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Direct conversion receiver with dithering local carrier frequency for detecting transmitted carrier frequency

  • US 5,091,921 A
  • Filed: 04/20/1990
  • Issued: 02/25/1992
  • Est. Priority Date: 04/20/1989
  • Status: Expired due to Term
First Claim
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1. A receiver for receiving a digitally modulated radio frequency signal at a transmitted carrier frequency, which has a frequency deviation, comprising:

  • a first voltage-controlled oscillator for generating a first local carrier;

    a mixer circuit for mixing the received signal with said first local carrier for converting the frequency of the received radio signal to a baseband frequency;

    a low-pass filter for filtering a baseband signal from said mixer circuit;

    a limiting amplifier connected to an output of said low-pass filter;

    a frequency detector coupled to an output of said limiting amplifier for generating a signal representative of a variation frequency of the received radio signal, said variation frequency equaling the deviation frequency of the transmitted carrier frequency minus an offset between the transmitted and first local carrier frequencies;

    an average circuit for averaging the signal from the frequency detector to produce a signal representative of the deviation frequency of the received radio signal;

    a comparator circuit for detecting whether the signal from the frequency detector lies in or outside of the neighborhood of a frequency represented by the signal from the average circuit; and

    a ramp generator for periodically generating a ramp voltage of constant amplitude when the signal from the frequency detector is detected as lying outside of said neighborhood and holding said ramp voltage at an instantaneous value when said signal is detected as lying in said neighborhood, said first voltage-controlled oscillator being responsive to an output voltage of said ramp generator to control the frequency of said first local carrier.

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