Gaussian and Click Noise in an FM/FM Multiplex Receiver

An FM/FM system is one in which the signal information frequency modulates a subcarrier, which, in turn, frequency modulates a main carrier for transmission. The detection process then involves two frequency demodulators in sequence. Using Rice´s click model for both FM demodulators, we obtain the average click rate and an approximate output spectrum that include the effect of the impulses generated by the carrier demodulator on the subcarrier demodulation at and below threshold. We then focus on the threshold behavior of these systems and the proportioning of their modulation parameters so as to provide certain desired characteristics. We find that due to the filtering action of the second demodulator, the number of output clicks from carrier generated spikes can be orders of magnitude smaller, yet their contribution to the output noise can be much greater than that produced by the first demodulation. In fact, this term is dominant in the thresholding of the subcarrier demodulators except for the most fully loaded multiplex. In the latter case, there is so little deviation available for each channel that the self-clicks produced by the subcarrier demodulation in the absence of carrier spikes become severe. We also find that each channel has a critical carrier modulation index below which the output clicks exceed the carrier generated clicks.