FFT block's use of sampling rate

GNU Radio
1

Hi,

In GRC I hooked up a signal source with a 100kHz sampling rate directly
to a FFT sink. I accidentally listed the FFT sampling frequency as
200kHz. I noticed that it did not complain and all it did was shift the
actual signal source frequency by a factor of 2. So my guess would be
that the FFT block is not actually using the sampling rate parameter in
the FFT calculations except to just scale the frequency axis. Is that
accurate?

Thanks,
Dave

2

On 04/18/2010 05:16 PM, David B. wrote:

Thanks,
Dave

To a first approximation, yes.

The other thing I think the FFT graphical sink uses the sample rate for
is to calculate the
required “keep-one-in-N” for maintaining the desired screen update
rate.

But the FFT itself onlys knows about a vector of samples, what they
mean, in a temporal sense
is rather outside the scope of the FFT calculation.

3

The graphical fftsink uses the sampling rate to scale the axis and to
determine the frame decimation to achieve frame rate. If the sample rate
provided is off, you could get a weird frame rate (refresh rate too low
or too high).

-Josh