Picoscope 4444 THD measurements

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Imbrondir
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Joined: Wed Jan 30, 2019 11:39 am

Picoscope 4444 THD measurements

Post by Imbrondir » Wed Jan 30, 2019 12:00 pm

My company often have to present microgrids 690V THD for electrical class approval, and interested in this product for measurements. However both THD as well as highest harmonic distortion are both part of this approval (example 5% total, and 3% single harmonic).

From screenshots and playing around with the Demo interface, the total harmonic seems quite easy to check, but is there any simple way of getting single harmonic measurements as a percentage value? Or is there a future planned support for this functionality?

I suppose the manual way would be to check the FFT output, check the highest amplitude besides the base frequency, then divide the square root amplitude with the base frequency amplitude, and finally divide highest amplitude harmonic frequency with base frequency to get band.

Gerry
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Re: Picoscope 4444 THD measurements

Post by Gerry » Fri Feb 01, 2019 3:37 pm

Hi Imbrondir,

A simple way of getting the % nth harmonic distortion would be to make two %THD measurements setting the 'Highest Harmonic' in the measurement options of the % THD mesurement to n for the first and n-1 for the second, and then subtracting the second value from the first.

I have created an example (see below) which shows the progression of harmonic distortion in a square wave as you add more harmonics to the calculation. The first is just the second harmonic, the next one second and third (don't forget that the scale is a decibel scale so the relative values as they appear in the plot are misleading), after that we have up to 8 harmonics, then 9, and finally the maximum that you can have, and as you can see it approaches the 48.3% for a square wave (because we haven't captured all of the harmonics). So if we were looking for the contribution of the 9th harmonic then this would be 42.87 - 41.41 = 1.46%.

Harmonic distortion of a square wave.psdata
(9.39 MiB) Downloaded 127 times
Regards,

Gerry
Gerry
Technical Specialist

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