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19797Re: [SDR#] Using SDR# as spectrum analyzer

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  • Shirley Márquez Dúlcey
    Mar 13, 2017
      In one sense, the 0dBFS value is not arbitrary. We're talking about digital receivers here, so the maximum possible input value that the A/D converter can represent is well defined. That value is 0dBFS.

      But in another sense it's arbitrary, because there is no defined relationship between that value and the amplitude of the signal coming from the antenna. If your hardware has a gain setting that affects the amount of amplification or attenuation before the A/D, as RTL-SDR dongles do, the relationship between 0dBFS and signal strength at the antenna is not constant but will change with the gain setting.

      Having the input signal ever exceed 0dBFS is very bad. The converter will clip it to 0dBFS because it's unable to represent any higher value. That clipping will cause severe audible distortion for most types of signal. If your signals are regularly exceeding 0dBFS you should adjust the gain if possible, incorporate attenuation in front of the receiver, or use a less effective antenna. Some A/D converters misbehave when signals approach 0dBFS so you might have to keep them a bit lower than that; -1 or -2 dBFS peaks are usually safe.

      I think you'll find dBFS is dB full scale, meaning signal level relative to the maximum arbitrary value reflected on the screen display.

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