The effects of an unwanted signal mixed with the carrier supply of ring and Cowan modulators

It is shown how the performance of a ring or Cowan modulator, when the carrier supply has an unwanted signal mixed with it, can be investigated by a frequency-analysis of its modulating function. Results obtained for particular cases lead to certain general conclusions, among which are the following:¿ (a) The primary modulating effect of an unwanted signal mixed with the carrier is largely independent of the resistance of the circuit supplying the carrier, even though this affects the shape of the modulating function very considerably. The amplitude of the unwanted component in the modulation is, relative to the wanted component, only about one-half of that in the carrier input. (b) Ring and Cowan modulators give a similar performance in this respect. (c) When the frequency of the input signal is equal to the carrier frequency, the output of the difference frequency between the carrier and unwanted signal is zero for an ideal modulator, and very small for one with a carrier supply of high resistance; as the resistance is reduced, the output of difference frequency increases, until with zero resistance it has a ratio relative to a main sideband of about one-quarter of the amplitude ratio of the unwanted signal relative to the carrier in the carrier-supply circuit.