Distortion in frequency-division-multiplex F.M. systems due to an interfering carrier

A theoretical investigation has been made of the distortion produced in f.m. frequency-division-multiplex trunk radio systems by the simultaneous reception of a wanted and an unwanted carrier, not necessarily identical in frequency. This form of disturbance can occur both between different systems and within a single system. It is found that, unless there is substantial failure of the receiver limiter action, the distortion generated in a particular channel is accurately proportional to the relative level of the interfering carrier. Thus, it is possible to plot curves of distortion in the worst channel against frequency separation of wanted and unwanted carriers, from which, for a given separation, a permissible level of interfering carrier can be deduced. Curves are presented for 240- and 600-channel systems when the interfering carrier is either modulated or unmodulated. Numerical results are shown for various carrier-frequency separations that can arise under the C.C.I.R. frequency plan for systems of six r.f. channels each carrying 600 speech channels. The distortion due to this mechanism can be divided into intelligible and unintelligible crosstalk. Reasons are given for the expectation that the former will not normally be important. Some measurements of distortion due to an unmodulated interfering carrier, using components of a 600-channel system, show good agreement with the theory when the frequency separation is not too large. For separations of the order of 9 Mc/s, or greater, distortion levels substantially above the theoretical were noticed. The discrepancy is considered to be due to departure in the demodulator from the ideal behaviour assumed (i.e. frequency insensitivity of the limiter and adequate broadness of band of filters, etc., and of the discriminator).