# Cycle time calculation

Hello every one,

I am using USRP1 with RFX2400 daughter card.

I have a system (any state of the art system, let say Atmel’s narroband
FSK
which works in 2.4 GHz band) transmitting a fixed packet cyclically (let
say
cycle time = T ms).

I want to use USRP system to sense the spectrum and calculate (or guess)
the
cycle time of this FSK system, and to do this I have following idea:

I am using fftsink2.py to sense the spectrum with FFT = 512 at a fixed
center frequency (which must be exactly same as FSK system). Then I set
the
decimation rate = 160 , hence getting B = 400 kHz (FSK’s B = 800 kHz).
USRP
start scanning the spectrum and in each scan it come up with 512 bins
and I
compare the max(of 512 bins) with a predefined threshold energy level.
When
I find this max energy above my predefined threshold value, I suppose
FSK is
detected, I save this number and start counting the number of scans
which do
not detect the FSK system (lets call it empty scan). After ‘n’ empty
scans
there is again a scan with FSK detection. In this way between two FSK
detected scans I calculate the value of ‘n’ empty scans. Now I already
now
that one 512 bin scan take ‘K micrseconds’, so I simply calculate the
cycle
time in the following way:

cycle tile = K(time for one scan) * n (no. of empty scans between two
detected scans)

BUT IT IS NOT WORKING!!!

If ‘p’ stands for peak (the scan which detect the FSK system), and dots
represent empty scans, then I was expecting an output as follows:

----n----< >-----n—< [all n’s should be equal
because cycle time is fixed]
p . . . . . . p . . . . . .p . . . . . . p . . . . . . p

mean that after running for long time I would get the value of n (n=6 in
above example scenerio), the value of n would be almost same between any
two
peaks as shown in above scenerio. BUT I get following results:

----n—< > n < >---------n----------< [all n’s are not
equal???]
p…p…p…p…p…p…p…p…p…

Means that there is no consistency between the value of n between
different
peaks.

*POSSIBLE REASONING: I guessed that one possible reason can be that
there
are multiple threads and the scan process is not really continues and
because of this some times the FSK system is not detected (because when
the
FSK system transmits the scanning thread is not executing??? Is it
possible)which makes some p’s to disappear and hence leaving the length
of
intervals between different p’s (value of n) unequal.???

1- Is the thread sharing time so significant that above given reasoning
(*POSSIBLE REASONING) has some worth.
2- What other possiblitities can be there
3- Can you suggest some different way to solve this problem, The problem
is
simply to calculate the cycle time of some system which is already
working
in the environment

## Thaks in advance and Best Regards

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