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Re: [perseus_SDR] Re: Fierce competition

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  • Leif Asbrink
    Hello Karel, You wrote: I remember one day in summer when the strongest sound could be heard around 500kHz and it was getting weaker and weaker below and
    Message 1 of 48 , Jul 12, 2013
      Hello Karel,

      You wrote: "I remember one day in summer when the strongest
      sound could be heard around 500kHz and it was getting weaker
      and weaker below and above this range."

      Interesting. Makes it even more mysterious I think.

      Next time please make a short wideband recording and
      also check other parts of the radio spectrum. If there
      is a maximum at 500 kHz you might find more maxima.
      Perhaps your antenna picks it up best at that
      particular frequency? The longwire might pick it up
      well at some higher frequencies...



      > Finally I found the files with my examples of discharges from 2009. You can download a ZIP file (2MB) with two recordings and one picture from Audition here:
      > https://app.box.com/s/z3zbszuctl5ptt7i3vyt
      > Conditions: a hot day in June with occasional dark clouds passing, no rain in my country side location.
      > The discharges were strong and clear, that is why I was supposing that they were originating very close to the longwire antenna or receiver (AOR AR7030) if not directly inside the receiver.
      > Karel Honzik, CZE
      > From: Karel Honzík
      > Sent: Wednesday, July 10, 2013 6:10 PM
      > To: perseus_SDR@yahoogroups.com
      > Subject: Re: [perseus_SDR] Re: Fierce competition
      > In fact I know these sounds very well. I always heard it when I was using my AR7030. I have not heard it since I use Perseus (but it does not mean anything). I heard it in different places in winter during snow gales or in summer when a black cloud was approaching. It was not raining (perhaps it was raining somewhere else, see Jürgen’s notes). I think I have some recordings, somewhere, I will try to find them. They sound more constant than what was presented here, like a motor changing its rpm. It was interesting to observe extremely magnified curves of this sound in Audition audio editor.
      > I was suspicious that the motor-like sound was originating in the circuits of my AR7030 when a static was coming along the longwire antenna into the receiver. It could be dangerous. But my receiver always survived I remember one day in summer when the strongest sound could be heard around 500kHz and it was getting weaker and weaker below and above this range.
      > I was even discussing this matter with a technician of AOR but his conclusion was that he has never heard something like that. Also other people who were listening to my recording suggested that it was a car passing my QTH. I think I was even trying to discuss it in an international internet group (which one?) but in fact noone was interested in this matter. Now I am pleased that there are more people (DXers) who came accross it than expected
      > Karel Honzik, CZE
      > From: Jurgen Bartels
      > Sent: Wednesday, July 10, 2013 2:18 AM
      > To: perseus_SDR@yahoogroups.com
      > Subject: Re: [perseus_SDR] Re: Fierce competition
      > Hi Guido,
      > > Very interesting story. Could it be that it sounds like the attached file.
      > I have exactly the same one, but *very* strong.
      > Now hold your breath:
      > This I heard before permanently moving here, and no antenna was errected. I
      > heard it with the ATS909 extremely loud, went around the house because I feared
      > something of our electricty was arcing and might set the house on fire. Then I
      > found that the further away from the electrical system I got the louder it
      > became.
      > Years later when I moved here the "arcing" still was here, and I discovered
      > that it began before it started to rain (or snow) here, or it stopped while it
      > was still raining. Studying the rain radar I discovered that I hear it when it
      > was raining/snowing 10-15km northwest of me (North of Wilhelmshaven). As soon
      > as the rain stopped there, the noise was gone. So I suspect something there,
      > but why is it here that strong? We don't have any powerlines above ground. An
      > engineer working at a power plant told me, that this sounds much different than
      > arcing of powerlines
      > I checked with a Perseus in Wilhelmshaven, my ant pointing to WHV, and another
      > pointing to the south of WHV. I could hear it on the first two, but not on the
      > last one. So it is definitely something there an dnot here
      > Here's how it sounds, it was NOT raining here at that moment
      > http://dx.3sdesign.de/unid/MW-Regen-Stoerungen-120112-1205.mp3
      > It was a short (small) shower there, you hear how it started and ended. I
      > watched on the rain radar that it was approaching the area, and just waited for
      > the moment to start recording. There was no other shower around.
      > On another day John Marsyla in Holland heard the same noise at the same time as
      > me.
      > Jurgen Bartels Suellwarden, N. Germany
      > Ant. hor: 29-45MHz 7-el, 45-87MHz 11-el, FM 15.11, Band-3:13-el, UHF:48-el
      > TV: Winradio G305 / Fly2000 + video noise filter & variable IF BW
      > FM: Downconverter + Perseus + Speclab as WFM demod.
      > MW: 30 x 4m EWE 320° with JB-terminator, Winradio & Perseus
      > http://zeiterfassung.3sdesign.de/station_list.htm
      > http://dx.3sdesign.de/tv_offset_list.htm
    • Leif Asbrink
      Hello Nico, ... I am afraid you apply a conventional model which is not applicable in the QRN-fighting context. Consider a sampling rate of 4 MHz. Apply a
      Message 48 of 48 , Jul 18, 2013
        Hello Nico,

        > If one computes the number of taps of a FIR decimation
        > filter with a decent performance (say 0.1 dB in-band ripple
        > and 100 dB alias image rejection) he discover a simple
        > rule of thumb:
        > N =(about) 4*D/(1-B/Fco)
        > where:
        > N is the required decimation filter number of taps
        > D is the decimation factor
        > B/Fco is ratio between the desired output alias free bandwidth and the output sampling frequency.
        > Since after filtering the decimator takes one output every D
        > input samples, the output impulse response is no more
        > than N/D samples long, that's to say:
        > N/D =(about) 4/(1-B/Fco)
        > Note that the length of the output impulse response
        > *does not* depend on the output sampling frequency, but just on the B/Fco ratio.
        > If such a ratio is high the output pulse can be quite long.

        I am afraid you apply a "conventional" model which is
        not applicable in the QRN-fighting context.

        Consider a sampling rate of 4 MHz.
        Apply a FIR filter that has say 0.1 dB in-band ripple
        and a -1 dB point at say 0.8 MHz. The -20 dB point should
        be at 2 MHz and the -100 dB point at 3.2 MHz. The alias-free
        range (-100 dB) would be +/- 0.8 MHz but a clever DSP software
        could compensate for the fall-off between say 0.8 and 1.6 MHz
        to provide a perfectly flat passband of 3.2 MHz or so. The alias
        suppression at the corner frequencies would be poor. Maybe 20 dB,
        but I do not think that would impair the noise-fighting.

        The useful bandwidth for receiving would be 1.6 MHz only and
        not any improvement over the 2 MHz sampling. The purpose of the
        faster sampling would only be to eliminate certain interference
        sources better.

        > In Perseus the decimation filter has been designed so that
        > the alias-free bandwidth is 80% the output sampling frequency
        > (1.6 MHz when the sampling rate is 2 MS/s) which is a good
        > compromise between the decimation filters complexity and
        > the efficiency of the digital signal processing made on the PC.
        > At such a B/Fco ratio you can expect that each output pulse
        > due to an istantaneous glitch at the receiver input is
        > approximately 4/(1-0.8) = 20 samples long whatever the
        > output sampling frequency is.

        > You can't really resolve it into a single pulse even if
        > the output sampling frequency were 40 MS/s. It will
        > always be 20 samples long.
        In Linrad, the PC software will take the fourier transform of the
        input data stream, divide it by the fourier transform of the
        impulse response of the hardware and multiply it by a "desired
        pulse response" This way the pulse length is made shorter than 20
        samples and at the same time the ~0.1 dB ripple is removed.

        The length of the pulse is determined by the "desired pulse response"
        which depends on the skirt steepness that the user has decided.
        The smart blanker knows the exact shape of the pulse and its length
        so it does not matter that the pulse is long in terms of samples.

        I am aware that very few operators use Linrad and that only
        a very small fraction of the users care to calibrate their
        systems properly. I have tried to explain the theory, but I
        do not think I have been sucessful at all. I am interested
        in static rain at high bandwidth because I have a feeling
        recordings would show a dramatic difference between the
        Linrad blanker and other blankers.

        > Of course 20 samples at 40 MS/s are a 0.5us interval,
        > which is a much shorter time interval than that obtained
        > if the sample rate were 2 MS/s but instead of increasing
        > the output sample rate one can obtain the same result
        > simply relaxing the B/Fco requirement.

        > If the B/Fco ratio were 60% instead of 80% the output
        > pulse lenght would be the half the original, if it were
        > 40% one third and if it were 20% one fourth of it, a
        > mere 5 samples interval (2.5us @ 2MS/s), which is even
        > the half of what one could obtain attempting to double
        > the output sampling frequency (and mantaining the
        > original 80% B/Fco ratio).
        > The penalty is that the the alias free bandwidth
        > is much less than the output sample rate...
        Yes:-) This is what I advocate. 4 MHz sampling and
        40% alias-free bandwidth. I also want the -10 dB point
        to be fairly high, maybe 80% of Nyquist.

        > but who cares if we would just be satisfied to (carefully)
        > clean-up a not-so-wide 200 kHz bandwidth out of a 2 MS/s
        > IQ stream?
        > And if it works, wouldn't it be better than obtaining the
        > same result using 4 MS/s maybe overloading a poor man CPU?
        As far as I undersdtand it is impossible to clean up a 200 kHz
        wide segment of a 2MS/s IQ stream if the (random) secondary
        pulses can not be resolved. From old experience as well as from
        the one and only wideband recording at my disposal a bandwidth
        of 1.6 MHz is marginal. It may or it may not work.

        > BTW, making a new 4MS/s DDC would not be impossible but
        > as I haven't implemented it yet I can't say that what
        > was initially conceived for a much smaller output sample
        > rate could sustain it (in 2008 I was even not sure that
        > the 2 MS/s rate could really work).
        Five years later it is very likely that a factor of two is OK:-)


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