Design of sampling mixer and A/D converter for high IF digitization

Abstract

The thesis discusses the issues and solutions in the design of an A/D converters for high IR signals in wireless communication systems. The first part of the thesis examines the problem of harmonic and intermodulation distortion in high frequency MOS sampling mixer circuits. It departs from traditional analysis for such circuits by using the method of Volterra series. Three different distortion mechanisms are identified and analyzed separately for the top-plate sampling mixer: the time-invariant distortion valid for 0 fall time of the gate LO signal, time varying distortion due to finite fall time and non-uniform sampling distortion due to abrupt cutoff. The thesis adapts and applies the method of Volterra series to all the three mechanisms and derives expressions for distortion for them. It then analyzes the practically important case of bottom-plate sampling. The second part of the thesis discusses issues in the design of a passive sigma-delta modulator operating at the high click-speeds necessary for adequate SNR and bandwidth. In particular, it discusses the design issues of a BiCMOS comparator consisting of a preamplifier and regenerative-latch with low input-referred noise and high clock-speed. Finally, the design issues and implementation details of an experimental, proof of concept, IF digitizer using a passive loop-filter based sigma-delta A/D converter in a 0.8 u m BiCMOS technology are discussed. The prototype achieves an SNR of 72 dB, HD2 of -72 dB, IM3 of -65 dB for a -3 dBm 100MHz IF input signal at a power level of 12.5 mW.