FFT distortion
FFT distortion
(OP)
I'm getting an interesting FFT result (see attached diagram) off a pulse that I am digitizing. You can see that I have a lower magnitude alias at 16 KHz. This isn't at 2 times the Nyquist frequency but at about 0.64 times the Nyquist frequency. Why at that frequency?
In our system, we piece the sample data together from a non-linear time delay, and then interpolate it back to fixed time samples before it gets processed through the FFT. I believe I read a few years ago that this type of FFT anomaly is due to distortion in the time samples. I can't find that reference though.
Does anybody have any leads on how to track this issue down? Good books to reference? A name for this phenomenon?
Thanks,
Z
In our system, we piece the sample data together from a non-linear time delay, and then interpolate it back to fixed time samples before it gets processed through the FFT. I believe I read a few years ago that this type of FFT anomaly is due to distortion in the time samples. I can't find that reference though.
Does anybody have any leads on how to track this issue down? Good books to reference? A name for this phenomenon?
Thanks,
Z





RE: FFT distortion
It looks a lot like "folding" (Wiki).
RE: FFT distortion
RE: FFT distortion
RE: FFT distortion
RE: FFT distortion
RE: FFT distortion
One perfectly reasonable approach to addressing them is to twiddle the controls to truncate the lower extreme of vertical scale so that these extremely weak artifacts disappear off the bottom of the display. Then you can go for lunch.
It's up to you if such tiny signals/artifacts at these extremely low amplitudes have any importance in the real world given your technical circumstances. At -80dB, it is probably unlikely that there's anything of real interest.
RE: FFT distortion
RE: FFT distortion
A mixing product does make sense. This data is oversampled and then 'interpolated' back to 40 uS (25 KHz) samples that go into the FFT. I can shift the 'image' to other frequencies (0-1.0 * Nyquist/2) by changing my oversampling rate. I'm thinking there is some non-linearity in the interpolation math. I had one programmer who thought he had solved this a few years ago in the math. Now I have a new programmer and new algorithms and the problem is back...
Z
RE: FFT distortion
"...in the interpolation math..."
You've opened up a new window there...
RE: FFT distortion
Well, then it shoudl be a simple matter to compare the two algorithms and determine where the bad juju is coming from.
Dan - Owner
http://www.Hi-TecDesigns.com
RE: FFT distortion
Z
RE: FFT distortion
1) truly there in the time history
2)something to do with your interpolation
3) something to do with your antialiasing filter
4) something to do with your DSP
I'd have thought if you posted a csv of the time history (the intial points, not your interpolated version) someone might check all that for you, it's a 10 minute job in matlab.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: FFT distortion
So far the best explanation that I'm getting out of all this is that I have some timing distortion in my sampling signal that mixes back in as a spur in the spectrum and causes a mirror (alias) around the spur.
Z
RE: FFT distortion
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: FFT distortion
Z
RE: FFT distortion
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: FFT distortion
1. evidently you are looking at the power spectra as there is no phase information displayed for your FFT
2. there are proven alternatives to FFT when variable time delays are involved, have you tried these, the artifacts in your spectra may be the result of your data manipulation
3. on close scrutiny of your spectral image, I am a bit puzzled by the random noise being so localized in the frequency as to appear pasted in, what sort of spectral window are you using?
RE: FFT distortion
1. That is correct, that is a power spectra being displayed.
2. Interesting. What other techniques are you referring to? We may be using them in other analysis.
3. Ha! You noticed my cut-n-paste window. You have a good eye for detail. This is simulated data that was reconstructed for this conversation. A system was not available to collect real data at the time I started this post.
As it turns out, we just tracked this down a few days ago. As stated earlier, our system collects data in real-time from a non-linear time delay system. When we switch 'time' domains from real-time to the delayed time, we have reference points in the delayed time base that we then need to interpolate between. This interpolation sets up a second sampling interval. We then experience inter-modulation-distortion products between the two sampling clocks and thus the 'images' show up at odd locations.
Z