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CIC decimation filter increases aliasing and imaging in the signal bandwidth?

Discussion in 'Electronic Basics' started by boki, Jun 1, 2004.

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  1. boki

    boki Guest

    Dear All,

    I am trying [cicdecimate] toolbox, my signal is a 2nd order single-bit
    sigma-delta modulator output, the sigma-delta modulator output
    spectrum is correct, but the decimation filter output increase the
    low-frequency noise, is this correct? I only use a CIC filter no other
    low-pass filter connected.

    CIC decimation filter increase aliasing and imaging in the signal


    Voice band sinwave -> 2nd order SDM -> CIC decimation filter

    SDM spectrum:

    CIC decimation filter spectrum:
    Noise++ in signal band, quantization noise--

    Thanks a lot.

  2. Joel Kolstad

    Joel Kolstad Guest

    Yes. If you decimate by a factor of k, the spectrum gets sliced up into k
    equally spaced pieces that are then all summed together. Hence, you get
    more noise aliased to both the passband and stopband, but of course if
    you're something like -40dB in the stopband this creates .087dB additional
    ripple in the passband and is generally not noticeable. On the other hand,
    each fold adds about 6dB noise to your stopband... ouch!

    This is a basic property of decimation that any book by Mitra or
    Oppenheim/Schafer will discuss (and go through the mathematical treatment
    of) in length.

    ---Joel Kolstad
  3. boki

    boki Guest

    Got it! Thank you very much.

    Best regards,

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