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Elimination of phase noise and drift incident to up and down conversion in a broadcast communication system

  • US 5,109,532 A
  • Filed: 01/30/1990
  • Issued: 04/28/1992
  • Est. Priority Date: 01/30/1990
  • Status: Expired due to Term
First Claim
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1. A communication system, comprising a transmitter including a modulator for frequency or phase modulating a baseband information signal at a modulation frequency, a first local oscillator for providing a first RF signal at a carrier frequency, and a first mixer for mixing the modulated information signal with the first RF signal to provide an upconverted signal for transmission;

  • and a receiver including a second local oscillator for providing a local RF signal at a frequency equal to the difference between the carrier frequency and an intermediate frequency, a second mixer for mixing the upconverted signal received from the transmitter with the local RF signal to provide a downconverted signal at the intermediate frequency, and a demodulator for demodulating the downconverted signal to provide the information signal at the baseband frequency, wherein the downconverted signal includes phase noise and drift caused by imperfections in said local oscillators;

    wherein the transmitter further includes means for adding a pilot frequency component to the modulated information signal in the transmitter prior to mixing the modulated information signal with the first RF signal, whereby the upconverted signal and the downconverted signal each includes a pilot frequency component; and

    wherein the receiver further includesa phase-locked loop for providing a signal at a frequency equal to the sum of the intermediate frequency and the pilot frequency by tracking the pilot frequency component of the downconverted signal; and

    means for frequency locking the second local oscillator so that the sum signal provided by the phase-locked loop is adjusted to a desired frequency;

    wherein the phase-locked loop includes a third mixer for mixing said sum signal provided by the phase locked loop with the downconverted signal to thereby provide a downconverted component for said demodulation at the difference between the pilot frequency and the modulation frequency that is free from the phase noise and drift caused by the local oscillators.

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