Some effects of delay and modulation-index mismatch on cochannel FM interference

The baseband distortion resulting from the cochannel interference of two FM carriers nominally carrying the same modulation is considered. Such distortion was observed to plague VHF mobile telephone service in Phoenix, Ariz. This distortion arises when the landline audio delay and modulation indices of the two transmitters are not closely equalized. Both sinusoidal and Gaussian modulation age analyzed. Curves presented indicate that baseband distortion power as large as 20 percent of baseband signal power may occur for an RF carrier amplitude ratio of 0.7 and an unequalized delay of the order of 200 µs when the minimum rms transmitter phase deviation is 2 rad. Increasing the deviation to 4 rad reduces the distortion to the 5-percent range. The desirability of large index modulation in this context is thus apparent.