Advanced Downconversion Mixers for CMOS Radio Frequency Integrated Circuits

Abstract

This thesis explores the design of advanced downconversion mixers in CMOS RF integrated circuits. First, a novel switched transconductor mixer with low power, low noise, and ultra-wide bandwidth is proposed. The transconductor stage with fixed DC operating point is switched by the ac-coupled local oscillator (LO) signal to realize the mixing so that a decade bandwidth is achieved with power-efficient LO buffer. The proposed mixer topology is investigated thoroughly with its bandwidth analysis, design process and a performance comparison to the traditional switched transconductor mixer is provided. The measurement results of the mixer demonstrate a 15.5 – 17.5 dB gain and a 4 –5.2 dB noise figure over 1 – 10 GHz, and the power consumption is only 8.3 mW from a 1.2 V supply.

Since a wideband mixer usually requires exceptional linearity to resist large in/out-of-band blockers, a novel feedforward linearization technique for the active mixer is presented. As it generates the low-frequency IM3 for the cancellation fully in the IF band, the technique can bring third-order input intercept point (IIP3) of the mixer that is robust against parasitic parameters and process variations. Furthermore, this method can be applied to any active mixers regardless of its circuit topology. The measurement results of a Gilbert cell mixer with this method applied has shown a 12.5 dB IIP3 improvement at cost of 4-mA current from 1.2 V supply. The adoption of this method does not introduce noticeable noise and gain degradation to the mixer.

Last, but not the least, a low-power, wideband, Gilbert-cell-based mixer with an on-chip balun is also presented. The mixer adopts a folded structure with its transconductor and the switching stages coupled with an on-chip, multifilament-transformer-based balun. With this balun integrated, the single-ended-to-differential conversion is realized within the mixer block. For the first time, the double-tuned resonator is utilized to expand the bandwidth of the mixer. The measurement results demonstrate a gain of 13.5 dB over 4 to 10 GHz, with amplitude and phase imbalances limited to only 0.9 dB and 2 over the whole band.