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Design of W-band Mixer with High Image Rejection Ratio in 0.1-μm GaAs pHEMT Technology

0.1-μm GaAs pHEMT 공정을 이용한 높은 이미지 제거도를 가지는 W-대역 혼합기 설계

WONSEOK CHOE (최원석, 서강대학교)

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In this thesis, design of W-band mixer with high image rejection ratio using 0.1-μm GaAs pHEMT technology is presented. Image rejection mixer is an essential circuit block for a receiver, which can improve noise performance of the receiver by suppressing unwanted signals and noise from image frequen...
In this thesis, design of W-band mixer with high image rejection ratio using 0.1-μm GaAs pHEMT technology is presented. Image rejection mixer is an essential circuit block for a receiver, which can improve noise performance of the receiver by suppressing unwanted signals and noise from image frequency. By using image rejection mixer, requirement of image rejection filter and number of down-conversion stages can be reduced. The image rejection mixer (IRM) suppresses image components by phase cancellation so that frequency spacing between image frequency and RF can be ignored, and this make IRM available for low IF applications. Image rejection ratio (IRR) is regarded as one of the most important performance parameter of the IRM, which represents power ratio between IF signals produced by RF and image. Since the IRM suppresses image components by phase cancellation, the IRR is directly affected by the amplitude and phase mismatches of the IF signals. A few IRMs published in W-band suffer from low IRR. It is expected that the increased parasitic effects in millimeter-wave frequency lead to difficulties for obtaining perfect amplitude and phase matches of the IF signals. In this thesis, we analyze the reasons of the signal mismatches, which never have been reported, can be observed in IRMs developed at millimeter-wave frequency, and it is verified by theoretical analysis, simulation, and experimental results. The mismatch of the IF signals can be produced by many reasons such as imbalance of LO, RF, and IF couplers or asymmetric structure of MMIC. Many reported IRMs focused on these problems. However, we focused on the isolation of RF and LO couplers and coupling between the two mixers of the IRM, because the two mixers can be placed close enough to be coupled in millimeter-wave frequency. The signal mismatches by the coupling are studied in Chapter 2. Theoretical analysis and simulation results show that the coupling between the two mixers can be led to serious signal mismatches and degradation of IRR even though there are no signal mismatches from the LO, RF, and IF couplers. The simulation result shows that the coupling of signal paths where the input signals are applied to with phase difference of 90° has dominant effect on the IRR. The analysis result shows that coupling between the two mixers should be less than -21 dB to obtain the IRR greater than 20 dB. Based on the previous analysis, IRM was designed using 0.1-μm GaAs pHEMT technology. A resistive mixer is used as a mixer core, and RF buffer amplifier is connected to mixer to compensate for the amplitude and phase mismatches and improve conversion gain and LO-to-RF isolation. The design method of the mixer, RF buffer amplifier, and IRM is specifically presented in Chapter 3, and the signal mismatch problem by the coupling was also analyzed in this chapter. Based on the simulation results, several solutions for this problem are presented, and one of the solutions is applied to a practical IRM design. The coupling between the mixers are predicted using full-wave 3D EM simulations, and the amplitude and phase mismatches of the signals are compensated for using RF buffer amplifier. The simulation results showed the IRR degraded due to the coupling can be recovered by the compensation. The simulation results of the IRM designed using GCPW transmission line to reduce the coupling between the mixers are also presented in this chapter. The simulation results show the improved IRR compared with the previous design, but the mismatch compensation seems still necessary. The effect of isolation of the couplers on the IRR was also studied. The simulation results show that the value of IRR has a limitation in practical design because of the limited isolation of the couplers. The designed MMICs have been fabricated using 0.1-μm GaAs pHEMT technology and the IRM module was fabricated using the fabricated MMICs. The fabrication procedure and the experimental results of the IRM module are presented in Chapter 4. The simulation results and experimental results show a great agreement, and the signal mismatch problem by the coupling was also observed in measurement results. The compensation method for the signal mismatch was verified by experiments, and the measurement results show excellent IRR of greater than 40 dB and conversion gain of -7.3 dB. The fabricated MMICs have been packaged into waveguide module for practical applications. The IRM waveguide module has LO and RF ports of WR-10 waveguide flange. To apply LO and RF signals through the WR-10 waveguide, a waveguide-to-microstrip transition was fabricated and connected to the MMIC using bonding wire. An extended E-plane probe transition was used for high reproducibility of the mixer waveguide module. The fabrication process and experimental results of the transition and IRM waveguide module are presented in Chapter 5. The measurement results show excellent IRR greater than 40 dB and conversion gain of around -11 dB. It is expected that the fabricated IRM waveguide module can be used for various applications such as Doppler radar, millimeter wave seeker, and millimeter-wave receivers, etc.