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Frequency tuning for a phase locked loop FM demodulator

  • US 5,446,411 A
  • Filed: 12/30/1993
  • Issued: 08/29/1995
  • Est. Priority Date: 09/29/1993
  • Status: Expired due to Term
First Claim
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1. Apparatus for setting up the tuning frequency of a phase locked loop including a voltage controlled oscillator, the apparatus comprising:

  • signal translation circuitry having a first input connected to receive a first voltage and a second input connected to receive a tuning voltage, the signal translation circuitry providing a control voltage to the voltage controlled oscillator which is dependent on the first voltage and the tuning voltage;

    frequency detection circuitry for comparing a frequency of a first signal output by the voltage controlled oscillator in response to said control voltage with a reference frequency and generating an output, wherein said tuning voltage is altered in response to the output of the frequency detection circuitry;

    a switch through which the tuning voltage is supplied and which is disconnected when the correct tuning voltage has been established; and

    a phase comparator connected to receive an FM modulated carrier wave as an input signal to the phase locked loop;

    wherein the voltage controlled oscillator has a second output which is supplied to said phase comparator to provide a voltage dependent on the phase difference between the second output of the voltage controlled oscillator and the input signal to the phase locked loop, the signal translation circuit providing to the voltage controlled oscillator a control voltage dependent on the voltage representative of the phase difference add on the preset tuning voltage when said switch is disconnected wherein said first voltage represents a demodulated output derived from said FM modulated carrier wave.

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