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Re: Noise blanker fails on adjacent strong signals.

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  • Leif Asbrink
    Hello Paul, ... We can do better than that. Linrad puts notch filters on all strong signals to avoid having them into the blanker. The pulses become higher and
    Message 1 of 6 , Feb 29, 2012
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      Hello Paul,

      > In SDR it would be nice to have a noiseblanker with
      > a user definable detection window.
      > If this detection window could be visualized, you
      > could easily avoid NB degradation by strong adjacent signals.
      We can do better than that.

      Linrad puts notch filters on all strong signals to avoid
      having them into the blanker.
      The pulses become higher and shorter as compared to
      what would happen if the bandwidth were limited as you
      suggest. That makes the blanker work better.

      Linrad is far from ideal however. We can do very much
      better (in principle)

      > It is a big benefit in Perseus to see that NBW clears
      > the waterfall window. Combined with means to avoid
      > strong signals it could be quite ideal.
      I think many operators have gone qrt because they
      feel the situation is hopeless. The best algorithms
      available today are far from the best that could be done.

      I have been asking for wideband files a great number
      of times on a large number od SDR mailing lists.
      From the response I get I must conclude that QRN
      problems occur very seldomly. That is quite contrary
      to my own experience from when I had an antenna.....

      > You can now partly avoid strong signals by choosing
      > 125kS/s and scrolling strong stations out of the
      > receiver passband.
      Sure. But when the QRN is static rain such a strategy
      would not work at all. The pulses are then spaced too close
      in time. Also powerline noise may be problematic. The
      pulses could be trains of a dozen spikes that repeat
      at 100 Hz. With pulses separated by 0.5 ms a 100 kHz
      filter that stretches each pulse to perhaps 0.02 ms would
      cause a small loss of the desired signal, but more importantly
      it would create AM sidebands with a lot of signal energy
      around every signal in the passband.


      73

      Leif / SM5BSZ
    • pd0psb@hotmail.com
      ... That is ofcourse even smarter! ... Take my word on it that the urbanized QRM situation is not alleviating :-) What I do notice is that a new generation of
      Message 2 of 6 , Mar 1, 2012
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        > Linrad puts notch filters on all strong signals to avoid
        > having them into the blanker.
        > The pulses become higher and shorter as compared to
        > what would happen if the bandwidth were limited as you
        > suggest. That makes the blanker work better.

        That is ofcourse even smarter!

        > I think many operators have gone qrt because they
        > feel the situation is hopeless. The best algorithms
        > available today are far from the best that could be done.
        > I have been asking for wideband files a great number
        > of times on a large number od SDR mailing lists.
        > From the response I get I must conclude that QRN
        > problems occur very seldomly. That is quite contrary
        > to my own experience from when I had an antenna.....

        Take my word on it that the urbanized QRM situation is not alleviating :-) What I do notice is that a new generation of radio enthusiats just don't know any better. They think noise levels have always been this high and are a natural part of HF listening.

        As I decided to purchase a second Perseus for local use it is more easy now to make recordings for me. I'll try to make some very specific NB challenges for you if the situation occurs.

        I also found some new forms of QRM. One is said to be genrerated by the widespread Wii playstations. It seems a very fast (over 1kHz) pulsation whine around 2-3MHz. Most NBs don't appear to keep up effeciency with high speed type pulses. (at these frequencies it is ofcourse harder to distinguish pulses from wanted modulation)

        Another form of QRM is multiple carriers with precize spacing over a very large spectrum area. These carriers are reported to be as dense as 4kHz or 8kHz spaced. It would be a good SDR addition if a wideband comb-filter solution for such situations was investigated.

        Best 73
        Paul
        PD0PSB

        >
        > 73
        >
        > Leif / SM5BSZ
        >
      • Leif Asbrink
        Hello Paul, ... Good:-) I look forward to that:-) ... If someone on this list can make a recording I would be very interested:-) ... Have a look here:
        Message 3 of 6 , Mar 1, 2012
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          Hello Paul,

          > As I decided to purchase a second Perseus for local use
          > it is more easy now to make recordings for me. I'll try
          > to make some very specific NB challenges for you if the
          > situation occurs.
          Good:-) I look forward to that:-)

          > I also found some new forms of QRM. One is said to be
          > genrerated by the widespread Wii playstations. It seems a very
          > fast (over 1kHz) pulsation whine around 2-3MHz. Most NBs
          > don't appear to keep up effeciency with high speed type
          > pulses. (at these frequencies it is ofcourse harder to
          > distinguish pulses from wanted modulation)
          If someone on this list can make a recording I would be
          very interested:-)

          > Another form of QRM is multiple carriers with precize
          > spacing over a very large spectrum area. These carriers
          > are reported to be as dense as 4kHz or 8kHz spaced.
          > It would be a good SDR addition if a wideband comb-filter
          > solution for such situations was investigated.
          Have a look here:
          http://sm5bsz.com/linuxdsp/spur/autospur.htm
          Compare figure 7 and figure 8

          The wideband comb-filter will only work if the spurs are
          very stable in frequency and amplitude.

          In case the multiple carriers cover the entire spectral
          range and are close spaced enough to fill the spectrum with
          more than perhaps 30 carriers it is probably possible to
          remove them with a noise blanker. So many equidistant carriers
          indicate a pulse train and if the carriers have the same
          levels the pulse train is likely to fit a noise blanker.

          Recordings with interference problems are always welcome.
          I can provide an ftp site for upload.

          Leif / SM5BSZ
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