Tuning system with provisions for calculating the local oscillator frequency from an aft characteristic

Claim 1

Tuning apparatus comprising: RF means for providing an RF signal having an information bearing carrier; local oscillator means for generating a local oscillator (LO) signal having a frequency controlled in response to a control signal; mixer means for combining said RF signal and said LO signal to produce an IF signal having an information bearing carrier corresponding to said information bearing carrier of said IF signal; automatic fine tuning means responsive to said IF signal for producing an AFT signal having an amplitude versus frequency response with a polarity and level representing the sense and magnitude of a deviation of the frequency of said IF carrier from a nominal value; amplitude comparator means for generating first and second signals when said AFT signal traverses first and second threshold levels corresponding to respective first and second frequency deviations of said IF carrier from said nominal value; and control means for generating said control signal to control the frequency of said local oscillator signal, said control means causing the frequency of said LO signal to change and thereby cause said AFT signal to sequentially traverse said first and second threshold level, said control means being responsive to said first and second signals for locating said first and second frequency deviations and for calculating a frequency of said LO signal corresponding to a correct tuning condition in accordance with a predetermined computation utilizing both of said first and second frequency deviations.