Wideband Monolithic Microwave InConverters with GaAs mHE

Viktor Krozer, Tom K. Johansen, Torsten DTechnical University of Denmark, Oersted•DTU, Dep

Oersteds Plads 348, 2800 Kgs. Lyngby, Denmark, Phone:+

Abstract— We present monolithic microwave inte-grated circuit (MMIC) frequency converter, whichcan be used for up and down conversion, due tothe large RF and IF port bandwidth. The MMICconverters are based on commercially available GaAsmHEMT technology and are comprised of a Gilbertmixer cell core, baluns and combiners. Single endedand balanced configurations DC and AC coupled havebeen investigated. The instantaneous 3 dB bandwidthat both the RF and the IF port of the frequencyconverters is ∼ 20 GHz with excellent amplitudeand phase linearity. The predicted conversion gainis around 10 dB. Simulated results are supportedby experimental characterization. Good agreement isfound between simulations and experiment is foundafter adjustment of technology parameters.

I. INTRODUCTION

Ultra-wideband MMICs based on mHEMT tech-nology operating over an instantaneous bandwidthof more 20 GHz are discussed in this paper. Theavailability of such MMICs reduces front-end com-plexity and enables reuse of components for differ-ent applications in receivers for radar, radiometerand communication applications. One of the keyelements in heterodyne receivers is the frequencyconverter circuit. Double balanced structures basedon Gilbert cell mixers have been used earlier andwideband operation has been demonstrated by theauthors in SiGe and GaAs technology [1], [2]. Sev-eral highly integrated active mixers based on FETtechnologies have previously been reported [3], [4].These active mixers have mostly been optimized fordown-conversion performance, whereas the activemixers demonstrated here can be used for both up-and down-conversion.

This paper focuses on fully integrated DC andAC coupled frequency converters comprised of aGilbert cell active mixer, balun circuits, balancedbuffer amplifiers, and combiners. Aspects of mixerdesign, such as balance considerations have beendiscussed in [1], [5]. In this paper we will discusstransistor scaling issues and stability problems. Sta-bility becomes especially important when moving tofaster technologies such as e.g. the GaAs mHEMT

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jurhuus, and Jens Vidkjærartment of Electromagnetic Systems,45-45253769, E-mail:vk@oersted.dtu.dk

H process from OMMIC employed here. Wempare the predicted performance versus ex-ntal results.

II. DC COUPLED MMIC FREQUENCY

CONVERTERS

aim of the DC coupled frequency converterdemonstrate ultra-compact wideband MMICon reaching the mm-wave range with com-

performance. In contrast to other circuits,out utilizes efficiently the wafer space and is

ompact. Fig. 1 shows the layout of the circuitng a Gilbert cell active mixer, two balunsLO and RF port, respectively, and a buffer

er with source followers and level shifters.ges are DC coupled and the DC level isd for optimum operation. The circuit is fullyntial on input and output. The input matchingeved by additional 50 Ω resistance togethershort transmission line and buffer amplifiers.

RF

RF

IF IF

LO

LO

Layout of an ultra-compact frequency converter. The× 2 mm.

The basic circuit for the fully differential fre-quency mixer is shown in fig.2 together with thebuffer circuitry. The RF and LO ports are bufferedand can be accessed either differentially or singleended. Especially for the LO port this is conve-nient if a quadrature oscillator is employed withdifferential output, which is frequently the case inintegrated down/up converters. The output IF portis also differential and is buffered through a bufferamplifier. The buffer amplifier is illustrated in fig 3.

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Fig. 2. Simplified circuit of the mixer.

A similar circuit has been used for the baluns, whichare integrated on another chip and which occupyroughly quarter of the space of the mixer.

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Fig. 3. Simplified circuit of the buffer amplifier integrated withthe mixer.

The predicted conversion gain performance versus

frequethe insports amm-wbandwseen tThis rIts origoutputThe liElimin

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ncy can be depicted from fig. 4 together withertion loss at the RF port. The LO and RFre almost identical and well matched up to

ave frequencies. Simulations predict a 0.2 dBidth of 12 GHz. However, it can be clearlyhat a resonance exist at around 37 GHz.esonance gives rise to instability problems.in is an interconnect line between the mixerand the source followers in the buffer circuit.

ne length is close to λ/4 at this frequency.ating the contribution of the line by reducing

Simulated conversion gain and insertion loss versusy for the frequency converter in fig.1.

e physical length or change in transmissionpedance, the circuit is absolutely stable overole frequency range.simulated resonance can also be detected in

rements, however due to the losses and a slightthe processing parameters the resonance doese rise to oscillations in measurements.surements have been performed with regardsuency conversion parameters. The RF portsion bandwidth was measured by sweepinge RF and LO signals at a constant IF fre-of 1.2 GHz. The IF bandwidth has been

red by keeping the LO signal at a constantncy and changing the RF signal frequency.sults shown in fig.5 have been corrected forble losses in the measurement system. Bothexhibit a relatively flat characteristic as a

function of frequency. Again the circuit works non-optimally due to non-optimum performance of thecurrent sources. Nevertheless, the operating 3 dBbandwidth for both ports is close to 15 GHz even inthis non-optimum case.

Fig. 5. The measured RF and IF bandwidth versus frequencyfor the frequency converter in fig.1.

One of the major issues in achieving the goodresults is the proper scaling of the transistors for thetransconductance and the switching stage. This pointwill be discussed in detail in the full paper.

III. AC COUPLED MMIC FREQUENCY

CONVERTERS

The AC coupled MMIC frequency converter isa modified version of the circuit presented in [1].It utilizes active baluns based on a common-gatecommon-source circuit. Input matching in the circuitis achieved with the common-gate FET of the activebalun [6]. The GaAs MMIC exhibit slightly widerbandwidth as compared to the SiGe HBT technol-ogy. They also exhibit a larger dynamic range. Theoutput of the mixer is DC coupled and inductivepeaking is employed for bandwidth enhancement,which is important due to the capacitive loading

of thedifferelevel s

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mixer output. The combiner is a modifiedntial amplifier with wideband performance,hifter, and source follower stages.conversion gain versus frequency is shown6. The predicted conversion gain is aroundand is expected to be flat up to frequenciesGHz. The measured values are significantlynt. First, the measured conversion gain wasined to be around -3 dB. Secondly, a strongncy dependence is visible. Detailed investiga-vealed that the common gate mHEMT of theexhibited a problem. Taking this into accountcluding a threshold voltage shift from theal VT = −1.0V to VT = −1.15V gavent agreement between measured and simu-alues. This is verified by the DC bias currentteristic versus the supply voltage as indicated

4 6 8 10 12 14 16 18

RF Frequency Response

RF Frequency [GHz]

MeasuredSimulated

Conversion gain versus frequency for the MMICy converter.

Conversion gain versus frequency for the MMICy converter.

nteresting feature of this circuit is among oth-

ers that the noise properties of the converter can beoptimized by appropriate choice of the common-gatetransistor periphery and bias conditions. Therefore,low-noise operation is feasible with such a circuit.However, the linearity is still a concern and will befurther investigated in the future.

IV. CONCLUSIONS

MMIC circuits with ultra-wideband operationhave been designed and fabricated. Results fordown/up-converters show a flat conversion gain witha 3 dB bandwidth in excess of 20 GHz. Theconverter circuits exhibit similar bandwidth at allports, including the IF port. Therefore, one canemploy these circuits as up/down converters withminimum redesign. Very compact circuits have beensuccessfully characterized. Good agreement betweensimulation and measurement is obtained if the pa-rameters are adjusted for technological spread. Prob-lems with some transistors have been observed inthe design, as well as stability problems, due to thelarge mismatch between the mixer output stage andthe source followers. The origin of the instability hasbeen identified and potential stability problems canbe alleviated by reducing the interconnect line lengthbetween the output of the mixer and the sourcefollowers of the buffer amplifier.

The presented circuits demonstrate the feasibilityof a compact fully integrated receiver front-end inGaAs technology spanning the frequency range fromDC to Ka band. The interesting aspect of this workthat the same chip can be used for up-conversion aswell as down-conversion.

REFERENCES

[1] T. K. Johansen, J. Vidkjær, and V. Krozer, “A HighlyIntegrated GaAs pHEMT Active Mixer for Wideband SARSystems,” in Proc. 2004 Gallium Arsenide and CompoundSemiconductor Conf., Amsterdam, The Netherland, 2004,accepted.

[2] V. Krozer, T. K. Johansen, T. Djurhuus, and J. Vidkjær,“Ultra-Wideband Monolithic Microwave Integrated CircuitSolutions in SiGe and GaAs Technology,” in Proc. Euro-pean Microwave Conf., Amsterdam, The Netherland, 2004,accepted.

[3] C.F. Campbell, “”A Wideband pHEMT DownconverterMMIC for Satellite Communication Systems”,” in IEEEMTT-S Digest, 1998, pp. 55–58.

[4] C.F. Campbell and J.M. Beall, “”Design and Performanceof a Highly Integrated Wideband Active DownconverterMMIC”,” in IEEE Radio Frequency Integrated CircuitsSymp., 2001.

[5] T. K. Johansen, V. Krozer, T. Djurhuus, and J. Vidkjær,“Monolithic Microwave Integrated Circuit Solutions forSynthetic Aperture Radar Applications,” in Proc. Mi-crowave Technology and Techniques Workshop: Preparingfor Future Space Systems, ESA/ESTEC, Noordwijk, TheNetherland, 11-12 May 2004.

[6] T. KMMrop200

. Johansen, J. Vidkjær, and V. Krozer, “Ultra-widebandICs for remote sensing applications,” in Proc. 2003 Eu-

ean Microwave Conference, Munchen, Germany, Oct.,3.