Noise in an FM System Due to an Imperfect Linear Transducer

In the course of designing FM systems, intermodulation noise is an important factor which deserves special attention. Many people have made contributions to this subject.1"10 In this paper, we extend their results 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. The multichannel baseband signal of an 1537 i 1556 TI-IE B E L L SYSTEM TECHNICAL JOURNAL, NOVEMBER 1966 FM system is represented by a band of gaussian random noise with flat power-density spectrum. The noise due to imperfect transmission medium can be calculated at any frequency in the baseband. 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. Extension to a higher-order distortion becomes unmanageable. A digital computer program concerning the intermodulation noise has been written. A typical problem can be solved at a very low cost. 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.