Electronics > Beginners

AC Line Waveform Distortion

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spiff72:
Hello all,

I used my Rigol 1054Z to measure the AC waveform in my house and I get a sort of shark-fix shape.  I attempted to use the FFT function on the waveform and ended up with the image shown.  Can someone confirm whether my conclusions are correct?  I used one of the other options under the FFT (Hanning?) when I took this image.


* The peak in the middle is the fundamental 60 Hz signal (I chose 60 Hz in the math menu)
* The following peaks are the 2nd harmonic (120Hz), 4th harmonic (240 Hz), and 6th harmonic (360 Hz - this peak is tiny)
* If the above is correct, then the graticules across are 100 Hz each RELATIVE to the fundamental, so the first graticle is at 100, not 100+60 (180 Hz)?
* The 2nd and 4th harmonic peaks are both about 27db down from the fundamental? 
What sorts of factors can distort the waveform like this?

IanB:
Based on a similar question I asked in the past, I'd say that it is not unusual to see the mains wave form distorted like this.

Probably the mains is a nice sine wave when it comes out of the generator, and it gets more and more distorted the closer it gets to the point of use (basically because many consumer loads are non-linear and they influence it more the closer you get to them).

https://www.eevblog.com/forum/chat/when-is-a-sine-wave-not-a-sine-wave/

spiff72:
Thanks.  I have a coworker with the same scope and he stated that when he does the same evaluation at home, he gets a very clean looking sine wave.  Obviously everyone's homes will be different, though.

Also, I think I need to repeat this test with the FFT again after seeing a few more tutorials.

JustMeHere:
How does your probe look on the test point?  Have you compensated it?  You can actually pick up the 60hz wave by probing your finger and pressing auto.

gf:
Looks like the mains line is not loaded with ohmic load, but heaviliy loaded with rectifiers (e.g. SMPS without or with poor power factor correction). One factor is likely that more and more people replace tungsten with led (and the power factor is certainly not the primary buying criterion for lamps or bulbs, but rather the price, or design of the lamp).

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