances. Overall mechanicalalignment between the LRU and the radiating ele-mentrequirestightcontroloflarge-tolerancestackups,and the resulting misalignment must be absorbed intheRFconnector.Therecaneasilybe15 to25 tol-erances,whichcombinetodeterminetheradialfloatrequired in theRF connector.Becausemost floatingRFconnectorsoffernomorethan0.030in.ofradialfloat, it is necessary to use a combination of tighttolerances,statisticaltolerancingmethods,andocca-sionallyspecializedassemblyfixturestoensureproperalignment.
Structuraldesignisasignificantissueforshipboardactive array radars. Traditionally the shipboard shockrequirement(Mil-Std-901)hasbeenthepredominantstructuraldesigndriver.Formostantennasystems,float-ingplatformbargetestingisrequired.Thesetestsimparta shock pulse on the order of 70 g at 11–14 Hz. Asecondary consideration is tomaintain adequate stiff-nessintheradiatingaperturetoensurethatarrayflat-nessrequirementsaremaintainedduringoperationalseastatesandshipboardstructuralvibrations.Thisflexuralstiffness directly contributes to the array error budgetandmustbeaccountedfor.Maintainingadequateflat-ness, particularly in larger,high-frequency arrays,willbecome increasingly difficult. One technique beingexplored isactivemeasurementandcompensation fordeflection.Inaddition,errorbudgetsmustaccountforlarge-scaledeflectionsoftheship’sstructuralmovementsuchasdeckhouseormastdeflections.
RELIABILITY AND LIFE-CYCLE COSTAchievinglife-cyclecostrequirementsiscriticalto
theacquisitionofanynewactivearrayradarsystemfortheU.S.Navy.Life-cyclecostconsistsofdevelopment,acquisition, installation, O&S, and disposal costs, ofwhichtheacquisitionandO&Scostsarethemaincon-tributors.Thekeystoreducinglife-cyclecostsaremin-imizing the cost of spares and, through fault-tolerantdesignoftheantennaarchitecture,minimizingthefre-quencyofmaintenanceactions.
The reliability of the antenna is measured in twoways: MTBF and mean time between critical failures(MTBCF).TheMTBFofalargephasedarrayisquitelow because of the large number of components. Itthereforebecomescriticaltoincorporatefaulttoleranceintothedesign.14,15Becauseof theredundancyoftheelectronics supporting each radiating element, activephased array antennas are inherently fault tolerantand can be readily designed to degrade gracefully.
ConsequentlytheyareprojectedtoachieveanMTBCFsufficient to support ship deployment periods with noscheduledmaintenance.
ThekeytoreducingO&ScostisthereliabilityandredundancyoftheindividualLRUs.TheLRUsmustbeoptimallysizedtominimizecost,partscount,andper-formanceimpact intheeventof failure,yetbeofsuf-ficientsizeforarraypackagingandmaintenanceaccessconsiderations.AntennaacquisitioncostsareprimarilydrivenbytheT/Rmodule;however,LRUassemblyandDC-to-DCconvertercostscanalsobesignificant.Keytoreducingthesecostsisattentiontoproduceabilityandeliminationoftouchlaborintheassembly.Becauseofthehighlyredundantnatureofthearchitecture,manyLRUs are produced in sufficient quantities to warranttruedesignforproduction.
Becauseofthecombinationofaninherentlyredun-dant architecture and highly reliable solid-state elec-tronics (notably T/R modules), active arrays are pro-jectedtoprovideimprovedreliabilityandreducedO&Scostsrelativetoconventionalarrayradarsystems.
CONCLUSIONActivearrayantennashaveemergedasafundamen-
tal technology for addressing evolving surface Navyradarsystemneeds.Newshipboardactivearrayradarscurrentlyunderdevelopment includetheAN/SPY-3multifunction radar, volume search radar for long-rangesurveillance,anddevelopmentalradarconceptsfor Navy Theater Wide Ballistic Missile Defense.Largely as a result of active array technology, thesenewradarsareprojected toprovidedramaticperfor-mance improvements as well as improved reliabilityandreducedO&Scostsrelativetoconventionalradarsystems.
Thisarticlepresentedanoverviewofthekeyaspectsofanactivearrayanditsvarioussubsystems.Thecur-rent state of the art was described, with particularemphasisonthecriticalT/Rmoduletechnology.T/Rmodulecostconsiderationswerealsoaddressed,giventhe significant focus on achieving affordable radaracquisitioncosts.
REFERENCES 1Frank,J.,andO’Haver,K.,“PhasedArrayAntennaDevelopmentat
theAppliedPhysicsLaboratory,”Johns Hopkins APL Tech. Dig.14(4),339–347(1993).
2Komiak, J., and Agrawal, A., “Design and Performance of OctaveS/C-Band MMIC T/R Modules for Multifunction Phased Arrays,”IEEE Trans. Microwave Theory Tech. 39(12), 1955–1963 (Dec1991).
3Jones, R., and Kopp, B., “Duplexer Considerations for X-Band T/RModules,”Microwave J.43,348–352(May2000).
4Agrawal, A., Clark, R., and Komiak, J., “T/R Module ArchitectureTrade-offs for Active Phased Array Antennas,” 1996 IEEE MTT-S Int. Microwave Symp., San Francisco, CA, pp. 995–998 (17–21 Jun1996).
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5Moore,C.,andKopp,B.,“PhaseandAmplitudeNoiseDuetoAnalogControlComponents,”Microwave J.41,64–72(Dec1998).
6Kopp,B.,“X-BandTransmit/ReceiveModuleOverview,”IEEE MTT-Symp.,Boston,MA,pp.705–708(Jun2000).
7Cordner, T., “GaAs Cost Drivers and an Approach to AchievingLow Cost Products,” Int. Conf. of GaAs Manufacturing Technol.,pp.153–159(1997).
8Mailloux, R. J., Phased Array Antenna Handbook, Artech House(1994).
9Lewis,L.R.,Fasset,M.,andHunt,J.,“ABroadbandStriplineArrayElement,” IEEE AP-S Symp. Dig., Atlanta, GA, pp. 335–337 (Jun1974).
10Monser,G.J.,“PerformanceCharacteristicsofNotchArrayElementsOver6:1FrequencyBand,”1987 Antenna Application Symp.,Univer-sityofIllinois(1987).
THE AUTHORS
ASHOKK.AGRAWALreceivedaPh.D.inelectricalengineeringin1979fromtheUniversityofNewMexico.HeworkedasaresearchscientistatMissionResearchCorporationinAlbuquerque,NewMexico,from1976to1982andasaPrincipalEngineeratLockheedMartinCorporationinMoorestown,NewJersey(previouslyRCA,GE,andMartinMarietta),from1983to1999.AtLockheedDr.Agrawalwasinvolvedinresearchanddevelopmentprogramsonvariousphasedarrayantennasandalsoledthedevelopmentofseveralactivephasedarrayantennaprograms.In1999hejoinedAPLandiscontinuingresearchanddevelopmentworkonactivephased array antennas. Dr. Agrawal has published numerous papers in IEEE andother journals andholdsfivepatentsonphased array antenna topics.His [email protected].
11Agrawal,A.,andHolzman,E.,“BeamformerArchitecturesforActivePhasedArrayRadarAntennas,”IEEE Trans. Antennas Propag.AP-47,432–442(Mar1999).
12Holzman,E.,andAgrawal,A.,“ActivePhasedArrayDesignforHighClutterImprovementFactor,”IEEE Int. Symp. Phased Array Technol.,Boston,MA,pp.44–47(15–18Oct1996).
13Deluca, A., Gentry, J., Thomas, D., Landry, N., and Agrawal, A.,“PhasedArrayAntennawithDistributedBeamsteering,”U.S.PatentNo.5339086(16Aug1994).
14Agrawal,A.,andHolzman,E.,“ActivePhasedArrayArchitecturesforHighReliability,”IEEE Int. Symp. on Phased Array Technol.,Boston,MA,pp.159–162(15–18Oct1996).
15Agrawal,A.,andHolzman,E.,“ActivePhasedArrayDesignforHighReliability,”IEEE Trans. Aerosp. Electronic Sys.35,1204–1210(Oct1999).
BRUCEA.KOPPreceivedaB.S.E.E.fromArizonaStateUniversityin1986andan M.S.E.E. from Stanford University in 1988. He was employed at Avantek inMilpitas,California,asamicrowaveandRFcomponentandsubsystemdesignengi-neerfrom1986to1990,workingonpassiveandcontrolcircuitdevelopmentusingprintedlumpedelements.Mr.KoppisamemberofthePrincipalProfessionalStaffat APL where he has been employed since 1990 developing T/R modules andassociatedpackagingandsolid-statetechnologiesforphasedarrayradarandcom-munications systemsapplications.Mr.Kopphasparticipated in thedevelopmentof 30 different T/R modules and has been the author or co-author of 17 papersrelated toT/Rmodulesand related technologies. Inaddition,he teachesa3-dayshortcourseonT/Rmodulecost,performance,[email protected].
MARK H. LUESSE, a member of the APL Senior Professional Staff in the AirDefenseSystemsDepartment,receivedaB.S.inmechanicalengineeringfromtheUniversity ofMaryland.Before joiningAPL in1989,heworked atAAICorpo-rationinHuntValley,Maryland,asadesignengineerdevelopingsimulationandtestequipment.AtAPL,hehasdesigned,analyzed,andmanagedvarioushardwaredevelopmentandfield test siteefforts.Mr.Luesse joinedADSD’sRadarSystemsDevelopment Group in 1996, working primarily on the mechanical design andpackagingoversightofactivephasedarrayradarandcommunicationsantennas.Hise-mailaddressismark.luesse@jhuapl.edu.
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KENNETHW.O’HAVERisamemberofAPL’sPrincipalProfessionalStaffandisSupervisoroftheAirDefenseSystemsDepartment’sRadarDevelopmentGroup.HereceivedaB.S.inelectricalengineeringfromVirginiaTechin1981andanM.S.inelectricalengineeringfromTheJohnsHopkinsUniversityin1984.Mr.O’HaverjoinedAPLin1984andhasbeenengagedinthedevelopmentofphasedarrayandactiveapertureantennasystemsandtechnologies forshipboardradarapplicationsandshipboardandairbornecommunicationsapplications.HeistheleadengineeratAPLforantennadevelopmentfortheNavy’sCooperativeEngagementCapabilityProgramandistheNavyIntegratedProductTeamleadforSPY-3radararrayantennaequipmentdevelopment.Hise-mailaddressiskenneth.ohaver@jhuapl.edu.