Automatic frequency control system

Claim 1

In a color television receiver of the type including an oscillator stage for generating a local signal to be heterodyned with a radiofrequency signal having color and audio information components in a mixer stage to produce an intermediate frequency output signal having corresponding color and audio information components and also including a shaped intermediate frequency band-pass response, a system comprising: a voltage-responsive variable capacitance device coupled to said oscillator stage providing the only means for fine-tuning adjustments of the oscillator signal frequency; automatic frequency control means having a resonant circuit for providing an output control voltage having a magnitude determined by the frequency deviation of the output signal from said mixer stage from the resonant frequency of said resonant circuit, said automatic frequency control means coupled to said variable capacitance device in a manner such that whenever said television receiver is in use, the capacitance exhibited by said device is controlled by said output control voltage; said resonant circuit including control means to tune said circuit to different frequencies within a band of frequencies by varying the capacitance of a voltage-responsive variable capacitance device operably connected in said resonant circuit; and first means for applying a voltage across said control means variable capacitance device including means for applying a preset voltage across said control means variable capacitance device to cause said resonant circuit to be tuned to a predetermined frequency within said band of frequencies during normal operation of said television receiver such that the intermediate frequency output signal color and audio information components are at a nominal position with respect to the television receiver shaped intermediate frequency bandpass response and means for applying a variable voltage across said control means variable capacitance device to vary the capacitance exhibited by said variable capacitance device to tune said resonant circuit across said band of frequencies during those conditions when it is desirable to shift the intermediate frequency output signal color and audio information components from the nominal position to a new position with respect to the television receiver shaped intermediate frequency band-pass response to obtain a more optimum combination of reproduced color and sound by said television receiver.