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Re: [softrock40] Re: DSP Image Rejection problem and some thoughts

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  • Fred Krom
    I had also a strong signal at 0Hz. After powering the SR40 with a 5V supply away from the computer and removing the earth connection (only one wire 14m
    Message 1 of 2 , Sep 20, 2005
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      I had also a strong signal at 0Hz.
      After powering the SR40 with a 5V supply away from the computer and removing the earth connection (only one wire 14m antena), there is only a very small puls left.
       
      I'm using a Delta 44 at 96KHz sample, own test software http://home.ict.nl/~fredkrom/pe0fko/AMSR/
       
      73, PE0FKO
      Fred
       
      ----- Original Message -----
      From: kd5nwa
      Sent: Wednesday, September 21, 2005 6:18 AM
      Subject: Re: [softrock40] Re: DSP Image Rejection problem and some thoughts

      Possible ground loops? Feed the power to the device from an
      independent floating power supply and see if the gunk in the center
      of the band goes down. Should not be too hard to try out, I would do
      it myself but I have not received my order yet. Better yet supply the
      power from a set of batteries.

      >Ahh the small board -- good question as to why is it so dang small -- I
      >think one of Tony's original goals for the gizmo was more of a
      >demonstration vehicle than an experimenter vehicle.   I for one wish it had
      >more working room, but I will say the experience has vastly improved my
      >soldering and rework in small places.
      >
      >The central hump you're seeing around 7.056 is normal.  The gunk you're
      >seeing around this is a consequence of the software architecture.  What is
      >happening is that the hardware takes the swath of RF centered and 7.056 and
      >downconverts  it such that 7.056 is essentially at DC.  Also down there
      >near DC is noise and 60 hz (and its harmonics) hum.  When one tries to tune
      >down near 7.056, the local software oscillator will be down in the 100's of
      >hz's.  The output of this will be Software LO +/- 60 Hz (and
      >harmonics).     Things will always be a bit messy near the center frequency
      >-- quieter less hummy sound cards can help, but I don't believe there's any
      >practical way to get rid of all the gunk in the middle.
      >
      >If you want to check out the hardware sans computer, and have a signal
      >generator and a scope you can put in a -30 dbm signal at 7.050 or so into
      >the antenna.  Look at the audio output on the I and Q channels -- it should
      >be about 6 khz (7.056 - 7.060) and on the order of about 0.8 Vpp or so
      >(don't take my nums as precision numbers, they are all from memory).  The
      >thing to look at is if both the I and Q channels are comparable in
      >magnitude -- if one is half the other something is not quite right in the
      >hardware.  You can also take a look at the phase relationship -- it should
      >be about 90 degrees between teh two channels.  While you've got the scope
      >out check the clocking signals to the FST3126.  If you've got a generator
      >and no scope, you might be able to compare the two channels with a good RMS
      >voltmeter.
      >
      >One last thing to check -- make sure you're using a sound card with a
      >stereo input.     Using a mono mic in input would give the symptom of not
      >being able to null the image.
      >
      >Hope this helps.
      >
      >Regards,
      >
      >Bill (kd5tfd)
      >
      >
      >
      >
      >
      >
      >Yahoo! Groups Links
      >
      >
      >
      >


      --
      Cecil
      KD5NWA
      <www.qrpradio.com>

      I fail to see why doing the same thing over and over and getting the
      same results every time is insanity: I've almost proved it isn't;
      only a few more tests now and I'm sure results will differ this time
      ...
    • kd5nwa
      Good, makes sense. The card is trying to read micro-volt signals, the ground used for power in a PC is horrible as far as noise is concerned. To make matters
      Message 2 of 2 , Sep 21, 2005
      • 0 Attachment
        Good, makes sense.

        The card is trying to read micro-volt signals, the ground used for
        power in a PC is horrible as far as noise is concerned. To make
        matters worse the power supply ground is connected to earth ground so
        there and lots of ways to get ground loops. The only way on a PC to
        eliminate a lot of that is to have the device not be powered by the
        filthy dirty signals of the PC.

        You should have one ground connection between the SR-40 and the PC,
        using the signal cable(power should not be flowing), and the power of
        the two units should be totally separate. That will cut down 60 Hz
        hum and it's harmonics.


        >I had also a strong signal at 0Hz.
        >After powering the SR40 with a 5V supply away from the computer and
        >removing the earth connection (only one wire 14m antena), there is
        >only a very small puls left.
        >
        >I'm using a Delta 44 at 96KHz sample, own test software
        ><http://home.ict.nl/~fredkrom/pe0fko/AMSR/>http://home.ict.nl/~fredkrom/pe0fko/AMSR/
        >
        >73, PE0FKO
        >Fred
        >
        >
        >----- Original Message -----
        >From: <mailto:KD5NWA@...>kd5nwa
        >To: <mailto:softrock40@yahoogroups.com>softrock40@yahoogroups.com
        >Sent: Wednesday, September 21, 2005 6:18 AM
        >Subject: Re: [softrock40] Re: DSP Image Rejection problem and some thoughts
        >
        >Possible ground loops? Feed the power to the device from an
        >independent floating power supply and see if the gunk in the center
        >of the band goes down. Should not be too hard to try out, I would do
        >it myself but I have not received my order yet. Better yet supply the
        >power from a set of batteries.
        >
        >>Ahh the small board -- good question as to why is it so dang small -- I
        >>think one of Tony's original goals for the gizmo was more of a
        >>demonstration vehicle than an experimenter vehicle. I for one wish it had
        >>more working room, but I will say the experience has vastly improved my
        >>soldering and rework in small places.
        >>
        >>The central hump you're seeing around 7.056 is normal. The gunk you're
        >>seeing around this is a consequence of the software architecture. What is
        >>happening is that the hardware takes the swath of RF centered and 7.056 and
        >>downconverts it such that 7.056 is essentially at DC. Also down there
        >>near DC is noise and 60 hz (and its harmonics) hum. When one tries to tune
        >>down near 7.056, the local software oscillator will be down in the 100's of
        >>hz's. The output of this will be Software LO +/- 60 Hz (and
        >>harmonics). Things will always be a bit messy near the center frequency
        >>-- quieter less hummy sound cards can help, but I don't believe there's any
        >>practical way to get rid of all the gunk in the middle.
        >>
        >>If you want to check out the hardware sans computer, and have a signal
        >>generator and a scope you can put in a -30 dbm signal at 7.050 or so into
        >>the antenna. Look at the audio output on the I and Q channels -- it should
        >>be about 6 khz (7.056 - 7.060) and on the order of about 0.8 Vpp or so
        >>(don't take my nums as precision numbers, they are all from memory). The
        >>thing to look at is if both the I and Q channels are comparable in
        >>magnitude -- if one is half the other something is not quite right in the
        >>hardware. You can also take a look at the phase relationship -- it should
        >>be about 90 degrees between teh two channels. While you've got the scope
        >>out check the clocking signals to the FST3126. If you've got a generator
        >>and no scope, you might be able to compare the two channels with a good RMS
        >>voltmeter.
        >>
        >>One last thing to check -- make sure you're using a sound card with a
        >>stereo input. Using a mono mic in input would give the symptom of not
        >>being able to null the image.
        >>
        >>Hope this helps.
        >>
        >>Regards,
        >>
        >>Bill (kd5tfd)
        >>
        >>
        >>
        >>
        >>
        >>
        >>Yahoo! Groups Links
        >>
        >>
        >>
        >>
        >
        >
        >--
        >Cecil
        >KD5NWA
        ><www.qrpradio.com>
        >
        >I fail to see why doing the same thing over and over and getting the
        >same results every time is insanity: I've almost proved it isn't;
        >only a few more tests now and I'm sure results will differ this time
        >...
        >
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        --
        Cecil
        KD5NWA
        <www.qrpradio.com>

        I fail to see why doing the same thing over and over and getting the
        same results every time is insanity: I've almost proved it isn't;
        only a few more tests now and I'm sure results will differ this time
        ...
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