A simple method of modulator analysis is given which enables the performance of a modulator with frequency-selective terminations to be determined so long as these terminations are resistive, zero or infinite at all significant frequencies. Most of the cases likely to be useful or unavoidable in practice have been worked out for the ring, Cowan (shunt-type) and series modulators, and the optimum terminating resistances (assumed equal) and minimum insertion losses have been tabulated in terms of the rectifier ratio and geometric mean resistance.