Spurious Tones in Upconverters Caused by IF Signal Multiplication

Spurious tones caused by the multiplication of IF signals up to the frequency range close to the upconverter output signal have been investigated both theoretically and experimentally. It is shown that these spurious tones experience very strong suppression as soon as modulation is applied on the IF carrier. This is caused by spectrum spreading, described previously in literature. In this paper, graphs of the magnitude of such suppression as a function of channel loading in a heterodyne microwave transmitter are given. It has been observed also that sufficient suppression is sometimes caused even by the FM noise or low-frequency fluctuations of the IF oscillator used. As a result of these findings it is suggested that the usual specifications, as applied to spurious tones in microwave transmitters, should be relaxed for this particular kind of interference.