Method and circuit for the frequency and phase locking of a local oscillator in television

Ce circuit permet d'utiliser un oscillateur (1) sans préréglage pour réaliser un asservissement sur la sous-porteuse de chrominance d'un signal de télévision couleur. La sortie de l'oscillateur est comparée en phase dans un comparateur (7) à un signal de salve (6) sur lequel on veut effectuer l'asservissement. La sortie du comparateur (7) est envoyée par l'intermédiaire d'un amplificateur différentiel (9) à un circuit de commande à seuil (21) qui agit sur l'entrée de comptage (up) ou de décomptage (down) d'un compteur-décompteur (24) dont la sortie est transformée en signal analogique par un circuit (25) agissant sur l'entrée de commande (2) de l'oscillateur. A conventional phase locking loop for locking a local oscillator in phase and frequency to a color burst signal in a television system requires a highly stable (i.e. crystal controlled) oscillator whose free running frequency must be individually adjusted to be as close as possible to the desired frequency. The present invention enables such adjustment to take place automatically in operation rather than as a separate manufacturing step. The output from the oscillator is compared in phase with the color burst signal to which it is to be locked. The output from the phase comparator is applied to one input of a differential amplifier whose output constitutes an error signal for application to a frequency controlling input of the oscillator. The error signal is also applied to a threshold control circuit which causes an up/down counter to count up or down when the error signal lies outside a predetermined range. The counter total is converted to analog form and is applied to the control input in addition to the error signal.