Intermodulation effects in FM and PM systems

With the increasing use of radio relay systems to transmit a large variety of time and frequency division information signals, noise and signal distortions that tend to reduce the information-carrying capacity of such systems take on added importance. In any multiplex system for either speech or data-type signals, noise produced by nonlinear elements carrying the active channels generally limits the maximum signal-to-noise ratio that can be obtained. Nonlinearities in baseband amplifiers, frequency modulation generators, discriminators, and distortions of the delay characteristic are the principal contributors of this modulation noise. Thermal or first circuit noise in any part of the radio system can be readily calculated. The following discussion is concerned primarily with the measurement, evaluation, and correction of delay distortion, klystron pulling effects, and the determination of optimum system loading. A number of references are included to assist in a further study of the subject.1¯⁷