US 4,053,933 A · Adaptive phase locked loop filter for television tuning

Abstract

A low-pass filter useful in association with a phase locked loop for controlling the frequency of a voltage controlled oscillator on the basis of an error signal resulting from a comparison between a reference signal and the oscillator's output. The low-pass filter comprises means responsive to the error signal for developing an output current for charging or discharging a capacitive network according to the frequency and phase deviations represented by the error signal. For large frequency and phase deviations extended charging or discharging currents provide high acquisition speeds whereas, as the frequency and phase deviation represented by the error signal decreases, drastically reduced charging or discharging currents result thereby effectively reducing the bandwidth of the filter for facilitating reference signal sideband supression as frequency "lock-on" is approached and when phase lock is achieved.

Claims11

Dependent claims

In a phase locked loop of the type having a phase comparator developing an error signal for controlling the output of a voltage controlled oscillator through a low-pass filter, the improved low-pass filter comprising:

amplifying means having a directional control input and a gain control input both connected for receiving said error signal, said amplifying means being operable for developing an output current having a direction and duty cycle determined by, respectively, the polarity of said error signal applied to said directional control input and the pulse width of said error signal applied to said gain control input;
and
a capacitive network connected for charging and discharging in response to said output current for controlling the output of said voltage controlled oscillator.

In a television receiver tuning system of the type having a phase comparator developing an error signal for controlling the output of a voltage controlled oscillator in response to the low-pass filter of a phase locked loop to achieve tuning of a selected television channel, said phase locked loop including a reference signal, the improvement wherein said low-pass filter comprises;

a. a capacitive network connected for developing a tuning voltage for controlling the frequency of said voltage controlled oscillator;
and
b. control means responsive to said error signal and operable as a current source or sink for rapidly charging or discharging said capacitive network toward a certain voltage corresponding to a particular frequency in said selected television channel;
c. said control means being also operable in response to said error signal and under the control of said phase locked loop as a pulse width modulated sample and hold circuit for adjusting said tuning voltage for achieving said certain voltage while facilitating reference signal sideband suppression.

1 more element

Dependencies are inferred from the claim text.

Adaptive phase locked loop filter for television tuning

Life of this patent

Adaptive phase locked loop filter for television tuning

Life of this patent

Abstract

Claims11