High-Performance V -Band Cascode HEMT Mixer

High-Performance V-Band Cascode HEMT Mixer and Downconverter ModuleJunghyun Kim,Student Member,IEEE,Moon-Suk Jeon,Dongki Kim,Jinho Jeong,Student Member,IEEE,andYoungwoo Kwon,Member,IEEEAbstract—A high-performanceFig.1.Equivalent=circuit schematic of a cascodemixer.Fig.2.Simulated S 11and S 21of the interstage matching network of mixerA.which case the higher order mixing products from the CG-FET complicates overall IMD characteristics.Generally,the lower CS-FET is connected directly to the upper CG-FET without any interstage circuits.Thus,various harmonics and spurious signals generated by the lower CS-FET are fed to the upper CG-FET without any filtering.As has been theoretically demonstrated in our previous work [1],these harmonics and spurious signals can degrade the conversion gain as well as the linearity.In this work,we have employed an interstage circuit between the two tran-sistors(-parameters of the interstage matching network havebeen simulated and are shown in Fig.2.The interstage net-work passes the IF frequency components while blocking the harmonics of RF and LO.For the LO signals,it also works as a reactive matching circuit,providing proper match to reduce reflections.Higher conversion gain with a lower LO power re-quirement is thus expected.This approach is applicable to the frequency ranges where one can easily realize a low-pass inter-stage network with proper rejection to theharmonics.Fig.3.I –V characteristics of a cascode-connected FET at a fixed V of 00.4V .Depending on V ,two different operating regions of interest can be found,as denoted with circles.In addition to the introduction of interstage matching circuits,bias optimization was performed to enhance the gain and/or lin-earity.The gate bias to the lower FET(biased for maximum transconductance(in Fig.1)determines the operating point onthe isexpected to have strong effects on the transconductance,output conductance,and their derivatives.As presented in [1]and [2],aV)results in a high conversiongain while “linear bias”(V)just outside the knee region allows optimum linearity,as well as a lower return loss of the LO port,corresponding to the IMD “sweet spot.”This kind of mixer can be unstable bysettingnear or above the knee bias [2].In order to guarantee that the mixer operates at its best under thetwowas chosen to be slightly below themaximum-B AND C ASCODEHEMT MIXERm and a width of80-band cascode mixers.The gate bias network includedatransmission line terminated with a large capacitor for IF short and an IFKIM et al.:HIGH-PERFORMANCE.Fig.5.Calculated and measured conversion gain and relative IMD3level of V -band cascode FET mixers at a fixed V of 00.4V and VV -band cascode FET mixer as a function of the RF frequency (a)at a fixed LO frequency of 59.3GHz and (b)at a fixed IF frequency of 1GHz.the knee bias(V)and the IMD “sweet spot”wasclearly observednearV .Also,mixer A (with interstage circuit)showed higher conversion gain and better IMD characteristics compared with mixer B.Biased for best gain region,the conversion gain increased by 2.7dB,from 3.6dB for mixer B to 6.3dB for mixer A.Biased at the IMD “sweet spot,”IMD3levels were reduced by more than 5dB for mixer A compared to mixer B.To the best of our knowledge,6.3-dB gain is among the highest conversion gains reported in this frequency range.The measured IMD results suggest that excellent intermodulation characteristics (IMD3of 46.4dBc)can be achieved with a reasonable conversion gain (4.1dB)when the mixer is bias tuned for the IMD “sweet spot.”Fig.5also shows good correspondence between the measurement and simulation for both conversion gain and IMD characteristics.The corresponding output third-order intercept point (OIP3)was as high as 11.2dBm with a low dc power consumption of 8mW.Fig.6shows the measured RF frequency dependence of the conversion gain of the cascode mixer.Fig.6(a)shows the con-version gain as a function of the RF frequencies with a fixed LO source(frequency808IEEE TRANSACTIONS ON MICROWA VE THEORY AND TECHNIQUES,VOL.51,NO.3,MARCH2003Fig.7.Measured LO-to-RF and LO-to-IF isolation characteristics of the V -band cascode FETmixer.Fig.8.Schematic of the overall integrated V -band downconverter module and the photographs of the individual chips.The measured small-signal return losses at the RF and LOports are typically9dB,respectively.The measured small-signal return loss data were not shown in this paper.Port-to-port isolation characteristics were also measured at a fixed LO power of 2.6dBm and are shown in Fig.7for the twodifferent-B AND D OWNCONVERTER M ODULEIn order to demonstrate the usefulness of the cascode mixer in realizing compact downconverters atthem GaAs pHEMT commercial foundry(GHz).The schematic ofthe-band oscillator is based on a parallel feedbacktopology and produced oscillation signals at 59.57GHz.Design target frequency was 60GHz.The output power was109dBc/Hz phase noisecharacteristics at a 10-MHz offset frequency.The photograph ofthe1.6dB and the frequency tuning rangewas from 59.4to 60GHz.In order to provide enough LO signal power(-band downconverter module is shown in Fig.11.Fig.12shows the conversion gain of the whole package including the loss of interconnection between the chips as well as the loss of the transition.The LO frequency was fixed at 59.57GHz.The overall gain of the integrated downconverter was higher than 20dB over the RF bandwidth of 4.2GHz(57.5KIM et al.:HIGH-PERFORMANCE-band cascode pHEMT mixer and the downconverter module using the mixer chip were presented. The conversion gain and linearity of the cascode mixer were improved through the use of low-pass interstage circuits and bias optimization.The mixer achieved a high conversion gain of6.3dB.The IMD“sweet spot”theoretically predicted by the authors[1]was also observed from the810IEEE TRANSACTIONS ON MICROWA VE THEORY AND TECHNIQUES,VOL.51,NO.3,MARCH2003Dongki Kim was born in Korea,in1975.He received the B.S.degree in electrical engineering from Sung Kyun Kwan University,Seoul,Korea,in2001,and is currently working toward the M.S.degree at the Seoul National University,Seoul,Korea.His research interests include modeling of active devices,design of power-combining structures,andMMICs.Jinho Jeong(S’00)was born in Korea,in1973.Hereceived the B.S.and M.S.degrees in electrical en-gineering from the Seoul National University,Seoul,Korea,in1997and1999,respectively,and is cur-rently working toward the Ph.D.degree at the SeoulNational University.His research interests include millimeter-wavepower-combining large-signal modeling of mi-crowave transistors,and MMIC/OEICdesign.Youngwoo Kwon(S’90–M’94)was born in Korea,in1965.He received the B.S.degree in electronicsengineering from the Seoul National University,Seoul,Korea,in1988,and the M.S.and Ph.D.degrees in electrical engineering from The Univer-sity of Michigan at Ann Arbor,in1990and1994,respectively.From1994to1996,he was with the RockwellScience Center,where he was involved in thedevelopment of various millimeter-wave mono-lithic integrated circuits based on HEMTs andheterojunction bipolar transistors(HBTs).In1996,he joined the faculty ofSchool of Electrical Engineering,Seoul National University.His currentresearch activities include the design of MMICs for mobile communicationand millimeter-wave systems,large-signal modeling of microwave transistors,application of micromachining techniques to millimeter-wave systems,nonlinear noise analysis of MMICs,and millimeter-wave power combining.。