A Frequency-Modulation Station Monitor

This paper describes the development, theory, and characteristics of a frequency-modulation station monitor developed in the general engineering laboratory of the General Electric Company. The monitor measures mean carrier frequency with and without modulation and the percentage of frequency modulation as a percentage of 75-kilocycle deviation. In addition, it provides a flasher and alarm circuit for overmodulation adjustable from 50 to 120 per cent and an audio-frequency fidelity monitoring circuit. The paper begins with a discussion of the requirements for frequency-modulation monitors and compares the problems involved with those involved in standard broadcast monitors. Several schemes which were examined for use as a frequency-modulation monitor are briefly described, including counter and frequency-division systems. The principles and design of the final monitor are discussed. The last part of the paper is devoted to a discussion of the salient development problems encountered, to a detailed description of the circuit, and to a discussion of the over-all specifications of the commercial sample.