Design, development and verification of the 30 and 44 GHz front-end modules for the Planck Low Frequency Instrument

1.5 MB·English

by  R. J. Davis

Design, development and verification of the 30 and 44 GHz front-end
modules for the Planck Low Frequency Instrument
Davis,R.J.1, WilkinsonA.1, Davies, R.D.1, Winder, W.F.1, Roddis,N.1, Blackhurst, E.J.1, Lawson,
D.1, Lowe,S.R.1, Baines, C.1, Butlin,M.1, Galtress,A.1, Shepherd, D.1, Aja, B.2,Artal, E.2,
Bersanelli,M.3, Butler, R.C.4, Castelli,C.5, Cuttaia,F.4, D’Arcangelo, O.6, Gaier, T.7,Hoyland,R.8,
Kettle,D.9, Leonardi,R.10, Mandolesi,N.4, Mennella,A.3,Meinhold,P.10, Pospieszalski,M.11,
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1 Stringhetti,L.4, Tomasi,M.3, Valenziano, L.4, andZonca, A.3
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1JodrellBank Centre forAstrophysics,Alan TuringBuilding,TheUniversityofManchester,

n
Manchester,M139PL, UK
a
J 2DepartamentodeIngenier´ıadeComunicaciones,UniversidaddeCantabria, AvenidadelosCastros

6 s/n. 39005Santander, Spain
2 3PhysicsDepartment,Universita` degliStudidi Milano,ViaCeloria16, 20133Milano,Italy

] 4INAFIASF Bologna, ViaGobetti,101,40129,Bologna, Italy
M
5Science and TechnologyFacilitiesCouncil,Swindon, Wiltshire,SN2 1SZ, UK
I 6IFP-CNR, ViaCozzi 53, Milano,Italy
.
h 7JetPropulsionLaboratory,Pasadena, California, USA
p
- 8InstitutodeAstrof´ısicadeCanarias, C/ V´ıaLa´ctea, s/nE38205-LaLaguna(Tenerife), Spain
o
9SchoolofElectrical and ElectronicEngineering,TheUniversityofManchester, Manchester,M60
r
t 1QD,UK
s
a 10DepartmentofPhysics,UniversityofCalifornia, SantaBarbara, CA 93106-9530,USA
[
11NationalRadio AstronomyObservatory,520EdgemontRd, Charlottesville,VA22903-2475USA

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3 publishedinJINST, 28 Dec, 2009
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. Abstract
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0 Wegiveadescriptionofthedesign,constructionandtestingofthe30and44GHzFrontEndModules(FEMs)for
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theLowFrequencyInstrument(LFI)ofthePlanckmissiontobelaunchedin2009. Thescientificrequirementsofthe
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missiondeterminetheperformanceparameterstobemetbytheFEMs,includingtheirlinearpolarizationcharacteristics.
:
v TheFEMdesignisthatofadifferentialpseudo-correlationradiometerinwhichthesignalfromtheskyiscompared
i witha4-Kblackbodyload.TheLowNoiseAmplifier(LNA)attheheartoftheFEMisbasedonindiumphosphideHigh
X
ElectronMobilityTransistors(HEMTs).Theradiometerincorporatesanovelphase-switchdesignwhichgivesexcellent
r amplitudeandphasematchacrosstheband.
a
The noise temperature requirements are met within the measurement errors at the two frequencies. For the most
sensitiveLNAs,thenoisetemperatureatthebandcentreis3and5timesthequantumlimitat30and44GHzrespectively.
ForsomeoftheFEMs,thenoisetemperatureisstillfallingastheambienttemperatureisreducedto20K.Stabilitytests
oftheFEMs,includingameasurementofthe1/fkneefrequency,alsomeetmissionrequirements.
The30and44GHzFEMshavemetorbetteredthemissionrequirementsinallcriticalaspects. Themostsensitive
LNAshavereachednewlimitsofnoisetemperatureforHEMTsattheirbandcentres.TheFEMshavewell-definedlinear
polarizationcharacteristcs.
1 Introduction
Observations of the Cosmic Microwave Background(CMB) provide unique information about the early history of the
Universesome380,000yearsaftertheBigBang.TheCMBpowerspectrumasafunctionoftheangularfrequency,l,gives
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criticaldataforcosmology. Uptothepresent,theangularfrequencycoverageisobtainedfromacombinationofground
and balloon (high-l) observationsof restricted areas and space (low-l) all-sky observations. Ground-basedinstruments
samplinghigh-lvaluesincludeCBI(Readheadetal.2004),theVSA(Dickinsonetal.2004),DASI(Kovacetal.2002),
QUaD(Cahilletal.2004)andACBAR(Kuoetal.2004);ballooninstrumentsincludeBOOMERANG(Jonesetal.2006)
andArcheops(Mac´ıas-Pe´rezetal.2007). Thespace-basedinstrumentsgivingall-skycoverageareCOBE (Smootetal.
1990)andWMAP(Jarosiketal.2003,2007;Hinshawetal.2007). ThePlancksatellitewillgivefullskycoverageovera
widelrangeinasingleinstrumentwithdatacollectedatallfrequenciessimultaneously.
FromtheearliestdaysofCMBresearchitwasrecognizedthatGalacticforegroundswereasignificantcontaminantof
theCMB.Atthelowerfrequencies,synchrotron(e.g.Haslametal.1982)andfree-freeemissions(Dickinsonetal.2003)
werewell-establishedforegroundsbuttheiraccuracyisinsufficientforprecisemeasurementoftheCMBandasindicated
below, Planck will be used to improve this. A spinning dust (“anomalous emission”) component (Draine&Lazarian
1998; deOliveira-Costaetal. 1999) was later identified; it dominated the foregroundsin the frequency range 15 to 40
GHz(Bandayetal.2003;Daviesetal.2006). Atfrequenciesabove 100GHz,thedominantforegroundisthethermal
∼
(vibrational)emissionfrominterstellardustwhichhasarangeoftemperaturesandparticlesizes(Desertetal.1990).HFI
willenableanaccuratemeasurementofthisforeground.
To derive the best estimate of the CMB sky distribution, it is necessary to have a wide frequency coverage in the
PlanckarmouryinordertoidentifyandremovetheGalacticforegrounds.ThePlancksatellitecoversthefrequencyrange
30 to 857GHz with two technologies. The LowFrequencyInstrument(LFI)consists oflow noise amplifiersat 30, 44
and 70 GHz. The High Frequency Instrument (HFI) uses bolometers at 100, 143, 217, 353, 545 and 857 GHz. The
frequencybandswhich are least contaminatedbyforegrounds,andhence bestforobservingthe CMB, are thosein the
octaveeithersideof100GHz. The70GHzchannelisthechannelwhereforegroundsareminimumbothinintensityand
linearpolarization.
Thiswidefrequencycoverageinadeepall-skysurveywillprovidecrucialinformationonalltheGalacticcomponents.
The 30 and 44 GHz channelsare particularly importantin determiningthe propertiesof the synchrotron,free-freeand
spinningdustcomponentswhencombinedwithnewground-basedsurveysatlowerfrequencies.
Allthereceivers/detectorbandsonPlanckbetween30and353GHzwillbesensitivetolinearpolarization. Special
attentionhasbeenpaidtothepurityofthepolarizationdetectionsystemsincetheE-modepolarizationoftheCMBisweak
( 2µK)andanyB-modepolarizationwillbeweakerbyfactorsof10ormore.Thelinearpolarizationofthesynchrotron
∼
emission is relativelystrong, so accurate measurementsovera wide frequencyrange will contributesubstantiallyto an
understandingofthestructureoftheGalacticmagneticfield.Carefulmeasurementsofthepolarizationinallthefrequency
channelsarenecessarytodetectandquantifythepolarizationofthefree-freeanddust–bothspinningandthermal.
Inthispaperwedescribethedesign,developmentandtestingofthecryogenicsection(thefrontendmodules,FEMs)
oftheLFIradiometersat30and44GHz. Thestructureofthepaperisasfollows. Section2describesthespecification
andrationaleoftheradiometerchainat30and44GHz,alongwithadescriptionoftheinputstructure(thehorns)andthe
backendmodulestowhichtheFEMsareattached.Section3describesthedesignofthevariouscomponentsoftheFEMs.
Section 4 gives the details of the FEM assembly and qualification. The radiometricperformanceof the FEMs is given
inSection5. Anoverallsummaryoftheexpectedperformanceofthe30and44GHzradiometersinorbitisprovidedin
Section6.
2 Specification of the Radiometer chain at 30 and 44 GHz
TheprincipalobservationalobjectiveofPlanckistoproducemapsofthewholeskyinthe9frequencychannelslistedin
Table1.Thetelescopeisanoff-axisaplanaticdesignwithadiameterof1.5m.Thespacecraftwillspinat 1rpmaround
∼
an axis offset by 85 fromthe telescope boresightso that the observedsky patch traces a large circle in the sky. The
◦
∼
FWHMbeamwidthateachoftheobservingfrequenciesisalsogiveninTable1. ThereceiversattheLFIfrequenciesare
basedonindiumphosphide(InP)highelectronmobilitytransistors(HEMTs)cooledto20Kbyaclosedcyclehydrogen
sorptioncooler. TheHFIdetectorsincludespider-webandpolarization-sensitivebolometersoperatingat0.1K.TheLFI
issensitivetolinearpolarizationinallchannelswhiletheHFIhas8polarizationsensitivedetectorsateachofits4lowest
frequencies.
ThesensitivityineachofthePlanckfrequencybandsisgivenintermsofthermsthermodynamicnoisetemperaturein
eachcorrespondingbeamareainTable1. Itisseenthatthehighesttemperaturesensitivitiesareatthelowerfrequencies.
TheLFIinstrumentwithitsarraysofcooledpseudo-correlationradiometersrepresentsanadvanceinthestateofthe
art(Bersanelli&Mandolesi2000;Mennellaetal.2003a)overthesensitivityofCOBE (Smootetal. 1990)andWMAP
(Bennettetal.2003). Itisparticularlydesignedtohavemaximumfreedomfromsystematicerrorsbothintotalpowerand
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Table1:OriginalspecificationofthereceiversandbolometersinPlanck.
Property Centrefrequency(GHz)
30 44 70 100 143 217 353 545 857
LFIaorHFIb LFI LFI LFI HFI HFI HFI HFI HFI HFI
Numberofhorns 2 3 6 8 12 12 12 4 4
Linearpolarization Yes Yes Yes Yes Yes Yes Yes No No
Resolution(arcmin) 33 24 14 10 7.1 5.0 5.0 5.0 5.0
∆T/T perpixel(StokesI)c 2.0 2.7 4.7 2.5 2.2 4.8 14.7 147 6,700
aLFIreceivesbothlinearpolarizationsandemploysindiumphosphide(InP)HEMTamplifers
bHFIunitsarespider-webandpolarization–sensitivebolometers
cGoalinµK/K1σforsquarepixelsofsidegiveninthe“resolution”row,achievableafter2fullskysurveys(14months)
Table2: Specificationofthe30and44GHzFrontEndModules(FEMs)asrequiredbythePlanckmission.
30GHzChannel(Kaband) 44GHzChannel(Qband)
Centrefrequency 30GHz 44GHz
NumberofFEMs 2 3
Bandwidth 6.0GHz(20%of30GHz) 8.8GHz(20%of44GHz)
Noisetemperatureoverband 8.6K,(6.1Kgoal) 14.1K,(10.4Kgoal)
Gain(bandaverage) 30dBmin/33dBmax 30dBmin/33dBmax
Gainvariationwithfrequency <5dBacrossband <5dBacrossband
Radiometricisolation 10%,(5%goal) 10%,(5%goal)
FEMgainvariationwithambienttemperature <0.05dB/K <0.05dB/K
FEMnoisevariationwithambienttemperature <0.8K/K <0.8K/K
FEMgainvariationwithtime <1dB/1000hrscoldrunning <1dB/1000hrscoldrunning
1/fkneefrequency <50mHz,(<20mHzgoal) <50mHz,(<20mHzgoal)
polarization.
Theoverallrationalefortheradiometerdesign,whichdiffersfromtheWMAPdesign(Jarosiketal.2003)isdescribed
in Bersanellietal. (2009). The LFI radiometerdesign is largelydrivenby the needto suppressthe fraction of 1/f-type
noiseinducedbygainandnoisetemperaturefluctuationsintheamplifierswhichwouldbeunacceptablyhighforasimple
totalpowersystem. Adifferentialpseudo-correlationschemeisadopted,inwhichsignalsfromtheskyandfromablack-
bodyreferenceloadarecombinedbyahybridcoupler,amplifiedintwoindependentamplifierchains,andseparatedout
byasecondhybridasshowninFigure1.
Eachfrontendmodule(FEM)islocatedinthefocalareaofthePlancktelescopewhereitisfedfromtwodual-profiled
feed horns,one directedat thesky andthe otheratthe 4-Kreferenceload locatedonthe surfaceof the highfrequency
instrument (HFI) unit. The FEM then receives the two input signals which pass through the first hybrid and are then
amplified. Afterappropriatephaseswitchingat 8kHzthesignalspassthroughthesecondhybrid.Thesignalsarethen
∼
sentvia 1.5mofwaveguidetothebackendmodule(BEM)atthe300Kstationofthespacecraftwheretheyarefurther
∼
amplifiedandthenwasmeasuredusingphasesensitivedetectors.TheBEMsat30and44GHzaredescribedindetailby
Artaletal.(2009, thisvolume),whilethe70GHzradiometersaredescribedbyVarisetal.(2009). Theoutputsignalis
thedifferencebetweentheskyandthereferenceloadwithgreatlyreduced1/fnoise. Thisinformationissenttothedata
acquisitionelectronics(DAE)moduleonthespacecraft.
ThespecificationoftheFEMparametersat30and44GHzissummarizedinTable2.
3 FEM Design
3.1 PhysicalLayout
TheoverallsensitivityofthePlanckLFI,ateachofitsmeasurementfrequencies,islargelydeterminedbythedesignof
itsFEMs. InordertomeettherequirementsofthePlanckprojectitwasnecessarytoachieveamplifiernoisetemperatures
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Figure 1: The basic block diagram of the radiometer chain. The signals from the horn and reference load are fed to
the hybridsvia the ortho-modetransducer(OMT).The dashed line surroundingthe hybrids, first amplifiers, and phase
switches delineates the front end module (FEM). The dashed line surrounding the second amplifiers, filters, detectors
and videoamplifiers definesthe back end module(BEM). The FEM and BEM are connectedby four1.5 m lengthsof
waveguide
lowerthanpreviouslyachievedwithmulti-stagetransistoramplifiers. Astheinstrumentisdesignedtomeasurethesky
temperaturein orthogonallinear polarisations, the inputportsof the FEM connectto the instrument’sfeed hornvia an
ortho-modetransducer(OMT).ReferringtotheblockdiagraminFigure1,eachFEMcomprisestwoinputhybridcouplers
(oneperpolarisation),fourlownoiseamplifiers(LNAs),fourphaseswitches,andtwooutputhybrids.
Toachievethelowestpossiblenoisetemperature,itwasnecessarytominimiseanylossinfrontoftheLNAsandthis,
togetherwithotherperformancecriteria,pointedtotheuseofawaveguidehybridcouplerattheinput. Thespecification
ofthewaveguidestoconnecttheFEMtotheBEMdictatedtheuseofanessentiallyidenticalcouplerfortheoutput.Itwas
decidedatanearlystagethatinordertofitalltheaboveelementsoftheFEMintotherestrictedvolumeavailablearound
the HFI instrument, a highlyintegrateddesignwould be required. This was notreadilycommensuratewith the mix of
transmissiontechniquesdesired,orwiththedesirabilityofbeingabletotesteachitemindividually. Amulti-split-block
solutionwasadopted,wherethefourLNA/phaseswitchmodulesweremountedtoendplates,sidebysidepairsofLNAs
beingarrangedaboveandbelow,inamirror-imageformataroundthecentreline. Theendplatesformedonewaveguide
faceofeachhybridcoupler,towhichablockcontainingthemachinedwaveguideswasattached. Theonlydifferencein
thedesignofinputandoutputhybridswasintherouteingoftheH-planewaveguidestotheexternalinterfaces. Figure2
showsanexplodedviewoftheFEMassembly.
Masswasamajorconsideration.Gold-platedaluminiumalloy(6082-T6grade)wasused,butpartthicknesses,mount-
ingscrewpositionsandtorquelevelshadalltobeconsideredtoavoiddistortionofthecriticalinterfaces,whereimperfec-
tionscouldresultinunwantedsignalleakage,crosstalkandloss. Toreducemass,unnecessarymetalwasmachinedaway,
leavinggoodcontactaroundwaveguidejoints,withsufficientstiffnessespeciallynearmountingpoints.
Themicrowaveabsorbingsurfacesofthe4KmatchedloadsaremountedaroundtheperipheryoftheHFIinstrument.
To accessthe two load hornwaveguidesconveniently,the H-planehybridoutputsterminateon the innersurface of the
FEMasassembledintheLFI.TheFEMmountingflangesweremachinedataprecisebeamanglefortheskyhorns.
Withupto20lownoisetransistorsand8phaseswitchdiodesperFEM,alloperatingat20K,severalrequirements,
some conflicting, had to be considered in the design of the bias circuitry. Since the expected in-flight cooling power
availabletotheentirePlanckfocalplaneunitisonly 1.1W,allpowerdissipationhadtobekepttotheabsoluteminimum.
∼
This set the requirement to use InP active devices operated in power starved mode. When testing the amplifiers it is
necessarytocontroleachindividualgateandthedrainvoltages. Whenthebestvalueswerefoundpotentiometerswere
usedtocommonmostofthecontrolwirestominimisethenumberofsupplylines. SincetheFEMsaredrivenby 1.5m
∼
ofwirelength,itwasnecessarytoterminateeachwireintheFEMwithelectricalprotectiondevices. Nuclearhardened
light-emittingdiodes(LEDs)werefoundtobesuitableandcanremoveanysharpspikeswhichareinducedontothese
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Figure2: ExplodeddiagramofFEMshowinginputandoutputdoublehybrids,4lownoiseamplifiersgroupedonepair
to each polarisation channel and associated side plates and divider plates. A single LNA is outlined in red, and the
polarizationpairsareoutlinedindashedlines.
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Figure3:MagicTHybridwaveguideasrealizedinPlanckFEMs
longlines.
ThemaximumallowabledissipationperFEMwasdictatedbythecoolingpowerofthePlanck20Ksorptioncooler
(Morgante&etal 2009) and the thermallosses down the FEM/BEM connectingwaveguidesand wiring. Such critical
limitationsweremetwithdrainvoltagesaslowas0.6V.Adedicatedbiastestpowersupplywithahighlevelofprotection,
capableofsupplyingupto5drain,5gateand2phaseswitchvoltagestoeachoffourLNAswasdesignedandbuilt. Both
gateandphaseswitchsupplieswereadjustableforeitherpolarity.
Assemblyandtesting(outsidethecryostat)wasdoneundercleanroomconditionsusingpre-cleanedparts. Inorder
toavoidanyleakagebetweentheradiometers,itwasnecessarytoassemblethedifferentpartsofthemoduleswithgreat
careastoalignmentofallfacesandflanges.
3.2 Hybrids
Theinputandoutputhybridcouplersutilisetheconventional“magictee”configurationanddifferonlyinthewaythatthe
inputandoutputwaveguidesareconfiguredin orderto interfacewith otherpartsof theradiometer(Figure3). Ateach
junctionisamatchingprobecentredbetweenthebalancedarms,butoffsetawayfromtheH-planewaveguideinterface.
Ansoft’s High Frequency Structure Simulator (HFSS) electromagnetic modelling software was used to determine the
dimensionsandoffsetofthe probeforoptimumhybridperformance. Inallcases, theperformanceswerecheckedwith
modeljigs. Thehybrids,beinganintegralpartoftheFEMbody,wererealisedusingsplit-blockconstruction,machined
fromaluminiumalloythengold-plated. InoperationtheFEMiscooledto20K.Thecombinedeffectofthewaveguide
design, and the low temperatureand consequenthigh conductivityof the gold, meansthat the inputhybridscontribute
negligible noise to the system. A typical plot measured at room temperature is shown in Figure 4. The insertion loss
abovethenominal3dBsplitwaslessthan0.1dB.
Thebalancedarmsofthehybridareterminatedinwaveguide-to-coaxialtransitionswheretheshort-circuitedendof
eachtransitionformspartofthehybrid,whiletheprobeformspartoftheLNA.ThisisshownintheFEMexplodedview,
Figure2. Aswiththehybrids,performanceofthetransitionswasoptimisedusingbothHFSSmodellingandtestmodel
measurements.Theamplifiersweretestedwithjigstoconvertfromstriplinetowaveguide.Theseconsistofashortpiece
ofwaveguidewithabackshortintowhichtheLNApinssit. Thelossofthejigsystemcomparedtothehybridconnection
whentheLNAswereintheFEMcontributedlessthan1K,whichwasourmeasurementerror. Thustheextralossofthe
hybridsmustbelessthan0.2dB.ThisresultwasextremelyimportantfortheperformanceofthewholeFEMasthefirst
hybridcomesbeforetheLNAs.
3.3 Low NoiseAmplifiers
3.3.1 LNAdesign
Each of the four outputs from the input hybridscouples via a tuned probe and short length of coaxial line to an LNA
constructedusingmicrowaveintegratedcircuit(MIC)techniques. EachLNAoutputconnectstotheoutputhybridviaa
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Figure4: 44GHz measuredhybridS-parameters. Allunwantedcross-talkparametersare lessthan15dB. Thewanted
signalsegS21areimperceptiblydifferentfromtheexpected3dBvalue.
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Figure 5: 30 GHz FM LNA and phaseswitch. The fourtransistorscan barelybe seen in the RFchannelrunningfrom
fromrighttoleft. Thephaseswitchisseenasablackrectangleontheleft.Thebiasvoltagestothetransistorsandcontrol
voltagestothephaseswitchescanseenfromtoptobottomofthefigure.
phase-switch.Figure5showsthe30GHzLNAandphaseswitchintheFEMRFsection.
ThedesignsoftheKa-bandandQ-bandHEMT-basedamplifiersdrewsignificantlyontheworkofPospieszalskietal.
(2000). A30GHzNRAOdesignhadbeenfurtherdevelopedoveranumberofyearsforuseatcryogenictemperaturesin
theJodrellBankObservatory(JBO)VerySmallArray(VSA)receiversoperatingonTenerife.Thisamplifierwasusedas
thestartingpointforthePlanck30-GHzLNA.ThecryogenichardeningwascriticalforthePlanckamplifiers,aswasthe
abilitytosurvivethevibrationduetothelaunch. DuetothesmallnessandlightnessoftheRFcomponents,g-forcesare
negligibleatthefrequenciesgeneratedduringtheArianelaunch,andthiswasnotinpracticeanissue(seeSection4.2).
3.3.2 HEMTandMICdesign
IndiumphosphideHEMTsmanufacturedbyTRW(nowNGST)areusedastheactiveelements,astheyprovideextremely
lownoiseperformancewithminimalpowerdissipation(aslowas5mWperLNAat30GHz),whichisvitalforthisspace-
borneprojectwithlimitedcoolingcapacity. EachLNAprovidesaminimumof30dBofgain,andthepairsofLNAsin
eachchannelarematchedinbothgainandphaseoverthe20%bandwidth. Inviewoftheneedforthelowestachievable
noisetemperature,thefinal30-GHzdesignuses80µmgatewidth,0.1µmgatelengthInPHEMTsprovidedtoJBOby
JPLPasadenafromtheCryogenicHEMTOptimisationProgram(CHOP)CRYO3waferrunforthefirststageandother
80 µm gatewidthdevicesfortheremainingstages. The44-GHzdesignuses60µm gatewidthHEMTsfromthe same
CRYO3waferforthefirststageand80µmCRYO3fortheremainingstages.
The CRYO3 wafers were found to be absolutely critical for this project. Not only did they give the lowest noise,
butalso theparticularwafer hadan unusuallythinpassivationlayerwhichprovedessentialforthe highestfrequencies.
At thetime of the PlanckconstructionthisparticularCRYO3 waferwas theonlyonein existencewith these particular
properties.
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Figure6:44GHz(Q-band)LNA,RFside.
3.3.3 LNAconstruction
Figure6showstheupper(RF)sideofaQ-band(44GHz)LNA/phaseswitchassembly,wheretheprobesformingapart
ofeachtransitiontothesymmetricalwaveguidearmsofthehybridcanbeseenprojectingfromeachend.TheRFinputis
ontheleft-handside,withthephaseswitchmountedonacarriertowardstheright-handsideofthechassis. Theremainder
oftheRFsideoftheLNA,includingtheintegralphaseswitch,isrealisedinMICmicrostripformatusingapurePTFE
substrate(CuFlon)forlowestloss.
The EM fields are containedas far as possible within a narrow channelto minimise the productionof higher-order
waveguidemodes.TheRFcircuitboardsarejustvisibleinFigure6asthingreylineswithgoldtracksdowntheircentres.
Effectivegroundingoftheseboardswasfoundtobeveryimportantandsilverconductivegluewasusedintheareabeneath
eachboard.
Some componentsof the biascircuitsare sited on the RF side. To minimisethe complexityofsupply wiring, each
amplifierwassuppliedwithasingledrain,twogateandtwophaseswitchvoltages. Oneofthegateinputssuppliedthe
critical first stage while the other inputwas commonto the remainingstages. To ensure that each HEMT received the
correctgatebiasasdeterminedbythecoldtests, apotentiometer,madeupofresistorarraychips,wasusedtodropthe
commonvoltagetothatrequired. Thesechipswereby-passedthroughoutthetestingphase,andonlybondedintocircuit
afterthefinalcoldtestdescribedabove.
Theremainderof theDCsection, comprisingpotentiometernetworksforthegatesupplies, a distributionPCB, and
protectiveLED diodes, ismountedin a compartmentbelowthe RF section, with feed-throughpinssupplyingpowerto
theladdernetworksabove. FromtheDCinterfacewhichisaNanonicsconnector,finePTFE-coveredwiresfeedsupply
voltagestothedistributionPCB.Thesearescreenedwithbraidinthe30GHzFEMandshieldedbyacoverplateinthe
44GHzFEM.
TwosetsofamplifierswerebuiltforeachFEM,onesetwiredfromtheoppositesidetotheother. Totherightsideof
Figure7,theLNAbiascircuitrycanbeseen,andtotheleft,thephaseswitchbias.
Duringtransitandhandling,protectivecoverswerefittedatalltimestopreventstaticormechanicaldamage.
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Figure7:Q-bandLNADCside
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