Method and apparatus for filtering interference in a modem receiver

摘要

A method and apparatus for filtering interference (e.g. transmitter feed-through and RF) in a modem receiver. The method of reducing transmitter feed-through (Tx-FT) interference at a receiver of a modem to obtain a desired receive (Rx) signal having a spectrum at a carrier frequency for input into a QAM demodulator includes attenuating the Tx-FT interference and passing the Rx signal at the carrier frequency using an analog filter. The output signal from the analog filter is converted to a digital representation by a analog-to-digital (ADC) converter. A digital band pass filter is used to pass the Rx signal at the carrier frequency and to attenuate the Tx-FT interference further. A mixer is then used to shift the carrier frequency of the spectrum of the Rx signal to an intermediate frequency. The mixer introduces at least one spectral transmitter feed-through image Tx-FT(I) and a receive signal image Rx(I). A digital low pass filter is used to pass the Rx signal at the intermediate frequency and attenuating the image spectrums Tx-FT(I) and Rx(I). A final decimation function is performed to downsample the Rx signal at a downsampling rate from the second digital filter to match the sampling rate of the QAM demodulator. Interference filtering is improved by dividing the functions between an analog filter prior to conversion and a pair of digital filters after conversion.