Automatic Frequency Control

A system is described for electronic control of the local oscillator frequency in a superheterodyne receiver for the purpose of centering the signal carrier in the intermediate-frequency band in spite of inaccuracies of manual tuning and oscillator drift. The system consists of two parts, a discriminator for converting frequency departures into direct voltage differences, and a control circuit for converting voltage differences into reactance variations of the oscillator circuit. The system thus tends automatically to control the intermediate frequency produced, so that maladjustments which otherwise would produce serious alignment errors will result in actual error of only a few cycles. The system also makes practicable remote tuning control and other simple mechanical tuning methods.