FFT from downsampled signal

Hello,

I'm having recorded data with Dewesoft, where I used a sampling rate of 50 kS/sec. The sampling rate has been chose, because we need to look at harmonic content up to about 15 kHz.

Now I also want to see the frequency spectrum of the signal in the area between 10 Hz and 100 Hz.

Unfortunately the frequency resolution is limited to df = 1,53 Hz (N = 16k). (32k or 64k cannot be selected.)

Is it somehow possible that I downsample my signals to e.g. 5 kS/sec and then make a FFT of it, where with N = 16k it would result in a frequency resolution of 0,153Hz?

Regards.

It's possible to reduce the data with offline math and then export those channels.

Best regards,

Primoz

Hi Felix,

Instead of reducing the sample rate, it is also possible to use 32K or 64K frequency resolution on the existing file. Please go to settings -> Performance -> Memory -> Sync DC buffer length -> increase to 10seconds.

Best regards,

Primoz

Hello Primoz,

thanks a lot!

Increasing the Sync DC Buffer length helped!

Regards,

Felix

Hello Primoz,

I saw this old post, and we have the same challange to have more resolution in a zoomed part of a FFT spectrum (0-100 Hz). Our original measurement have a samplerate of 25600 Hz (bandwith 10 kHz). I tried to reduce the data (kind of downsampling) and I exported it, as you discribed it in the post above. I asume the exporting is needed for export with anti-aliasing filter?

When opening the exported file, I have still the same resolution as in the orignal file (max res 64K lines, df=0,195) . I can adjust in the FFT setup the resolution (see attachment, df=0,1 Hz, 128k lines), but I can't select this settings in FFT setup in review mode (max 64k lines). Is there a solution for this?

Regards,

Leon.

Hello again,

In the meanwhile I was reading old posts, and I found a few that talked about export rate. So I thought, maybe this could help me. In the attachment you can see that on one channel I decreased de exportrate from default (25,6 kHz) to 12,8 kHz and number two is original. But after exporting and zooming in on the timesignal you can see, that both signals have the same samplepoints. Can you explain this?

Regards,

Leon

Dear Leon,

thank you for contacting us.

When opening the exported file, I have still the same resolution as in the original file (max res 64K lines, df=0,195) . I can adjust in the FFT setup the resolution (see attachment, df=0,1 Hz, 128k lines), but I can't select this settings in FFT setup in review mode (max 64k lines). Is there a solution for this? The FFT preview is not controlled through the mathematic FFT setup. The FFT preview Widget only has the option to set the lines to max 64K lines. To display the FFT calculated with the FFT Math please use the 2D Graph.

Regarding the export that has the sample rate of 25,6 kHz but the export rate is set to 12,8 kHz. The export rate setting only applies to export to other file formats TXT, Excel etc. and does not apply to the .dxd export. To change the sample rate of the recorded file (to downsample the data) you can create a Math channel with the signal that you would like to downsample and in the Time axis choose Sync Timebase and SRDiv to 2, please refer to the attached image. With downsampling by just applying the sample rate divider the donwnsampled signal will have aliases. That is why before downsampling a signal a low pass filter has to be applied. Then you can export these downsampled. If you are exporting to other file formats such as TXT, Excel etc this is automatically done and the resampler type can be specified.

Kind regards.

Dear Gregor,

Thank you for the explanation but I still can't figure it out. I can create the low pass filter (FIR with Fc2=400 Hz) and then the mathchannel as you discribed. In the recorder (time domain), I can see that the number of samples is halved, but I can't make a FFT of it in a 2D graph. In a FFT-preview I can see the df=decreased and the spikes are sharper (2 times more resulotion).

If I set the SRDIV from 2 to 4, there is no FFT visible any more.

Is it possible that you can add a Dewesoft setup as an example in this topic? The original recording is Fmax=10kHz/ SR=25,6 kHz and I want to make a FFT 0-100Hz, with a resolution 0,1Hz or better.

Kind regards, Leon.

Dear Leon,

We have changed the Sync DC buffer length (Settings -> Performance -> Memory -> Sync DC buffer length) to 50 seconds and then created a FFT math channel with resolution of Df [Hz] = 0.1 Hz. Please notice that this math FFT is viewed on the 2D Graph. For the comparison there is also a FFT Preview Widget (left of the 2D graph) where the resolution is df = 0.195 Hz. Please also notice that the FFT on the 2D graph is not displayed immediately but after some time, this is due to the difference on how FFT preview and FFT math calculate FFT. The FFT math displays values after they are calculated for defined Line resolution. The FFT Preview takes the available data and displays the FFT.

Kind regards.