TV voltage tuning circuit - has digital frequency control circuit which counts intermediate frequency cycles and compares count with preset value

摘要

A TV receiver circuit includes a tuning circuit tuned by a variable capacitance diode whose controlling voltage is regulated by a digital automatic frequency control. The latter includes two 12 bit counters which count respectively the cycles of an inter intermediate frequency signal derived from an intermediate frequency amplifier and a reference oscillator . The two counts are compared in a comparator and the resulting signal is used to modify the content of a register of the tuning circuit. The register receives preselected frequency selection signals from a read only memory and the controls the diode voltage accordingly, through an integrator. A deviation of 9.57 KHz from an intermediate frequency of 30.2 MHz can be detected and compensated for.