Export frequency series - wrong FFT size

I am running into this bug where I try to export a frequency series after having set an FFT size of 32k. When I open the CSV file to check it says: FFT bins: 16384. When I plot the file i can also guess it exports in 16k instead of 32k cause I see a higher noise-floor.
I have tried to restart, update the software, but nothing. It was working fine last week.
Can you please help?

Thank you.

Hi @albertoteenage,

I think it is working correctly. Let’s consider a 1K time series:

1K real samples from ADC.
Converted to 2K complex samples by adding a zero to every other sample. Now we have 2048 samples

FFT on 2048 complex samples. This gives a mirrored spectrum. Take the lower half gives 1024 complex, and take the magnitude of those give 512 reals.

So, a 1K time series of reals sampled at 48K ultimately produces 512 bins between 0 and Nyquist, but 1024 bins overall. The resolution between 0 and Nyquist is Fs/N, or 48K/1024 = 46.88 Hz. And 46.88 Hz * 512 = Nyquist

What software version are you running? Has that changed? I don’t think there’s been a change to export. But I think the current behavior you are reporting is expected.

A source of confusion perhaps is the “FFT” reading on the display. That can be ambiguous as it’s not clear if taking about input complex, input reals, output complex output reals, output half spectrum, etc.

Hi @matt,
Thank you for your reply, I see what you mean, but I am referring the the actual number of real samples exported here. Please check the picture below:

As you can see the left example is my “last week” measurement with 32k fit size exported. You can note that the frequency steps of the example on the left are finer with respect of the case to the right, so there are more real FFT points exported (more resolution and less noise).
The right size is my “yesterday” measurement exported with the same settings of the first case, but you can see that the size is reported as 16k and the frequency steps are halved (less resolution and more noise).
In both cases the “FFT size” in the QA software was set to 32k, but I am obtaining two different results!

There must be something else going on…

Hi @albertoteenage, I’m at a loss to explain it. I just checked the source and the FFT bin count is pulled from the completed data ‘package’ and so what is displayed on the screen should be the same as what is exported. I thought maybe an acquisition was done, and then the FFT size was changed. But if I try that I see “infinity” is exported in the data fields. Not ideal, but at least its clear something went wrong.

In any case just to be clear (for my own benefit as this gets confusing at times), the expectation is that when the graph display shows 16K FFT, it should emit 8192 bins, and each bin is 2.92 Hz in resolution.

Please let me know if this happens again, and I’ll also monitor. And thanks for reporting.