A time-domain RF steady-state method for closely spaced tones

Abstract

Verifying circuits with two or more closely-spaced driving frequencies is important in RF and wireless communications, e.g., in the design of down-conversion mixers. Existing steady-state calculation methods, like harmonic balance, rely on Fourier series expansions to find the difference-frequency components typically of interest. Time-domain methods are, however, better suited for circuits with strong nonlinearities such as switching. Towards this end, we present a purely time-domain method for direct computation of difference tones in closely-spaced multi-tone problems. Our approach is based on multiple artificial time scales for decoupling the tones driving the circuit. Our method relies on a novel multi-time reformulation that expresses circuit equations directly in terms of time-scales corresponding to difference tones. We apply the new technique to an RF-CMOS mixer to predict baseband bit-streams and down-conversion gain and distortion, in two orders of magnitude less CPU time than traditional time-stepping simulation.