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missing 70 dB

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  • xeloradio
    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,
    Message 1 of 2 , Jan 1, 2008
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      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.

      details: the soundcard is a delta44, noise level displayed is around
      -125 dBm with no input. The overall system, preamp, mixer, softrock,
      works well for real signals. The calibrated signal is from an AN9951
      DDS with programmable power output from I0CG. A calculation of
      received signal strength from TV stations gives roughly the same 70 dB
      issue.

      Dave
      K0IMH
    • adibene
      ... 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
      Message 2 of 2 , Jan 1, 2008
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        --- 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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