Clamping circuit for a PLL tuning system

• US 4,903,329 A
• Filed: 06/15/1987
• Issued: 02/20/1990
• Est. Priority Date: 06/15/1987
• Status: Expired due to Term

• Current Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Original Assignee: Motorola, Inc. (Motorola Solutions, Inc.)
• Inventors: Ecklund, Lawrence, Marik, Charles J.
• Primary Examiner(s): Safourek, Benedict V.
• Assistant Examiner(s): Kuntz, Curtis
• Application Number:
  US07/061,757
• Time in Patent Office: 981 Days
• Field of Search: 455/260, 455/265, 455/316, 455/317, 455/257-259, 455/139, 455/164, 455/182, 455/264, 455/234, 455/183, 331/4, 331/11, 331/14, 381/4, 381/13, 381/15
• US Class Current: 455/182.1
• CPC Class Codes: H04H 20/49 for AM stereophonic broadcast systems

Abstract

An AM Stereo receiver is described which includes an AGC having an output which indicates whether the received signal has significantly decreased in level and having a clamping circuit for controlling the voltage at the input of a VCO. The clamping circuit is further defined as having a window detector for sensing a change in the voltage at the input of the VCO according to a predetermined threshold; a selectable controller, responsive to the window detector, for adjusting the voltage at the input of the VCO; and a window reduction circuit, responsive to the output signal of the AGC, for altering the predetermined threshold to provide the controller with a more sensitive indication of a drift in the voltage at the input of the VCO. The receiver additionally includes a conventional locked loop driver for steering the VCO according to the phase of the received input signal. When the AGC output signal indicates that the signal quality of the received signal has significantly decreased in level, or when the window detector indicates that the system is tuned outside of an established narrow "tuned" window, the locked loop driver is disabled to prevent the locked loop driver from introducing a beat note into the VCO.

Claims

1. In a receiver receiving a received signal, a clamping circuit for controlling the voltage coupled to the input of a voltage controlled oscillator (VCO) comprising:

window detection means including;

first means for sensing a change responsive to the frequency of said received signal in the voltage coupled to the input of the VCO;

second means for establishing a plurality of thresholds with respect to the change in voltage; and

third means, responsive to said second means, for indicating the sensed voltage has exceeded at least one of said thresholds; and

clamping means, responsive to said window detection means, for selectively controlling the voltage coupled to the input of a VCO according to the thresholds established within the window detector means,

wherein said window detection means includes a differential driver having an input coupled to the voltage at the input of the VCO and having a pair of current mirrors each respectively operating responsive to the differential driver,

wherein said window detection means includes a resistor network, coupled to the pair of current mirrors, for setting the thresholds,

wherein one of said current mirrors from one pair crosses over one of said current mirrors from the other pair.

2. In a receiver which includes sensor means for detecting a decrease in the level of a received signal, a clamping circuit for controlling the voltage coupled to the input of a VCO, comprising:

window detection means for sensing a change responsive to the frequency of said received signal in the voltage at the input of the VCO according to a predetermined threshold;

window reduction means, responsive to the sensor means, for altering the predetermined threshold; and

clamping means, responsive to said window detection means, for selectively controlling the voltage coupled to the input of the VCO,

wherein said window reduction means responds to said sensor means by lowering the predetermined threshold such that the clamping means responds to smaller changes in the voltage at the input of the VCO,

wherein said window reduction means further responds to said sensor means by signalling the clamping means to initiate said selective control of the voltage at the input of the VCO.

3. In a receiver which includes sensor means for detecting a decrease in the level of a received signal, a clamping circuit for controlling the voltage coupled to the input of a VCO, comprising:

window detection means for sensing a change responsive to the frequency of said received signal in the voltage at the input of the VCO according to a predetermined threshold;

window reduction means, responsive to the sensor means, for altering the predetermined threshold; and

clamping means, responsive to said window detection means, for selectively controlling the voltage coupled to the input of the VCO,

further including phase driver means, responsive to the sensor means, for controlling the voltage at the input of the VCO when the phase locked loop is locked to the received signal.

4. In a tuning system of a receiver which includes sensor means for detecting a decrease in the level of a received signal, a clamping circuit for controlling the voltage at the input of a VCO, comprising:

a baseband discriminator for indicating the direction that is required to tune the receiver to the received signal;

window detection means for sensing a change responsive to the frequency of said received signal in the voltage at the input of the VCO according to a predetermined threshold; and

clamping means, responsive to said window detection means and said baseband discriminator and the sensor means, for selectively controlling the voltage at the input of the VCO at at least two separate rates,

wherein the clamping means includes first charge pump means, responsive to said baseband discriminator, for controlling the voltage at the input of the VCO at a first rate and also includes second current pump means, responsive to said window detection means, for controlling the voltage at the input of the VCO at a second rate, which is substantially less than said first rate.

5. In a AM stereo receiver which includes sensor means for detecting a decrease in the level of an AM received signal, and a lock signal for indicating when the receiver is locked to the received signal, a circuit for controlling the voltage at the input of a VCO, comprising:

a baseband discriminator, responsive to absense of the lock signal, for indicating the polar direction that is required to tune the receiver to the received signal;

phase driver means, responsive to the sensor means, for controlling the voltage at the input of the VCO when the receiver is locked to the received signal;

window detection means for sensing at least two degrees of voltage change at the input of the VCO according to one or more predetermined thresholds; and

window reduction means, responsive to said sensor means and said window detection means, for altering one of said thresholds such that one of the degrees of sensed voltage change is lessened;

tune indicator means, responsive to said window detection means, for indicating when the receiver is tuned to the AM received signal;

clamping means, responsive to said window detection means, said baseband discriminator and the sensor means, for selectively controlling the voltage at the input of the VCO at a first rate selected by the baseband discriminator, and at a second rate selected by the window detection means.