The input impedance of a rectifier modulator is different in its circuit relationships from that of a passive linear network; for a given input frequency it depends on the value of the load impedance at all the frequencies which can exist in the circuit and often also on the impedance of the signal source from which it is measured.The calculation of the input impedance appears to be impracticable in the general case; but in special cases where the terminating impedances can be restricted to zero or infinity or to a pure resistance at the various frequencies which can exist in the circuit, and where the resistance/time function of the rectifier can be expanded as a Fourier series independent of the signal frequencies—i.e. controlled solely by the carrier—calculation is possible, and a selection of results is tabulated.