Short-term phase stability of audio signals in mobile radio vehicle-base links

An investigation into the phase perturbation of an audio tone transmitted over conventional frequency-modulated mobile radio links at frequencies of 72, 170, and 462 MHz is described. Data from stationary and moving vehicles in a variety of urban routes and in mountainous terrain are presented. The effects of noise and of multi-path propagation are identified. The results are interpreted in light of the requirements of a particular automatic vehicle monitoring (AVM) system, which is described briefly, but should be of interest to others concerned with the transmission of phase-sensitive data.