Sideband-Operated Automatic Frequency Control for Reception of Suppressed-Carrier SSB Voice Signals

It is suggested that frequency errors in the reinserted carrier in SSB voice reception, such as result from doppler shift of the incoming signal or oscillator drift, may be corrected by an AFC arrangement utilizing the sidebands themselves. The technique depends on the fact that the loudest speech sounds are rich in harmonics of the vibration rate of the vocal cords. This harmonic spectrum can be established, essentially independently of receiver tuning, by diode rectifying the incoming sideband energy while it is at radio or intermediate frequency. The AFC compares the spectral content of the output of the normal SSB (or product) detector with that of the diode detector; it then adjusts the frequency of the reinserted carrier until frequency components of the two audio signals properly overlap. Reinserted-carrier-frequency correction may be accomplished either by a phase-locking procedure or by a control depending on frequency rather than phase difference. The latter method appears to be able to reduce deviations as large as 1 or 2 kHz by an order of magnitude or better. The technique should be applicable when one terminal of a link is a rapidly moving aircraft or satellite and when the doppler shift is unpredictable or time varying. Another application is in marine, aeronautical, or mobile radiotelephony where economic considerations prevent use of frequency standards of adequate stability. The AFC circuit could be niade compatible with AM.