| 摘要 |
A television tuning system has a voltage controlled oscillator and frequency converter, cooperating to convert a receiver broadcast signal to an intermediate frequency signal and an intermediate frequency filter having a predetermined frequency response. Automatic frequency control means operative upon the voltage controlled oscillator to maintain correct frequency conversion include a limiter coupled to the intermediate frequency filter producing a constant amplitude output signal which is coupled directly to one input of a multiplier and to a surface acoustic wave filter having a predetermined frequency-dependent phase shift characteristic. The filter characteristic defines quadrature phase points at the picture carrier frequency and a selected frequency substantially midway between the picture and sound carriers. The filter output is coupled to the remaining multiplier input. A low pass filter couples the output signal of the multiplier to the voltage controlled oscillator thereby providing automatic frequency control. The output voltage characteristic of the multiplier includes zero crossings at the intermediate frequency picture carrier frequency and at a frequency between the intermediate frequency picture and sound carriers.
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