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RE: [softrock40] Re: Interference from PC?

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  • Bill Tracey
    I believe the gunk you see near the center frequency with or w/o hardware attached is an artifact of the signal processing architecture, and noise near DC on
    Message 1 of 41 , Sep 18, 2005
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      I believe the gunk you see near the center frequency with or w/o hardware
      attached is an artifact of the signal processing architecture, and noise
      near DC on the audio input (ie 60 hz hum,. etc)

      How the software tuning works is by having a software mixer fed with the
      local oscillator to dowconvert the signal of interest. In the case of the
      SoftRock, you have the hardware QSD downconverting and quadrature sampling
      a 48 khz swath of RF centered on 7.056 Mhz. To listen to a signal at 7.060
      MHz, the software mixer is set to run with a local oscillator at 4 khz to
      mix down the 7.056 Mhz centered signal. As you tune closer to the center
      frequency, the freq of the software local oscillator gets lower and
      lower. Very near the center frequency, the local osc will be in the 100's
      of hz, Mixed with 60 hz (and harmonics) hum this will tend to product
      responses at 60Hz (and harmonics) +/- SoftwareLO etc -- thereby getting
      you the gunk around the center frequency.

      I will have to admit that my understanding of this phenomenon is not as
      crisp as I would like it to be. As Tony said in a prior post, there's some
      hope that some of this can be eliminated with DSP magic. One approach
      would be to record and characterize the noise characteristics with no input
      and then try and remove them with DSP magic when fed with a real
      signal. One could do this in either the time domain or frequency domain,
      although I'm not sure how one would maintain sync with the noise signal in
      the time domain. My DSP skills are not strong enough (yet) to really know
      how feasible such an approach would be.

      Another (potentially naive) approach might be to try a different tuning
      approach. If one is interested in a sig at 7.060, I'd think one could take
      an FFT of the input signal, zero the FFT bins outside the passband of
      interest, shift the spectrum over by 4 khz and then IFFT the result to
      recover the signal of interest. I think this should work, but not having
      tried it really can't say for sure. Also not too clear in my mind if this
      would be better than the software mixing approach currently used as you get
      closer to DC.

      One thing to be aware of -- in a more complete SDR with a frequency agile
      downconverter, you typically don't tune in software down near DC. For
      example, on the SDR 1000, the tuning is always such that you're
      approximately 11khz above DC for software tuning. The reason for this
      being approximate is that the DDS tuning is limited to tuning to freqs
      where the DDS generates a minimum of spurs.

      I will be the first to admit I'm somewhat of a neophyte with DSP. SDR is
      an area rich with opportunities for learning and experimenting, and we've
      now got hardware and software accessible to the amateur community to
      experiment with. Hoping folks get in there and do some experimenting and
      learning.

      Cheers,

      Bill (kd5tfd)


      At 01:25 PM 9/18/2005, Stan Rife wrote:
      > Tony, one thing I noticed, when just playing around with the SDR
      > software, was the 600 Hz tone that comes from the soundcard (?) at the
      > 7.056 freq. after the Fixed HW oscilator is set up per the instructions.
      > This is, of course, without the SoftRock hardware installed, as I do not
      > even have my kit YET. (boo hoo).
      > Is this oscilator tone, at that freq., something that will always be
      > there? I saw a procedure in the manual that said something about going
      > above 7.056, as much as the test oscilator is below 7.056 and adjusting
      > for a null. I am not quite clear on all of this. I do have an XG-1, and I
      > assume that is what this is refering to.
      >
      >
      >Stan Rife
      >W5EWA
      >Houston, TX
      >K2 S/N 4216
      >
    • Bob Hillard
      Rein, When I operate on the low bands (40M thru 160M) I have to turn off my PC s. In fact I sometimes have had to unplug them completely, as the power supply
      Message 41 of 41 , Oct 29, 2005
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        Rein,

        When I operate on the low bands (40M thru 160M) I have to turn off my
        PC's. In fact I sometimes have had to unplug them completely, as the
        power supply isn't completely off when the On/Off switch is in the Off
        position.

        I found the problem to be caused by the computer's switching power
        supply. In fact I lost a power supply in one of my computers recenly,
        and replaced it with a better grade supply, and then was happy to find
        that the noise from that computer was gone.

        Try unplugging your computer completely and see if that quiets things
        down.

        Bob WA6UFQ

        --- In softrock40@yahoogroups.com, "observer35" <rein0zn@i...> wrote:
        >
        > Hi all,
        >
        > Just new to the group.
        >
        > Does anybody experience interference from the PC on 40 meters?
        >
        > What about noise and garbage on the 5 V USB line?
        >
        > 73 Rein W6/PA0ZN
        >
        > http://www.nitehawk.com/linrad_dat/
        >
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