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Re: [sdr-radio-com] Re: Spur in the middle of the spectrum...

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  • Alan
    ... From: Henk [pd5dp] Subject: RE: [sdr-radio-com] Re: Spur in the middle of the spectrum... ... The Delta 44 series does not usually have a spike at the
    Message 1 of 14 , Dec 1, 2010
      ----- Original Message -----
      From: "Henk [pd5dp]"
      Subject: RE: [sdr-radio-com] Re: Spur in the middle of the spectrum...



      >
      > TNX Terry. I am looking forward to that. I am still a novice regarding the
      > SDR thing. At the moment I am using a M-Audio Delta44 as my soundcard, but
      > also this soundcard has some kind of noise in the middle of the spectrum.
      > I
      > tried also yesterday my onboard soundcard. The result was poor. Much worse
      > than the M-audio Delta44.
      >

      The Delta 44 series does not usually have a spike at the zero Hz position.
      Anything round the centre is usually due to ground loops that should be
      reduced by careful attention to grounding and ground paths.
      In any case it is best to avoid this area for reception, in fact a spike
      helps remind you of where it is!
      I'm not sure about spur reduction, are these actually spurs? What is the
      purpose of spur reduction? I thought it was associated with some DDS.

      73 Alan G4ZFQ
    • Leif Asbrink
      Hello Henk, ... (from your first posting on this thread) ... (from your second posting on this thread) It is non-trivial to get rid of the center spur in a
      Message 2 of 14 , Dec 2, 2010
        Hello Henk,


        > At the moment I am using a M-Audio Delta44 as my soundcard, but
        > also this soundcard has some kind of noise in the middle of the spectrum. I
        > tried also yesterday my onboard soundcard. The result was poor. Much worse
        > than the M-audio Delta44.
        (from your first posting on this thread)

        > In my case I have some spur in the middle of the bandpass of my SDR. Simon
        > has got an option to reduce this. It is a little better, but it is still
        > present and it is anoying me a little.
        >
        > Does someone know why this is present there and what a solution is to reduce
        > this? Is it something about the adjustment of my SDR on the hardware side?
        (from your second posting on this thread)

        It is non-trivial to get rid of the center spur in a direct conversion
        radio that uses a PC soundcard.

        The spur is caused by several different mechanisms in the hardware.
        The way to remove the spur is to first reduce it to a modest
        level and then apply front end RF gain that rises the antenna noise
        level well above the spur level.

        I can not judge from your image what the dominating problem
        might be because the resolution is too low and I can not read
        the frequency scales. As a first step I suggest that you
        replace the antenna with a dummy load. Then look at the
        spectrum and you should see the spur but no other signals.
        In cas this simple test shows an improvement you suffer
        from ground loop problems. Try to connect the antenna
        ground to the dummy load ground. If that affets the spur
        level you can eliminate that problem with an RF transformer
        at the input.

        The Delta 44 suffers from design errors so it has a rather
        strong enter spur even if nothing is connected to it.
        This link shows how you can modify the card to eliminate
        the errors:
        http://www.sm5bsz.com/linuxdsp/hware/delta44a.htm
        Figure 1 shows the card in its original shape with
        nothing connected to the input. The center spur is symmetric
        and consists of two peaks separated by 10 to 20 Hz.
        (Different in the two hannels)

        The spur is 1/F noise that would cause a very high level
        at frequency zero, but there is a digital high pass filter
        in the Delta 44 that starts to attenuate at 20 Hz or
        so (I do not remember the exact response) The high
        pass filter attenuates 0Hz infinitely an that causes
        the double peak.

        The dominating source of 1/F noise is poor decoupling
        of the 5V DC supply lines to the A/D converters. See
        figure 4.

        The modifications lower the center spur by about 20 dB
        as you can see here:
        http://www.sm5bsz.com/linuxdsp/hware/delta44.htm
        Figures 1 and 2 have the center spur at 48 kHz.
        The modifications clean up the spectrum in several other
        ways too.

        I suggest that you disconnect the cable from the Delta 44
        and look at the spectrum. Then connect only the ground
        pin(s) from your radio hardware. In case you see a difference
        you suffer from the ground loop error. Step 1 here:
        http://www.sm5bsz.com/linuxdsp/hware/d44step.htm

        Your mixer is likely to produce 1/F noise and there might
        be pick up of magnetic fields and many other things. They
        can all be mastered and give you nice spectra like these:
        http://www.sm5bsz.com/linuxdsp/run/pmmx266.htm

        There is an infinitely deep notch at the enter but it is so
        narrow that it is unlikely to cause any problems. If the
        carrier of an AM station would fall exactly on it a normal
        AM detector would fail however.

        73

        Leif / SM5BSZ
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