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

[Ed Criscuolo]

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