Digital measurement of FM transmitter quality

A description is given of how a passive mixer can be used to replace a professional FM demodulator and distortion meter for FM transceiver measurements, when used with an automatic test system carrying a normal complement of signal generators and a timer/counter. The system measures two signal-to-noise and distortion (SINAD) power ratios for the FM transceiver by first causing the transmitter to be modulated by a standard 1000 Hz audio test tone and then downconverting the frequency of the transmitter carrier to a lower and more convenient value. A simple timer/counter is then used to measure a series of alternate periods between zero-crossing points of the frequency-converted transmitter output waveforms. These data points, taken over varying periods of time, are then converted into a series of equal interval data points by the author's proposed algorithms, and then the data are Fourier transformed into a frequency-domain representation of the FM transmitter modulating waveform. The required SINAD ratio and other useful data is then derived from this transformed data.<>