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Method and apparatus for filtering interference in a modem receiver

  • US 6,240,150 B1
  • Filed: 05/12/1998
  • Issued: 05/29/2001
  • Est. Priority Date: 05/12/1998
  • Status: Expired due to Term
First Claim
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1. An apparatus for reducing transmitter feed-through (Tx-FT) interference at a receiver of a modem operating at a first sampling rate to obtain a desired receive (Rx) signal having a spectrum at a carrier frequency for input into a QAM demodulator operating at a second sampling rate, said apparatus comprising:

  • (a) an analog filter having a filter mask for attenuating the Tx-FT interference to a first prescribed level and for passing the Rx signal at the carrier frequency;

    (b) a converter for converting output from the analog filter to a digital representation;

    (c) a first digital filter having a mask for passing the Rx signal at the carrier frequency and for attenuating the Tx-FT interference to a second prescribed level;

    (d) a mixer for shifting the carrier frequency of the spectrum of the Rx signal to an intermediate frequency, said mixer introducing at least one spectral transmitter feed-through image Tx-FT(I) and a receive signal image Rx(I);

    (e) a second digital filter having a mask for passing the Rx signal at the intermediate frequency and attenuating the image spectrums Tx-FT(I) and Rx(I); and

    (f) a decimator for downsampling the Rx signal at a downsampling rate from the second digital filter to match the second sampling rate of the QAM demodulator.

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