Custom-made receiver mixer MMIC front-end for X-band radars

Rasmus Schandorph Michaelsen

Research output: Book/ReportPh.D. thesis

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Monolithic Microwave Integrated Circuit (MMIC) Mixers in Silicon-Germanium (SiGe) technology for direct conversion suitable for X-band Doppler radars have been investigated. The mixers are passive double balanced diode mixers, as these have the properties of good linearity and low noise, together with isolation between all ports. An investigation into how Local Oscillator (LO)-leakage and DC-level influences the flicker noise of the mixers was conducted. It showed that the DC level is critical in regard to minimize the flicker noise, where a zero volt DC level gives minimum noise. And that the DC-level is a function of LO-leakage due to self-mixing. This led to the idea of mixers where the LO-leakage can be controlled. It is proposed to use a balun with tunable phase balance to control the LO-leakage. A novel Marchand balun configuration which allows this is described. Experimental results showed a tuning range from −183◦ to −175.8◦. This circuit is used as the crucial building block in an experiment comparing three mixers. Two mixers with tunable balun on the LO port, one with diode connected HBTs and the other with Schottky diodes. And a reference mixer for comparison, with diode connected Heterojunction Bipolar Transistor (HBT)s, but without the tuning circuit. These are all manufactured in a SiGe technology. Unfortunately the diode connected HBT mixer was short circuited on the control voltage, only allowing comparison between the Schottky mixer with tuning and the HBT mixer without this option implemented. It is shown that by tuning the balun one can enhance linearity and isolation of the mixer, but not 1/f noise. They both have a conversion loss around 9 dB. The HBT mixer has a flicker noise corner around 10 kHz while the Schottky mixer has one around 250 kHz. The low Quality factor (Q) inductors used for the Marchand baluns and matching have rather high loss. Techniques to overcome this loss are investigated. Suggestions for an active balun and a novel Marchand balun with complex matching are given, and two mixers are designed with these, with promising results for Doppler radar applications
Original languageEnglish
PublisherTechnical University of Denmark
Number of pages144
Publication statusPublished - 2016

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