Re: [Discuss-gnuradio] Can threading disturb sensing continuety

[kaleem ahmad]

Hi,

I have one more confusion about this sampling rate of USRP. 

At FPGA level it is 64MSps and when we decimate (D = 4...256), the sampling
rate is reduced to 64MSps/D. Now I am confused that if the actual sampling
rate of USRP is changed from 64MSps to the new decimated sampling rate or
the USRP is still sampling at 64MSps and only internal sampling rate (i.e
just for channel selection) is changed.

Addition to this as maximum value of decimation rate is 256 so sampling rate
can be reduced to 250ks/s, if I want to set it to 3ksps then how it is
possible.

Best Regards



Ed Criscuolo-2 wrote:
> 
> 
> 
> kaleem ahmad wrote:
>> Thanks Ed,
>> 
>> ADC sampling rate = 64MHz
>> it gives resulution time = 1/64MHz = 156 ns
>> if we choose N=512 for FFT then total observation time for one scan(one
>> complete FFT) is = 156ns x 512 = 8 micro sec
>> if the maximum possible (expected) cycle time is 200ms then:
>> it needs 200ms/8microsec = 2500 continues FFT scans to cover the range of
>> frequencies that correspond to 200ms cycle time.
>> 
>> Is above calculation correct?
> 
> As Firas already noted, you should be using the decimated sample rate
> NOT the ADC rate.
> 
>> 
>> The other possibility to increase the observation time is to increase the
>> FFT size, but to cover entire 200ms would need 200ms/1/64MHz = 12800000 =
>> FFT size (Which is not possible! Is it ???)
> 
> There's no need to go that big.  The problem is that you are sampling
> too high for the cycle time you're looking for.
> 
> As an example, if the cycle time will be between 1 and 200 ms, this
> corresponds to a frequency range of 1KHz to 5 Hz. So you only need a
> minimum sample rate of 2 K-samples/sec.  Let's use 3 so we have some
> margin.  Now, simply take your high-rate data stream, run it through
> a 1.5 KHz low-pass filter (for anti-aliasing) and downsample the data
> to a 3 Ksample/sec rate.  A 512 bin FFT now has a resolution of around
> 5 Hz per bin.
> 
> @(^.^)@  Ed
> 
> 
> 
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