Checking your mains supply waveform can be done quite nicely and safely using an old low voltage AC output wallwart transformer (7.5vac being typical of the breed used to power the older domestic ethernet hubs and switches of two decades back - 99% of which can be safely powered off a 6 to 15vdc smpsu wallwart with a suitable wattage rating).
Unless they're
very Chinese (draw in excess of a couple of watts unloaded), even a modest level of saturation current, due to not quite sufficient copper being splurged on its primary, won't visibly add to the apparent clipping of the mains waveform as I discovered after witnessing this for the first time some 15 years ago on the "1 volt 50Hz" calibration source of an ancient (as in probably produced a year or five before I was even conceived) 5MHz CRT oscilloscope when I kicked the APC SmartUPS2000 onto battery power (pulled its mains plug - it was supplying backup power for my basement radio shack and workshop, including the boat anchor 'scope and I wanted to check out how close to a true sine wave my UPS was courtesy of this 1v calibration secondary winding on the CRO's mains transformer).
What I'd witnessed was a flat top with a slight downslope on the raw mains supply which was swapped to a perfect sine wave when the UPS was kicked into circuit. On close inspection I could discern the 5KHz PWM ripple but, other than that, it was a perfect example of what I'd naively expected our 240v 50Hz mains supply to look like.
I repeated this test using a newly acquired laptop and a 6VA 12vac 500mA BT wallwart to provide a galvanically isolated copy of the mains waveform feeding a simple resistor attenuator network to reduce it to circa half a volt to feed into the line input on the laptop. I used CoolEdit Pro to record several minutes of the mains waveform both direct and from the UPS whilst on battery power alone or powered from its charging brick (which, afaicr, made little, if any difference).
In this case, I also noted the same down-sloping flat top that I'd seen with that ancient boat anchor CRO which I'd assumed to be the actual wave shape of the mains supply until finally acquiring a second oscilloscope around 18 months ago.
The difference being that unlike the boat anchor I'd bought just over 40 years ago from a local government surplus dealer (long since retired) for something like 30 to 50 quid, this was a brand spanking new SDS1202X-E for way less in real terms of what I couldn't afford to pay for a 2nd hand Tektronix boat anchor with a modest 50MHz (possibly just 25MHz) bandwidth rating. Suffice to say that the totally flat topped (no slope at all) came as a bit of a puzzle until I switched to ac coupled mode and saw once again the same trace as I'd seen on the CRO and the recorded waveforms captured via an ac coupled sound card.
The lesson is clear. For those who have a suitable sine wave output UPS to hand, compare the mains power against the UPS power traces and FFT spectrum plots. Unless you're powering your garden shed workshop via a hundred metre extension, it's unlikely you'll be able to discern any contribution made by even the biggest and baddest of desktop gaming computers using an older non-PFC corrected ATX psu. Virtually all of that 'distortion' comes from the supply grid itself.
When I initially saw that distortion on my ancient CRO's 1v 'calibration' source, my first thought had been the effect of the 170 odd watts powering it causing the flat top distortion effect. This misconception was immediately dispelled the moment I transferred the power source to the UPS. My similar suspicion over that 6VA BT wallwart transformer being likewise dispelled when I repeated the mains versus UPS test CoolEdit Pro recording sessions.
Further to this, I re-ran such a mains versus (other thing) test just over a year ago, where the 'other thing' was a cheap Liddle 1000W inverter genset which gives out a perfect 230v sine wave. If you happen to possess an emergency inverter genset but no sinewave output UPS (unlikely I know
), you'll still be able to compare your local mains supply against a much purer source of 50/60 Hz power as a sanity check on what your 'scope (CRO or DSO) is telling you even if using the low cost safety option of a low voltage isolating mains transformer which might otherwise leave you second guessing about its effect on the purity of the waveform.
Whilst it's true that most mains transformers will start attenuating the higher odd order harmonics (say from the 9th onwards), this will have little effect on the quite visible distortion typically displayed on 'scope traces of most domestic mains voltage supplies.
[EDIT 2020-06-07]
I managed to track down those CoolEdit Pro recordings I'd made of the mains voltage wave form way back in 2009 (not quite as far back as I'd thought). I've attached a few cutdown examples which you can open in a decent audio editor, such as CoolEdit Pro or Audacity, which will display the audio waveform quite nicely (especially so with CEP).
Just bear in mind the 4 to 8Hz HPF effect from the sound card's line input pathway to its ADC on the 'flat topping' which gives rise to the downward slope on positive peaks (vice-versa for negative peaks), simulating what you'd typically see on a modern DSO switched to AC coupled mode.
PS. The second file shows the over-volting effect of trying to filter the HF cogging effect of the stator winding slots by connecting a 15uF capacitor across the generator's output. I'd not anticipated this effect and had set the recording level optimally with absolutely no margin to cater for these events which caused the clipping at FSD (rather than in the line input buffer amp driving the ADC).
I only mention this fine distinction because I'd never been able to track down a single PCI soundcard that did
not clip in the line input buffer amp some 2.5dB below the actual FSD limit as result of the idiot manufacturers just blindly following the sound chip manufacturers' reference circuit, complete with the 6dB sensitivity reduction option[1] unsuited to a unipolar biased buffer amp fed from only a single 5v supply rail. As you can see, I still hold a grudge against these manufacturing idiots.
[1] An option provided to lower the noise floor in the resulting digitised audio, given a buffer amp with sufficient output voltage swing capability (i.e an opamp fed off a bipolar supply of +/-5 to 12 volts or an amp fed off the +12v supply rail).
PPS. I added a screenshot showing a section from that 5 second clip in CoolEdit Pro.
PPPS. I found that 12vac 500mA BT wallwart I'd originally used to record those audio samples and used it to get a 'scope trace capture which I've added to the list of attached files (SDS00288.png)
PPPPS. I've just noticed how much worse that 'scope screenshot looks (more trapezoidal than sine). It's just possible that doing an auto-calibrate routine (7 or 8 minutes afaicr) might improve the looks of the trace seeing as the DC coupled option relies on two signal paths being blended together in the correct proportions to properly fake the equivalent of a DC to 200MHz amplifier response. In the case of a pure sine wave, it wouldn't make much difference to the wave shape but in this case (a flat topped sine wave) it might well do given enough drift in the gain of the DC and AC pathways.
I've appended yet another four DSO screen shots after running the auto-calibrate routine. It doesn't seem to have made a blind bit of difference. The oddball shift in the trigger point seems to be due to my messing with the acquisition options which, as usual, left the display in a rather shaky state until I fiddled some more with that troublesome acquisition menu having, as I'd mistakenly thought, restored it back to normal. Understandably, the acquisition options aren't a feature I'd like to interact with without a good compelling reason to do so.
JBG
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