Frequency synthesizer tuning system with manual fine tuning control

摘要

A television tuning system employs a frequency synthesizer system for establishing the tuning of the receiver. A first programmable frequency divider controlled by a reversible counter is connected between the output of a reference oscillator and a phase comparator to which the output of the local oscillator, after passing through another programmable frequency divider, also is applied. The phase comparator output is a tuning voltage used to control the tuning of the local oscillator. A logic circuit is coupled to sense predetermined relationships of signals from a picture carrier detector, a sound carrier detector, an AFT signal, and the presence of vertical synchronization signal components for changing the count in the reversible binary counter to adjust the first programmable frequency divider to compensate for channel frequency offsets which may occur in excess of the pull-in range of the AFT circuit. To enable operation of the receiver in a fringe area, where it may be desirable to intentionally mistune a channel slightly, manual fine tuning control logic circuitry is employed to disable the frequency offset logic circuit and to permit changing the count of the reversible counter by the viewer to manually fine tune the receiver as desired.