Author Topic: FFT setting for harmonics on AC ?  (Read 833 times)

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Offline SNDTopic starter

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FFT setting for harmonics on AC ?
« on: June 26, 2019, 03:43:45 pm »
Just wondering if you guys can point me in the right direction for maybe a tutorial video or such on properly setting FFT parameters. I searched youtube a bit and it helped some, but its not quite there yet for what I want it to display. I have a Siglent SDS-1104X-E and Micsig D20003P probes, set to 200X.  Trying to probe for harmonics/noise 5khz or less down to 3rd harmonic if possible, on 60Hz AC circuit. Yes I do know all about the dangers of working on AC, not new to it.
So far the settings I've tried are still showing too many lines that I don't believe mean anything useful yet and were not really changing with the system that I know to cause noise on the line turned on or off. Could the 200x attenuation make it harder for the scope/FFT to pick up these harmonics? 
I'd like it to just show a bar graph essentially of the highest noise peaks present on the circuit. From what I had read about these scopes and FFT ahead of time it sounded capable of what I'm trying to do, actually doing it so far is a bit trickier.

thanks
 

Offline rstofer

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Re: FFT setting for harmonics on AC ?
« Reply #1 on: June 26, 2019, 04:23:00 pm »
It certainly doesn't seem like I can go to such low frequencies on my Rigol DS1054Z.  Given that the lowest Hz/Div I can select is 5 kHz, I don't see where this is going to work out for low order harmonics and the higher order harmonics will generally have a 1/n relationship to the amplitude of the fundamental.  For a perfect square wave, the 7th harmonic will have an amplitude of 1/7 the fundamental.

Where I CAN do such low frequencies is with the Analog Discovery 2.  In the attached display, I just put wire in one side of the CH1 differential inputs and touched it with my fingers.  There's real science going on here!

 

Offline Fludo

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Re: FFT setting for harmonics on AC ?
« Reply #2 on: June 27, 2019, 05:01:45 pm »
First I will say that low cost 8 bit scopes with the FFT option have never produced any meaningful harmonic data for me.  I also have that MICSIG differential probe and the output was not clean enough to resolve small amplitude harmonics. 
 


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