Validity of first-order f.m. distortion theory as applied to trunk radio systems

Much of the published work relating to distortion in frequency-modulation trunk radio systems has been, and continues to be, based on a so-called `first-order theory¿. In the course of some correspondence published a few years ago, the validity of this first-order theory was questioned in a general way, although no evidenec was given to show whether or not serious errors arise when the theory is used to evaluate the performance of trunk radio systems. It thus becomes important to check, in a representative range of situations, the accuracy of the distortion levels predicted in such systems by the first-order theory, since these results have been widely used in system design. This is done in the present paper, using a recently developed Monte Carlo method involving direct computer simulation of the distortion process. The agreement between the first-order theory and the computer simulation method is found to be satisfactory. For one special form of transfer characteristic, a nonlinear f.m. distortion term fails to appear in the first-order theory. The distortion level in this case will normally be very low; for its evaluation it is necessary to consider the dominant term from higher-order theory.