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Re: S meter calibration -- HDSDR

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  • warrenallgyer
    In normal operating circumstances I would completely agree with you Bill. In the case of HDSDR the reason I do use the calibration is you have the ability to
    Message 1 of 10 , Jun 27 3:25 AM
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      In normal operating circumstances I would completely agree with you Bill.

      In the case of HDSDR the reason I do use the calibration is you have the ability to expand the scale of the spectrum display for single dB resolution. If you have calibrated it properly then the numbers on the vertical scale reflect that. Again, in normal QSO operations that does not have a lot of merit. However, in a controlled environment where the source impedance is set by the signal source or (better) a good set of attenuators, this calibration gives a very accurate indication of absolute power levels and an even better measurement of relative levels.

      The meter itself on HDSDR seems to me to be relative useless except for a quick indication.... and I never have understood why it sits at S6 on a quiet band.... a topic for another day.

      BTW.... to an earlier comment... I have bypassed the input attenuators in the two low ranges of my RXII just to preserve band to band calibration. It is good to check it on each band but I find mine holds calibration remarkably well from 3-30 MHz.

      Warren Allgyer - 9V1TD
      >
      > Hi,
      >
      > I don't really believe in S-meter "calibration". It's only useful to see
      > that one signal is stronger than another and maybe some wild notion
      > about how much stronger (or weaker). Yeah..I try to set mine somewhere
      > near 50 uV, too, but the inputs to receivers is rarely ever 50 ohms and
      > even if it is 50 ohms or padded to 50 ohms when you diddle the S-meter
      > it is NOT going to be 50 ohms when you expose it to 'the wild'. Worse,
      > it will change with frequency and band and change by a LOT.
      >
      > I give out RST reports based on what I hear and NOT what the S-meter
      > needle indicates. At my location I am not "plagued" with manmade noise
      > even though I have some. How much varies from day to day and hour to
      > hour but the noise is spread out up and down the bands so there are
      > quiet areas - sometimes large quiet areas - all over the bands. I
      > routinely listen to Q5 signals that do not tickle the S-meters. So would
      > that be some kind of nonsense like a 509? I heard that on the air once
      > or twice (grin). When I apply DSP filtering in the audio channel I copy
      > signals that I can't hear without the DSP and even some of them out from
      > under noises made by naughty television sets - loud and clear. I suspect
      > that SDRs use a little bit of DSP, so the S-meter isn't really critical.
      > Some of my radios don't even have an S-meter and it isn't missed.
      >
      > 73,
      >
      > Bill KU8H
      >
    • Bill Cromwell
      ... Hi Warren, I have started using some of the DSP soundcard software as test bench equipment. In that environment I agree 100 per cent with your comments.
      Message 2 of 10 , Jun 27 4:46 PM
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        On 06/27/2013 06:25 AM, warrenallgyer wrote:
        >
        >
        > In normal operating circumstances I would completely agree with you Bill.
        >
        > In the case of HDSDR the reason I do use the calibration is you have
        > the ability to expand the scale of the spectrum display for single dB
        > resolution. If you have calibrated it properly then the numbers on the
        > vertical scale reflect that. Again, in normal QSO operations that does
        > not have a lot of merit. However, in a controlled environment where
        > the source impedance is set by the signal source or (better) a good
        > set of attenuators, this calibration gives a very accurate indication
        > of absolute power levels and an even better measurement of relative
        > levels.
        >
        > The meter itself on HDSDR seems to me to be relative useless except
        > for a quick indication.... and I never have understood why it sits at
        > S6 on a quiet band.... a topic for another day.
        >
        > BTW.... to an earlier comment... I have bypassed the input attenuators
        > in the two low ranges of my RXII just to preserve band to band
        > calibration. It is good to check it on each band but I find mine holds
        > calibration remarkably well from 3-30 MHz.
        >
        > Warren Allgyer - 9V1TD
        >

        Hi Warren,

        I have started using some of the DSP soundcard software as "test bench"
        equipment. In that environment I agree 100 per cent with your comments.
        My head is wrapped up tightly around operating radios on the air and my
        comments will probably always reflect that. I think I said something
        like "in the wild". And I do attempt to setup my radios with S meters to
        that 50 uV spec - knowing that it won't really matter so very much when
        I expose the antenna terminals to the real world of HF operating.

        BTW - I do have a SoftRock kit waiting for some ham time allocated to
        the bench to put together (SoftRock-40-R on 20 meters). I seem to have
        found ways of abusing the DSP software (especially Spectran) so maybe
        I'll be able to subvert the SDRs, too. Wondering what pleasant surprises
        await.

        73,

        Bill KU8H
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