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AC Line Waveform Distortion

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capt bullshot:

--- Quote from: spiff72 on April 28, 2019, 01:57:05 am ---What sorts of factors can distort the waveform like this?

--- End quote ---

This particular waveform (picture from your original post) does look quite flattened.
The usual reason for this is:
- your line has a relative high impedance, either a literally long line or a weak transformer can cause that.
and
- your load consists for large parts of rectifier-input devices

Edit:
power line distortion isn't measured in dB but rather in % of the rms or fundamental voltage, and only the harmonic bins are shown on linear axes.
If you want to do this measurement right, you'll probably end up using a rectangular window over an exact multiple of power line cycles (typically a 200ms window - 10 periods @50Hz / 12 periods @60Hz - is used).

Components at non-harmonic frequencies can be caused by ripple control systems or loads with fluctuating power consumption (e.g. some kind of modulation of a resistive heater, or an arc furnace at large scale).

spiff72:
Thanks.

I think I am going to retry the measurement again later with the same number of cycles on the waveform displayed on screen as were shown by bdunham7 and see if my plot looks more like his.  I suspect that this is the way that he got 10k sam/sec instead of the 2k/sec that I was getting.

Most of the info i have been seeing only suggest using the "Memory" option for the FFT operation rather than "Trace" to increase the number of datapoints, but in practice i wasn't getting very good results this way.  I think that I just need to fiddle with the settings a bit more to see what changes.

As per capt bullshot's suggestion - I will try to do what he suggested as well using a 200ms window for the trace and see what that shows.

bdunham7:
I don't think you can do 200ms.  There are 12 divisions and your window seems to be 12X your timebase setting, unless there is some other way to change it.  Still, 120ms works for 50 or 60Hz.

I only got decent results at these low frequencies using "Trace" and either the Blackman or Hanning windows.  For those of you following along but without a DS1054Z, the noise at -60-70db is high, but not as obscuring as you might think.  In real time, the noise is flickering while the harmonic humps hold pretty steady.  You can easily see them and pick them out, even on "clean" power like mine.  I think we can conclude that for all its faults, the FFT can readily distinguish between clean power (mine) and marginal or not clean power (spiff72).

This capture is my last, best attempt to illustrate and what we can see and measure.  This is using a 4kHz LPF, although there wasn't much difference between that and the scopes own 20MHz LPF.

spiff72:
I think I misspoke there - my intent was to try to replicate your full-width FFT by experimenting with the timebase by getting the number of cycles visible in the trace to be around 12 periods (20ms/div or 25 would get me close - whichever the scope will let me do).

I am still trying to understand the sample rate shown for the FFT, as well.  You showed 10k/sec in your plot, while I only had 2k/sec.  I am thinking this is a function of me having about 30 periods visible when I did it, and you had fewer.  I have to play around with this some more, too.

bdunham7:

--- Quote from: spiff72 on June 06, 2019, 04:26:54 am ---

* 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.

--- End quote ---

1054z, but not latest firmware (I'm one behind)

100MHz and 500uV options installed

I'm not sure about the FFT sample rates or even if the terminology is correct here.  The display in the FFT window says "KSa/S".  I would think it would just be "KSa", or the number samples in each window.  Thus 2 KSa would limit the bandwidth to 800Hz, thus the point where your graph cut off.  My 10 KSa would allow 4KHz and thus my display went to the end of the window, or 1.2KHz.

I struggled to get back to 600Hz center and 100Hz/div as it wasn't giving me a 100Hz option at 20ms/div.  I had to use the 10ms timebase.  I used 1.2M memory points as well, but I'm not sure it was necessary.

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