[hpsdr] hpsdr] About the Dynamic Range of Receivers

[John B. Stephensen]

There are also intermodulation distortion products generated by the ADC. The levels are never specified for audio ADCs and only occasioally specified for high-speed ADCs. The IMD levels are always higher than the quantitization noise and are critical if most of the filtering is done via DSP.

73,

John
KD6OZH

  ----- Original Message ----- 
  From: KA2WEU at aol.com 
  To: hpsdr at hpsdr.org 
  Sent: Tuesday, August 08, 2006 18:19 UTC
  Subject: [hpsdr] hpsdr] About the Dynamic Range of Receivers


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    About the Dynamic Range of  Receivers

    It has nothing to do with the instantaneous dynamic range, which is
    defining
    the radio's capabilty to discern a weak signal (near the noise floor) in
    the
    presence of a much larger signal. The maximum possible  ratio of the two
    signal's levels is called instantaneous dynamic range).
    The overall instantaneous dynamic range of the receiver is not limited by
    the nyquist dynamic range of the 16-Bit ADC (about 95 dB).

    N1UL:

    Please note ADC's quantization noise having a part in the receiver's
    overall noise figure (NF): At the ADC input the thermal noise from the
    front end and the quantization noise will add up. To keep the ADC's part in
    overall NF as low as possible (<1dB), the gain from antenna to ADC should
    be set so that for the front end noise will dominate the quantization noise
    from ADC by about 10 dB. This will further reduce the overall instantaneous
    dynamic range of the receiver !

    DSP is decimating the ADC sampling rate from 100 kS/sec down to rates
    appropriate to the actual
    CW / SSB signal bandwidths of (50)300 ... 3100(4500) Hz (meaning  Fs=
    781.25 / 1.5625 / 3.125 / 6.25 / 12.5 kS/sec)./N1UL

    Noise coming from ADC (including the  quantization noise) is reduced by
    just
    the ratio of ADC's nyquist bandwidth (=50  kHz) to the signals bandwidth,
    thus
    giving a corresponding gain in dynamic range  of  ...     >
    > _______________________________________________




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