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Re: [softrock40] SSB demod, was Re: carrirer oscillator

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  • Robert McGwier
    This is not what we do on the Softrock40. The 11 kHz IF is used in the SDR-1000 in order to do exactly what Frank says and that is avoid 1/F, DC, junk, noise,
    Message 1 of 31 , Sep 30, 2005
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      This is not what we do on the Softrock40. The 11 kHz IF is used in the
      SDR-1000 in order to do exactly what Frank says and that is avoid 1/F,
      DC, junk, noise, etc. at 0. We are attempting to maximize the performance.


      On the softrock40, we have one oscillator on the board and it is not
      steerable. It provides a 48 kHz (theoretically) wide signal. We want
      to be able to tune ANYWHERE in that band. So we in fact live with the
      little hump of nasty at 0 frequency and tune the software oscillator
      from -24 to 24 kHz around the center frequency. We get single sideband
      demodulation by using a complex tap weight based bandpass filter using a
      very clever mathematical trick which allows us to do huge, fantastic
      filters with little more computation than much smaller (and poorer
      filters) done by the normal convolution.
      In the newer Windows release which should be out some time next week
      (depending no Eric's work schedule), PowerSDR will carry SR40 support
      in the base Windows code, and you can select SDR-1000, Softrock 40,
      and DEMO (no hardware required) mode from the setup panel after install
      or the installation wizard during install.

      Bob
      N4HY


      Frank Brickle wrote:

      >--- In softrock40@yahoogroups.com, Ramakrishnan Muthukrishnan
      ><vu3rdd@g...> wrote:
      >
      >
      >
      >>I couldn't understand how the mixing down to the magic number 11250
      >>
      >>
      >is
      >
      >
      >>obtained.. Would you mind explaining it a bit more?
      >>
      >>
      >
      >Sorry, typo. it's 11025. I got it right in a subsequent paragraph :-)
      >
      >
      >
      >>What I thought was that the signal at 7.056 gets downconverted to
      >>
      >>
      >zero
      >
      >
      >>IF and because of the soundcard sampling rate of 48 khz, we are able
      >>to tune +-24khz..
      >>
      >>
      >
      >11025 * 2 = 22050.
      >22050 * 2 = 44100.
      >
      >11025Hz is the center of the presumed "good" passband on typical
      >soundcards capable of handling audio CD rates.
      >
      >You don't want to downconvert to 0 because most soundcards have
      >bandpass filtering that roll off towards 0 and the Nyquist frequency.
      >If you downconverted to 0, you'd be filtering off most of the signal
      >before it got to your signal processing.
      >
      >Therefore you downconvert such that the signal of interest is in the
      >"best" part of the soundcard passband.
      >
      >The subsequent mixing stage (complex oscillator at -11025 or whatever)
      >then moves the signal down to 0 once it's in digital form. Then the
      >remainder of the processing takes place.
      >
      >73
      >Frank
      >AB2KT
      >
      >
      >
      >
      >
      >
      >
      >Yahoo! Groups Links
      >
      >
      >
      >
      >
      >
      >
      >
      >
      >


      --
      Laziness is the number one inspiration for ingenuity. Guilty as charged!
    • Bill Tracey
      When tuning an SDR 1000, if spur reduction is turned off, the DDS is tuner 11025 khz below the frequency of interest and the software (DttSP) oscillator is set
      Message 31 of 31 , Oct 1, 2005
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        When tuning an SDR 1000, if spur reduction is turned off, the DDS is tuner
        11025 khz below the frequency of interest and the software (DttSP)
        oscillator is set to -11025 khz to recover the frequency of interest. This
        works well but for the spurs from the DDS.

        If spur reduction is turned on (the normal case) the DDS is tuned to the
        nearest low spur point (as documented by some of the AD docs) and then
        DttSP oscillator is set to an appropriate value to recover the signal of
        interest. This generally results in a DttSP oscillator value between -9.5
        and -12.5 khz.

        On the SoftRock all the tuning is done with the DttSP oscillator and it
        ranges over +/- 24 khz - giving a tuning range of 48 khz.

        Regards,

        Bill (kd5tfd)
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