Analysis and Design of a Low Power 1.2V CMOS Downconversion Mixer Utilising Substrate Biasing

Abstract

This thesis presents detail theoretical analysis of downconversion Gilbert cell mixer with the improvements on major performance parameters by utilizing different substrate biasing techniques. By modifying the threshold voltage of the switching core, the LO transistors perform more ideally as a perfect switch. It improves the active mixer performances in conversion gain, noise and linearity performances. The techniques are implemented on a 1.2 V low power CMOS downconversion mixer for performance comparisons between simulation and measurements result. They are realized in TSMC 0.18 um CMOS technology. It shows that body-biasing techniques help to increase the switching efficiency of the Gilbert mixer. And a mixer with a better switching provides better performance. With no additional power consumption, the no body effect technique in Design B has shown a 1.5 dB higher in conversion gain, 2 dBm higher in IIP3, and a 0.5 dB lower in NF performance. With the varying biasing technique implemented in Design C, it shows an improvement of 22 dB in conversion gain. Both Design B and C have less than 2 mW power consumption and are suitable for Bluetooth applications. This thesis introduces a stage-by-stage procedure for designing a Gilbert mixer; design tradeoffs at each stage are also discussed.