Automatic frequency tuning system

An automatic frequency tuning system according to the present invention includes an antenna, a tuner circuit, a video SAW filter, a video intermediate frequency amplifier, a video detector, a video amplifier, a video PLL circuit, an AFT control circuit, a microcomputer and a memory. In addition to the above members, the automatic frequency tuning system further includes a reference PLL circuit for reducing variation in a free running frequency of a video voltage controlled oscillator, a comparator for determining a magnitude relationship between a received video frequency and a standard video frequency, a mixer for mixing an output signal of the video control frequency and an output signal of a reference voltage controlled oscillator and retrieving sum and difference signals for frequencies of the outputs, and a mixer low pass filter for supplying only a frequency difference signal to a subsequent stage.