Surface wave AFC discriminator with two zero crossings

Claim 1

In a television tuning system, for receiving a broadcast signal having frequency-spaced sound and picture carriers, said tuning system having a voltage controlled oscillator producing a heterodyning signal, a frequency converter responsive to said received broadcast signal and said heterodyning signal translating said broadcast signal to an intermediate frequency signal having frequency-spaced intermediate frequency picture and sound carriers, and an intermediate frequency filter; automatic frequency control means producing a first control effect due to said intermediate frequency picture carrier and a second control effect due to said intermediate frequency sound carrier, said automatic frequency control means comprising: a surface acoustic wave filter having a frequency-dependent phase characteristic such that a quadrature phase shift is imposed upon signals at said intermediate frequency picture carrier frequency and signals of a predetermined frequency between said intermediate frequency picture and sound carriers; multiplier means having first and second input channels, producing an error signal having a zero value when said input signals are in quadrature and opposite polarity maxima when said input signals are in-phase and counter-phase; first coupling means coupling said intermediate frequency filter to the input of said surface acoustic wave filter and to said first input channel of said multiplier means; second coupling means coupling the output of said surface acoustic wave filter to said second input channel; and third coupling means coupling said multiplier means error signal to said voltage controlled oscillator.