Common-Channel Interference between Two Frequency-Modulated Signals

Two frequency-modulated signals received in the same frequency band produce an output from the receiver which is simply a combination of both signals and a beat note whose frequency is modulated in accordance with both signals. The difference of signal strength required for the reduction of crosstalk from the weaker signal is less in frequency modulation than in amplitude modulation, but is the same for different bandwidths of frequency modulation. The required difference can be further reduced by the use of a limiter in the receiver. The beatnote interference remains as background noise fluctuating with the modulation of both signals. This noise is reduced by wideband frequency modulation. Simple expressions for the detector output in several cases enable the identification of frequency effects which are unavoidably detected as distinguished from amplitude effects which can be removed by a limiter. Common-channel interference is readily tested by oscilloscope patterns. These show the normal operation with or without a limiter, and also the effects of departure from the normal, such as detuning.