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17224Re: missing 70 dB

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  • adibene
    Jan 1, 2008
      --- In softrock40@yahoogroups.com, "xeloradio" <sealions@...> wrote:
      >
      > I have a question about what is displayed as dBm in Winrad,
      > SpectrumLab, etc. Using a Softrock lite at 10.7 MHz, If I put in a
      > calibrated signal of -40 dBm, the peak is shown at -70 dBm. I would
      > expect to see both the -40dBm I put in, and a gain in power of 40 dB
      > from the op-am voltage gain of 100 in the Softrock,,ie 0dBm total.
      >

      Dave,

      I cannot speak for the other programs, but the dB (not dBm)
      indication in Winrad is relative to the saturation value of the ADC on
      the sound card.

      It is impossible to have an absolute indication, as it depends on
      factors outside the control of the program, as e.g. the setting of the
      level sliders in the Windows mixer panel. Or, if you use a Delta 44,
      you will have noticed that in the Delta control panel you can set the
      input level either to -10dBV, or Consumer, or +4dBu.

      So, in order to have the dB indication of Winrad corresponding to the
      dBm level of the input signal, a calibration step would be necessary,
      that should be performed on every installation, and repeated each time
      the hardware configuration would change.

      73 Alberto I2PHD
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