CPW X-Band Dielectric Resonator Stabilised Oscillator

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    CPW X-BAND DIELECTRIC RESONATORSTABILISED OSCLLATORChangKahHyong

    Microwave Laboratory,NationalUniversityof Singaporel0 Kent RidgeDrive,Singapore511

    Abstract - An X-band tuneable GaAs FET dielectricresonator stabilisedoscillator has beenconstructed. Thecircuit isfabicated usingcoplanar waveguide CPr\ MICtechnologt,with a designed rocedure hat takesadvantageof the acilities availablefrom the uy of HP85I}C vectornetwork analyser, and a microwaveCADprogram, EEsofhlllbla,lws streamliningand simplifyingthe task The DROhas a SSBnoise is -96 dBc/ IIz ot 100 kl{z of carrierfrequenqt. Theavailablepwer is an averageof +13 dBm,with a corwersioneficiency of 12.6%. A mechanicallytuneablerequency mge of 124MHz with an outputpowervariation ofless than 2 dBhas beenachievedINTRODUCTION

    Since the advent of solid-state technology, oscillatorsconstructedusing mioostrf techniqueshave continuouslybeenreported Recently,with ttrepushto high frequenciesand monolithic technology,coplarur waveguides CPWs)have experienced growing interestdue to their 4pealingpropertiesand advantages ver miqosfips [1], such asno-via holes neede4 easyshunt and series connections,andeasy adaptation and corryatibility with the MMICtechnology.Unfortunately,up to now the commercialCAD design oolsdo not contain enoughaccuratemodels for line elements,components nd discontinuities n coplanar echnique andthereforecfucuitdesip is very difFsull in this technology.In thefuture, with moreCPWmodelsavailable n CADsoftware, a large historical microsfrip des4n database anbe t4ped. CPW will prove a very successfirl echnologyindee4 both n tennsof perfonnanceand cost

    OSCILLATORMODEL

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    OrclllrtorModrl

    LINEAR CAD DESIGNPROCEDUREThe block schematic of a rdlection stabilised oscillatorwith feedback is shown in Figure l. Due to theunavailabilityof the arge signalSaarametenof the activeelement" he tansistor modelused s thesmall signalmodelof theFET flr0l61lhbyFujitsu)ntheformofanSjarameter data file. Series feedback is added by areactancen the common source)ead. This is provided nthis caseby the open-end-gap PWstub.

    StepOneObtain he one-portS-parameters f theresonatorrom theHP85I0C AutomaticVectorNetworkAnalyser,orfrequencies rom 0.045GHz to 20 GHz.This setof S-parameters, hich givesarelatively highloadedQ-factoranda reasonrblyhigb reflectioncofficient(seeGraph ) atthe rcsorumcerequency 9.75GHz), ssavednto a data ile in Touchstoneru onnat for use n theCAD sirmlation.

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    Graph 1 Mognitude Plot (in dB) of the Reflectionoefficientsof the DR Coupled to the CPWLine for the Setof $Parameters ChosenStep TwoWith the resonator incorporatd the series feedback stubsare then optimised for maximum negative resistance,R6.

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    Theoptimisationprocsss carriedout in three stages.Thefirst stagenvolves carryingout a parameter weep or thelength of the series eedback stub at f*", and finding thestub length that givesthe maximum value for the negativeresistance,Rr".The secondstage involves the use of the optimisationfacility in Libra to achiwe the goal morc accurately.The'tune' facility is alsoused o see he effects of varying thestub ength on thenegative esistance alue.

    Graph2 Graphof Resistancc61 gainstFrequencyfor VaryingSeries eedbacktubLengthFinally, the resistrnce, Rr* , looking into the FET withseries eedback and with the DR at the input port of theFET with feedback) tom the ouQut port of the FET, isploted against frequency, frrom 0.45GHz to 20Glfz, asshown n Graph2, to see f thenegative esistances indeedpresntat f*, and whether he stub ength s the optimisedvalue. The result is positive, with a maximum negativeresistancef -17.680 at 9.75GHa which s the resonancefrequency of the DR configuration chosen earlier. Theoptimised stub length for eadr of the source erminals isfound o be 136mils.

    StepThreeThe irput stability circles are plotted for a range offrequency in the vicinity of L' This is to ensure hatinstability canoccur at the irput port in the frequency angeof interest 9 GHz to I I GHz. The resultsare plotted intogether with the trace of f., the reflection coefficientlooking towards heDR from the FET irydportf, must lie in the potentially rmstableareason the SmithChart o obtainnegative esistance t the nputport.

    StepFourTheouput matchingcircuit is designedo transformRL the50Q()oad mpedanceo Zsp, zuch hatRop+j Xop: -RfJ3 -j Xf.r (1)After start-rp, the negative resistanceRi* wiil decrease.Equation (1) gives lllaximum power transf,er o the loadassuming lnt fte mapitude of the negative conductancedecreasesinearly with increasing arylitude [21. Thisapproximation has been found to give good results inpractice.StepFiveThe ouput-rnatching shunt stub is combinedwith the dcbiasing network in order to reduce the nurnber ofdiscontinuities along the output transmission line. Theoutput stability circles are plotted (seeGraph 3), and theoutput impedance,Zopmust lie in the potentially unstableregimes on the Smith chart within the frequency ange ofinterest.

    Graph 3 Output Stability Circlesand Impedanccs orFrequencies etween9.7 GEz- 10GEz

    Step SixThe oscillator stability is testedat frequenciescloseto {o"[3] by displaying 1/T;" andf, on a Smithchart. As libra isunableto plot l/T;, the results are presentedusingMDS(seeGraph4).

    ";:':;-- -i :::: ' ::.:: '- _ r -::..._l:

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    r l ' a t r ( - l . l l l l t - t l _ l 9 l ? - l 6 E _ l l )I 1 . t . i 0 t f r !r r . l

    r ! - Z l r l 5 6 ? . ! ! t - l l t ll . T S a t - r l iU - ! . ? ! 0 t+ 0 !I ? - a

    L r t r l . l l . . 5 l l , l tI ' . . . { . 1 / l . f . t . . S t I , l )

    Graph 4 Plots of 1/f6 and ,

    Graph4 shows hat he 1/T-(f; and ,(f) loops ntersecteachother, and heir changeswith frequencypoint inoppositedirections. This will ensure tability at only onefrequency.Step&ven (opfional)If anon-linearmodelhasbeenderived, hen he oscilliatorcanbe simulatedusing a non-linearenalysisprogram. Thischecks tequenryand ouQutpower. It is alsopossible ouse suchaprogram o plot thepredicted1/f;,,dependencyrpon anplitude and denti$ the angleof intersectionbetween /Ii" (v,Dand , (f) on the Smith.trrrt a1rcangleshouldbeas close o 90"aspossibleo minimise AM to PMnoiseconversion3].

    PRACTICAL CONSIDERATIONS

    Fig, 2 Mark Layout

    Thecircuit was abricated nRT/Duroid6010.2 seeFig.2),with a dielectric constant of 10.2.and a thickness of 25mils. A photographof the assembled scillator is shown nFigure3.

    Fig 3 f inal Cirruit in Metal Eousing(fop lid notshown)Thegroundplane s purposelydesigred o be ratherwide toavoid discnepanciesrom the assunption made, i.e., thegroundplane s infinite. As this is only a prototype,andnotfor commercialmassproduction" he size of the circuit is ofsecondaryuryortance.The conponentswere ater solderedonto the zubstrate, ndthe whole circuit was placed in a metal housingspeciallydesignedo suit a CPW circuit The Metal housing shieldsthe circuit frromoutside nterfernce,and reduces adiationloss. The top and botom covers were designedo be lowenough suchthat no wavegride mode of resonance ouldoccur n either he upprpart or the lowerpartof the casingbut not too low lest the cbaractristic mpedanceof thewhole circuit would be atrecied 4].A thin film microwaveresistor was usedto tenninate the50Cf ine at the gate npul This line was corpled to a DR,a high Q material made by Thompso4 with unlnowndielectric constantand teryerahue coeffrcienl Apart fromthe GaAs FET, flr016ft and the 50O thin film microwaveresistor, only two other discrtecorponeils wereused anormal chip resistor or self{iasing and an ATC capacitorfor dc blocking. All the cornponents sedwere solderedonto the zubstrate.The ATC capacitorand he chip resistorhavebeenassumedideal n the simulationexercise. But at l0 GHz, they c:tnnolonger appeardeal. In fact the measued reactance f theATC capacitor s induclive at frequencieshigher than 330MHz. It can only behoped hat the inductance alue s stilllow at l0 GHz, or else t will serve as a high impedancechoke o the microwavesignal.To testthe completecircuiq theornput of the oscillatorwasconnectedto a vector spectrum analyser to check foroscillationq and spurious oscillations were observedataround 3 GHz. One interestingobservation was made.

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    When the bonding wires were all removed" spuriousoscillation ccurred t3.5GHz. ThisThis phenomenondisappearedwhen bonding wires weresoldered around the T-junctions. near the two sourceterrrinals, gateterminal, and the drain terminal across hetransistor leads, however, no oscillation was observed.Then theresistorwasmovedalong he line andthepositionof the DR was also con$antly adjusted. Oscillationoccured when he resistorwas at the end of the CPWseriesfeedbackstrb. The resistorwas modelledas deal rn Libra,but in actual fact its high frequenry behaviour could becryacitive in nahre, especiallyby virtue of the fact that it isa normal chipresi$or.After some rial and errors egarding hepositionof theDR,the optimal oscillation was obsewedat frequenciesowerthan l0 GHa with or.trput ower greater han 12dBm, andwith no spurious. Howwer, when the circuit was testedagain the next day, the power became oo low (less than0dBm) for the oscillator o be usefirl, md the absentof thecasing id would make he oscillation disrypeared-Later itwas discovered that one of the bonding wires wasdislocated rom the ground plane. When all the bondingwires had beenreplaced,andthe circuit board surfacewascleanedusing acetone, he frequenryof oscillationbecamehigher than l0 GHz This shows how important thebonding wires are for a CPW circui! wi&out them thecircuit could betotally different from the ntendeddesign.Thepositionof the DR that gave he highestouQut power,optimal tuningrangeandhigh frequencystability wasftxed(seeGraph 5 for the output of the oscillator), and furtheranalysiscouldthen be carriedout to assessts performance,which is discussed hortlv.

    A - r " t F - N e o d uH - 1 C O l F l r n

    M K n 8 . 5 0 d E l m1 0 d t s . , ' 1 | f r - 6 2 1 3 e G H z

    C E t l T r j F t O . 1 e a 1 - - J R G r r r 6 P A f l S . O O O M H zf a B w i o k , l - \ - / f r h ' 3 o r . r l z F h ' F 5 ( ) . O m s

    Graph 5 Output FrequencyasDisplayedby Spectrumof the Oscillator

    OSCILLATIONFREQUENCYAND OUTPUTPOWERNo spuriousoscillationhas been observed.A tuning rangeof l2l MHz (10.0728GLlz- 10.1963GHz) s achievedwith ouput powervariationsof less han 2 dB. which isaround l.2oh of the power correspondingo the hmingmnge. A tuning rangeof 25'7 vftlz (10.0728GHz-10.33GHz) is achievedwith an outptilpowervariationof 3 dB.The intended requencyof oscillation s around10GHz,brfthe result obtainedaboveshows hat the frequencies re allhighs than l0 GHz for the position of the DR chosenandfixed. It is observedhat for at this positionof the DR thepower output and the frequenry stability of the circuit astimepassess optimal. It is possibleo locatea position orthe DR where the oscillation is tuneable to aroundl0.00cllz, bu the power outpts will not be ashigh as thecurrent configuratioq and the SSB phase noise of thecircuit is not quiteappealing seeGraph6).

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    M K F ? . 6 7 c J B TI . 9 9 5 ) a a r : l t z

    ? . 6-Z'r ' i 9l -

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    :rnd the small-signal S-parametersused in the desigt stageof the oscillator.

    SINGLESIDEBANDNOISEOne of the most hportant performancecriteria used ingading an oscillator is perh4s the single sidebandphasenoise. Themethodusedhere s by no meflrs accuate,sinceaccuratemeaswement f phasenoise requires complicatedequipment The synthesisedspectrumanalyser is themethodused n thisproject.The carrier-to-noisesideband evel is measureddirectlyusing he nstnrment'sdifferential cursors seeGraph7).A f T = N l 2 0 c t 8H r 1 0 o d B m

    C E N - : E t r l L O , e . ? 7 A . ! l ; C . t . , S P A I { 1 , O O O M H Zf t B d ! \ ) k t - l ) z x v n ! S O < ) | l : S W t r B 4 O m 3

    Graph 7 Display of the Vector Spectrum Analyser Used nMeasuringSSBPhaseNoise

    The result is then converted from its measured bandwidth toa I Hz bandwidth. The results are plotted in Graph 8.

    Graph 8. Single Sideband Phase Noise for fo$ l0-2 GHzThe oscillatorhasa singesideband oiseof -96dBcat 100kHz from the carrier. This figure s not a goodone, and canstill be nprovedfurtherby lightly coupling he DRO to theCPW,but the ouQutpowerwill be compromised.

    TI{E HARMONICS CONTENTThe TDRO has only an unwanted harmonics at20.2 GHz.Its frst harmonic level is low. -39.5 dBc (-(9.l7dB +30.33d8c)) (SeeGraph9).

    A T T E N A O d B M K F _ 3 O . 3 3 O B MH L 1 0 o d B m r o d g / 2 0 1 3 G H z

    C E N T E F I Q . ! 7 E H Z S P A N A O , 5 4 G H Za B w 1 . o M H z V B W 1 . O M H z 6 W P a a o m g

    Graph 9. Harmonics Contentof the TDRO Output

    DEPENDENCEOF EFFICIENCYON DRAINVOLTAGEPlots of Output Pffi and Efflclnca ys Vd

    Graph 0 Efficiencyf heTDRO sDrainSupply oltageThe maximumefficiency =P*/(Ia.VJ x 100%) occursatVa= 5 V, whichcorrespondso a 12.60/"fficienry.

    FREQI"'ENCYSTABILMY WITH TEMPERATUREThe TDRO was subjectedo an ambienceemperaturerom25'c to 70 c.The result shows hat the GaAs FET TDRO has a +20ppm/"C frequency tability.which s notconparableo the

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    nonnalfigrne for aFET, whichusuallyhas ess han5 ppm/"C of tenperaturestability.The power ouQul on the otherhand, s lessdependent ntemperahre,havingavariation of only I to 2 dB when hetemperature ncreases hroughout the whole range (seeGr4h ll).

    Plot!of Fpquocy atrd Ou$ut P(m.s Anbbil Tmp.ntuE

    oscillation s obsenred ver the rangeof 0.045GHz - 50GHz.The present coupling condition gives the optimumfrequency stability over time. widest tuneable frequenryrangeandoptimal outputpower. ts SSBnoise s -96 dBc/Hz at 100 kHz off carrier ftequency. The availablepoweris an average f +13 dBm with a corn'ersion fficiencyoft2.60

    ACKNOWLEGEMENTAcknowlegments made o theDept.of ElectricalEngineering or ftrnding hisproject.Special hanks o [tof.P.S.Kooi andProf.Q.J.Xr! for their nsights ndguidance.

    REFERENCESGUPTA K. C., GARG.R.. and BAHL. "Microstriplines and slot-lines."Artech House, nc, Norwood,A/L4,1990.G. Gonzale4"Microwave TransistorAmplifiers."Prentice all.p196,1984.BOWLES, J.W, *The Oscillator as a ReflectionAmplifier: an intuitive approach to oscillatorDesign,"vfiuowave ournal, p.83-98. une1986.M.S.Leong, .S.Kooi, .L.Satya rakas[ Effectofa Conducting Enclosure on the CharacteristicImpedanceof Coplanar Waveguides",LrtcrowtweJournal,Aug. 1986.Darko Kajfez and Pierre GuillorL DielectricResonators, rtechHouse,p369,1986.

    C. Tsironis and V.Pauker, "Temperaturestabilizationof GaAs MESFET oscillators usingdielectric resonators." IEEE Trans. lvficrowaveTheory Tech., vol. MTT-31. pp.3t2-31.1.Marchr983.

    10.'r6400010.1ffiOO10.16000010.15800010.15600010 1540fl)10.15200010.15000010.'148000

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    25 36 40 45 50 55 60 65 69AmbieotTmFntuF, deg C

    Graph ll Frequency Stability and Power Output of theTDRO for Temperature Variations between 25deg. Celcius to 70 deg. CelciusNwertheless, his poor frequencystability due o variationsin tempemtures expected s t hasnwer been cateredo inthis projec! due o the unavailability of the DR temperaturesensitivityfigure and dfficulties in measuring he phaseofthe eflection oefficient f theactivedevice 5]. Themetalhousingwasonly fabricated owards he end of theproject,but we need hemetalhousing o accunilelycharacteriseheloadedQ faclorof theDR.It has beennoted from the analysisgrvenby 16l thiat, Qmay be selected by adjusting the coupling coefficientbetweenthe DR and the CPW line at will, and that acertainvalue of Q, will makethe tefiperature stability ofthe circuit as small asnegligible.Since t is only possible o fine-hrne emprature ensitivityof the circuit by changing he position of the DR so a tovary q, if we want to obtain a temperature-stableoperation, t will require hegluedDR beremoved rom thesubsfate from time to time. Howwer, this has not beencanied out due o time constraints.

    CONCLUSIONA reflection-tne dielectric resonatoroscillator designedbasedon the CPW MIC technology hasbeen zuccessfrrllyfabricated and tested. It has a constant output power of12.50dBm - 14.3dBm,and a mechanicaluning rangeof10.0728GHz - 10.1963GHz. Only one harmonicsoccurringat 20.1 GHz. at -39.5 dBc. No spurious

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