Clamping circuit for a PLL tuning system

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.

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.

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.

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.