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 From: Firas Abbas Subject: Re: [Discuss-gnuradio] Cycle time detection for an unknown cyclic radio system Date: Tue, 26 May 2009 12:14:43 -0700 (PDT)

```Hi,

You have to explain well in order to get help, for example you said :

[ for 200ms cycle time my array look like [27, 200, 27, 27, 27, 27, 276, 27],
here count = 27, other values are actually some errors ]

What is [my array]? how we suppose that we know your array? And what are these
numbers? and how you get them? What was the input signal? what was its
frequency, modulation,...etc?

Then post again.

BR

Firas

> Subject: Re: [Discuss-gnuradio] Cycle time detection for an unknown cyclic
> Date: Tuesday, May 26, 2009, 10:57 AM
>
> Can someone at least comment on first part of the problem,
> I mean where I am
> getting this 'factor' from
>
>
> Thanks
>
>
>
> >
> > Dear All,
> >
> > A few weeks ago I was involved in a discussion about
> calculating the cycle
> > time of an unknown system present in the area by
> sensing the spectrum with
> > the help of RFX2400+USRP1 system. For details you may
> like to have a look
> > on previous mails on this thread.
> >
> > http://www.nabble.com/cycle-period-detection-of-a-cyclic-periodic-transmitter-td23171564.html#a23174531
> >
> > As Firas suggested to choose a decimation factor with
> respect to duty
> > cycle of the cyclic system. I implemented it in this
> way but I am facing
> > two problems:
> >
> > 1- I am using following formula to calculate the cycle
> time:
> >
> > if
> >
> > ADC_rate = 64MHz, D=decimation rate, fft_size, count=
> total no. of
> > fft_scans where signal is not present
> >
> > Then following would give me the value of cycle time:
>
> >
> > gives the time resolution or time gap between two
> points in fft bin. 1e3
> > for conversion in ms.
> >
> > with D=128, fft_size=512, I scanned the spectrum for
> more than one times
> > and stored the results in an array, then I choose
> maximum occerence in
> > that array as 'count' in above formula e.g.
> >
> > for 200ms cycle time my array look like [27, 200, 27,
> 27, 27, 27, 276,
> > 27], here count = 27, other values are actually some
> errors
> >
> > for 100ms cycle time my array look like [13, 13, 87,
> 13, 13, 13, 13, 167],
> > here count = 13, other values are actually some errors
>
> >
> > for 50ms cycle time my array look like [6, 6, 6, 6, 6,
> 6, 6, 6], here
> > count = 6.
> >
> > From this array with multiple scans I was able to see
> that some cyclic
> > behaviour is correctly detected. BUT when I put all
> values in above
> > mentioned formula then I found that to get exact value
> of cycle time I
> > must multiply my answer with a 'factor' ranging from
> 7-8 (some times 7.25,
> > or 7.4, or 7.5 etc). But this 'factor' is not a fixed
> value and variation
> > in its value depend on the cycle time. So final
> formula becomes
> >
> > [1/(ADC_rate/D)]*fft_size*count * factor *1.e3
> --->  The value of factor
> > is changing on following pattern:
> >
> > for 200ms cycle time factor ~ 7.25 would give me exact
> > for 150ms cycle time factor ~ 7.35 would give me exact
> > for 100ms cycle time factor ~ 7.5 would give me exact
> > for 50ms cycle time factor ~ 8 would give me exact
> >
> > I cant understand what this factor is and where it is
> comming from. Is
> > there any processing in USRP which I am forgetting?
> Can someone help me to
> > figureout this factor.
> >
> > 2-My second problem is that, I am able to detect cycle
> time for any duty
> > cycle with the help of 128, and 256 decimation rates ,
> but with any other
> > decimation rate my answers are quite random and I cant
> > in my above mentioned array. Or simply you can say as
> it was looking from
> > theretical side that for any specific decimation rate
> I should get a
> > cyclic answer but the only thing which would change
> was the no. of
> > detected and undetected scans. But its not happening
> in this way. e.g with
> > any other decimation rate except 128, 256, I am
> > follows:
> >
> > for 200ms cycle time my array look like [20, 2, 187,
> 17, 87, 65, 13, 167],
> > which dont provide me any clue. Similarly for any
> other cycle time I am
> > getting random values.
> >
> > Is there any thing wrong with selection of decimation
> rate? can some one
> > help me to figure out where I am doing wrong.
> >
> > Thanks and Best Regards
> >
> >
>
> --
> View this message in context:
> Sent from the GnuRadio mailing list archive at Nabble.com.
>
>
>
> _______________________________________________