A few weeks ago I was involved in a discussion about calculating the
time of an unknown system present in the area by sensing the spectrum
the help of RFX2400+USRP1 system. For details you may like to have a
previous mails on this thread.
As Firas suggested to choose a decimation factor with respect to duty
of the cyclic system. I implemented it in this way but I am facing two
1- I am using following formula to calculate the cycle time:
ADC_rate = 64MHz, D=decimation rate, fft_size, count= total no. of
where signal is not present
Then following would give me the value of cycle time:
[1/(ADC_rate/D)]fft_sizecount*1e3 —> where [1/(ADC_rate/D)] simply
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
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,
here count = 27, other values are actually some errors
for 100ms cycle time my array look like [13, 13, 87, 13, 13, 13, 13,
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
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
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
its value depend on the cycle time. So final formula becomes
[1/(ADC_rate/D)]fft_sizecount * factor *1.e3 —> The value of
changing on following pattern:
for 200ms cycle time factor ~ 7.25 would give me exact answer
for 150ms cycle time factor ~ 7.35 would give me exact answer
for 100ms cycle time factor ~ 7.5 would give me exact answer
for 50ms cycle time factor ~ 8 would give me exact answer
I cant understand what this factor is and where it is comming from. Is
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
decimation rate my answers are quite random and I cant get a cyclic
in my above mentioned array. Or simply you can say as it was looking
theretical side that for any specific decimation rate I should get a
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 getting answer like follows:
for 200ms cycle time my array look like [20, 2, 187, 17, 87, 65, 13,
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
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