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Re: [Discuss-gnuradio] question regarding usrp_fft.py

From: adib_sairi
Subject: Re: [Discuss-gnuradio] question regarding usrp_fft.py
Date: Wed, 26 Aug 2009 21:10:35 -0700 (PDT)

adib_sairi wrote:
> Good day every one,
> I am trying to calculate the sensing time for my system with using
> usrp_fft.py as the core of my sensing method. my fft bin is 256 with Fusb
> = 4MHz and my bandwidth is 1MHz. 
> with this parameters, each sample of 256 bin FFT will take 1 second. am i
> right? means, between one evt.data[0] to the next evt.data[0] will take 1
> second. this is due to the maximum USB BW that i use for the sensing.
> but i can see that in the usrp_fft GUI, the update of the plot is much
> faster than 1 second. so i think my calculation is wrong but i am not sure
> what i miss. 
> i see in some paper, they divide the Fusb to the number of their bin. for
> example if the Fusb is  4MHz and the FFT bin used is 128, then they divide
> 4MHz with 128 which give them 31.25kHz of sensing speed (0.032s). but i
> think we cannot divide like that because the FFT is being done in GNU
> Radio. USRP dosent know about this is it? what bin number we choose i
> think the speed is the same (1 second) because the speed of the data flush
> from USRP to PC is only depend on the decimation we choose not the FFT
> bin..FFT bin is only take place inside GNU Radio.. am i right?
> so what is the actual way to calculate the sensing time if i am using
> usrp_fft as my core (to be more specific, i use the fft_nongl as the basic
> code and do a modification from there). 
> please help and guide me for this.. thank you very much in advance..
> Adib

i have one more question with usrp_fft.py. in the GUI, it show that if i
choose my decimation to be 16, the Fusb become 4MHz (due to 64MHz/16) and
the BW of the measurement should be 1MHz right? but why in the x-axis it
show that the frequency range of measurement is 4MHz; if my fc = 2.45GHz, it
give the minimum value of x-axis = 2.4480GHz and the maximum value of x-axis
= 2.4520GHz (BW = 4MHz)? is it this labeling is miss or is i had miss
somewhere?  i suppose the BW of the display should be 1MHz right?


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