Electronics > Beginners
AC Line Waveform Distortion
spiff72:
I hope this thread hasn't died - I would really like to hear others' thoughts on the waveforms (and possible sources of the distortion).
I tried to re-run this FFT on my AC waveform, and got the attached. I screen grabbed all the settings pages in the MATH menus to show how I had the FFT set up to run.
I also ran a similar pass on a 60 Hz square wave from my function generator for reference. Unfortunately this scope doesn't seem to provide very granular access to the settings (Hz/div, etc because it seems dependent on the scopes settings for the horizontal timescale), and as a result, I don't have the same Hz/div setting on the AC wave vs the square wave.
I think i successfully got the FFT plot to show DC/0Hz at the left edge of the screen, and it looks like the 3rd and 5th harmonics are obvious high points. The high peak at about 700Hz seems odd?
The square wave looks reasonable to me with the gradual falloff of odd harmonics.
bdunham7:
Looks like a pretty good FFT for what you have. I'll have to try your setup. Your THD from the 3rd and 5th is probably pushing the 5% limit, but that 700Hz spike is more interesting. It would be about 6Vp-p and that isn't a small amount for a non-harmonic that high up. And then you have all those smaller spikes at 40Hz intervals starting at 220Hz. If you can, start turning stuff off and see if you can find a source in your house.
spiff72:
I might have to make a weekend project of turning off breakers in the house while monitoring the waveform and the FFT plot until it either goes away or at the very least rule out anything within the house (if I can't get rid of it with only one circuit running).
Just need to get everyone out of the house so dad can get some more "mad scientist" time without driving everyone crazy. :D
bdunham7:
Here is my power with as close to your setup as I could get. I have 10K samples and a full screen, you have 2K and not a full screen. I was unable to easily figure out what setting changes that. You can see my 3rd is way down and 5, 7 and 9 are all much lower than yours as well and reflect less than 1% THD. The second picture is with the probe disconnected so you can see what the background is--and why the FFT on this scope is more interesting than useful.
spiff72:
Bedtime here, but a few question about your settings:
* are u using the same 1054z scope on the latest firmware?
* Is it modded to get 100MHz bandwidth? Mine isn't, and not sure if that makes any difference in this case since frequencies are very low
* Is the timescale on the horizontal what determines the different sample rates (or at least the limits that can be chosen)? I couldn't get the same center and Hz/div when I tried to retest the square wave.
I will try again with your settings tomorrow.
Thanks again for the reply!
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