An Extended Analysis of RF Interference in FDM-FM Radio Relay Systems

In a conventional microwave receiver a number of critical frequencies exist such that, when an interfering RF signal is introduced in their vicinity, a spurious IF response results. The existing methods for computing intermodulation due to an interfering carrier are found to be only partially satisfactory, in that, they do not explicitly define the manner in which the receiver mixing mechanism modifies the RF interference with respect to the level relative to that of the wanted carrier, frequency displacement in the IF from the primary IF response, and modification of the modulation index of the interference modulation. This paper extends, in a general manner, the theories of RF interference to include interfering signals falling on or near all major critical frequencies of the receiver. It is shown that, with the relative conversion efficiencies of the mixer available from empirical data and an understanding of the manner in which the mixer will modify the frequency displacement and modulation index of the interfering signal, these interference situations can be analyzed within the scope of a previously defined general interference equation. As an illustrative example some interference problems in a 1200-channel system are analyzed and substantiated with measurement data.