Analysis and design of current-commutating CMOS mixers

The recent advances in low-cost CMOS fabrication processes have rendered them appropriate for the realization of high-frequency analog communication circuits, traditionally implemented in more expensive technologies such as bipolar or Gallium Arsenide. These CMOS implementations have the significant advantage that they can be more easily integrated with the low frequency analog and digital circuitry. The demand for short design cycles imposes the need for fast optimization of the high-frequency analog circuit blocks. Such a circuit, present in the front end of any communication system, is the mixer which performs frequency translation of the carrier signals. Because one of its inputs is the strong local oscillator signal, its operating point is periodically-time-varying. As a result, the analysis of its operation is considerably more complicated than that of the linear time-invariant blocks. The subject of this thesis is to analyze the operation of one commonly used class of mixers, those which employ a switching transistor pair to commutate the signal current. The objective is to provide results in a form that can be applied by Radio Frequency circuit designers to systematically optimize their designs.; In the first part of this thesis the mixing operation is described and practical mixer nonidealities and related performance metrics are introduced. Several mixer topologies in CMOS technology are discussed and the current-commutating CMOS mixers, for which the results of this research apply, are emphasized. In the second part an analysis of the nonidealities which define the mixer dynamic range, namely the noise and the nonlinearity, is performed. The contribution of every internal and external noise source to the output noise is calculated and the mixer noise performance is predicted. The noise performance of a CMOS inductively degenerated transconductance stage is investigated in depth. Consequently, the nonlinearities of the CMOS transconductance stages are analyzed. These results are applicable besides mixers to other blocks that employ transconductance stages, such as low noise amplifiers and power amplifiers. Finally, the nonlinearity of the switching pair is investigated. In all cases the results are provided in terms of simplified analytical expressions or graphs of normalized parameters. A simple transistor model with continuous derivatives of any order in all operating regions is adopted and compared with more sophisticated simulator models. In the third part, the design of some single-balanced active mixers is presented as a demonstration of the application of the theoretical results derived in this thesis.