An AM-FM Electronic Analog Multiplier

This paper describes an electronic multiplier wherein the input voltages are made proportional to the respective indices of amplitude and frequency modulations of a radio frequency carrier with the aid of feedback stabilization. The desired product of the input voltage is obtained from a Foster-Seeley discriminator whose output is independently proportional to each modulation index after the carrier component is subtracted out. Performance of the AM-FM multiplier is essentially independent of vacuum tube characteristics by virtue of the feedback stabilization and is limited primarily by the stability of the discriminator parameters. Other features of the multiplier are: ease and rapidity of adjustment; conventional AM and FM receiver circuitry; long-time multiplication error of less than 1 per cent of full scale output; usable bandwidth (flat amplitude response and less than 15 degrees phase-shift for highest frequency component) approximately 1 per cent of carrier frequency.