Automatic frequency tuning system

An automatic frequency tuning system comprising:

• a first phase synchronous circuit including a first voltage controlled oscillator circuit, a first phase detector circuit for comparing a phase of an output signal of the first voltage controlled oscillator circuit to a phase of a received video intermediate frequency signal, and a first low pass filter for smoothing an output of the phase detector circuit and feeding back a first frequency control voltage to the first voltage controlled oscillator circuit;
• a second voltage controlled oscillator circuit synchronized with a frequency received from a highly stable frequency source externally located and oscillating at a standard video intermediate frequency;
• a mixer for outputting a mixture component obtained by mixing a frequency output from the first voltage controlled oscillator circuit and a frequency output from the second voltage controlled oscillator circuit;
• a second low pass filter for passing only a low frequency component extracted from the mixture component;
• an AFT control circuit for receiving an output signal of the second low pass filter, counting a frequency difference between a standard video intermediate frequency and the received video intermediate frequency, and outputting a digital signal corresponding to the frequency difference,
• wherein even if the received video intermediate frequency is changed to a maximum extent, a difference between an oscillation frequency output from the first voltage controlled oscillator circuit and an oscillation frequency output from the second voltage controlled oscillator circuit is set so that a magnitude relationship between the oscillation frequency of the first voltage controlled oscillator circuit and the oscillation frequency of the second voltage controlled oscillator circuit is not changed.