Noise in an FM System Due to an Imperfect Linear Transducer

An approach to the calculation of intermodulation noise in FM systems due to imperfect transmission media is presented in this paper. The technique is essentially that originating with Carson and Fry. In this paper we extend a formulation due to Rice to include an arbitrary continuous pre-emphasis characteristic as well as an arbitrary gain and phase shape transmission medium which are representable by low-order polynomial series in radian frequency. Series approximations are carried out far enough to ensure accurate results for transmission characteristics normally encountered in broadband microwave radio systems. In many cases, only the second- and third-order noise is significant in broadband microwave radio systems. Hence, the analysis carried out in this paper considers only the second- and third-order distortion terms. A digital computer program concerning the intermodulation noise has been written. This analysis and the digital computer program are of aid in the design of microwave radio systems. With a slight modification, the calculation of noise due to AM -to-PM conversion caused by transmission deviation can also be accomplished. An optimum design of the pre-emphasis network may be achieved by using the computer programs through an iterative approach.