Current source loop filter with automatic gain control
AssigneeCyrix Corporation
InventorMark E. Burchfield
Life of this patent
Estimating the term…
Abstract
A current source suitable for use as a loop filter in a phase-locked loop having two aspects of automatic gain control one for prohibiting the voltage controlled oscillator from stopping as the input voltage approaches the limits of the oscillator and another to compensate for current limiting drain to source voltage drops.
Claims8
A loop filter with automatic gain control operating between a positive and a negative voltage supply rail, comprising:
- (a) a current source having first and second inputs coupled to up rand down control signals and first and second current outputs;
- (b) a first low pass filter having an input coupled to the first current output and an output providing a first bias voltage;
- (c) a second low pass filter having an input coupled to the second current output and an output providing a second bias voltage;
- and,
- (d) the current source reducing the first current output in response to the first bias voltage approaching the positive voltage supply rail and increasing the second current output in response to the second bias voltage approaching the negative voltage supply rail.
In a phase-locked loop including a phase comparator and a voltage controlled oscillator, a loop filter comprising:
- (a) a current source having first and second inputs coupled to up and down control signals from the phase comparator and first and second current outputs coupled to the voltage controlled oscillator;
- (b) a first low pass filter having an input coupled to the first current output and an output providing a first bias voltage;
- (c) a second low pass filter:having an input coupled to the second current output and an output providing a second bias voltage;
- and,
- (d) in response to the first bias voltage approaching a positive voltage supply rail, the current source reducing the first current output and in response to the second bias voltage approaching a negative voltage supply rail, the current source increasing the second current output, so that the voltage controlled oscillator maintains oscillation.
In a phase-locked loop with a loop filter providing first and second bias voltages to a voltage controlled oscillator and operating between positive and negative voltage supply rails, a method for automatic gain control comprising the steps of:
- (a) supplying a first and second current to a first and a second low pass filter respectively;
- (b) reducing the first current in response to the first bias voltage approaching the positive voltage supply rail;
- and,
- (c) increasing the second current output in response to the second bias voltage approaching the negative voltage supply rail.
Dependencies are inferred from the claim text.