strange results from THD measurement of mains

[pete_dl]

I have followed the video on mains THD measurement. I used the ground differential measurement method on the wall outlet in the UK.

It reports the largest frequency being at 25hz and sometimes jumping to 34hz. This makes no sense to me, as I would have thought that a distorted 50hz sine wave would only have harmonics above 50hz.

I have attached a screen grab.

Have I made a configuration mistake ?

Regards,
Peter.

MIgrab3.jpg (250.75 KiB) Viewed 63360 times

[VirtinsTech]

Does this problem exit in Earth Grounded Single End Measurement?

[pete_dl]

I have now tried Earth Grounded Single Ended Measurement, and it still says the peak frequency is 25hz

[pete_dl]

I have also tried with my own generated sine wave. I created a WAV file with a 100hz pure sine wave and verified it with a frequency analysis app. I then played it through a DAC and into the DSO-2810R. The MI software reports a peak at 50hz and sometimes jumps to 68hz.

[VirtinsTech]

Did you see any abnormal in the captured waveform in the oscilloscope?

For example, from the screenshot you provided in the first post, the waveform in the oscilloscope did show a frequency of about 34 Hz. To check whether the DSO hardware is faulty or not, you can use the DSO to measure the signal output from an external signal generator.

[pete_dl]

the waveform looks fine. I have independently confirmed the input signal is 100hz sine wave.

What the device seems to report is half the correct frequency and sometimes 0.68 of the correct frequency.

When I drive it with 100hz, it reports either 50hz or 68hz

when I drive it with 50hz UK mains, it reports either 25hz or 34hz. 34hz is 50hz * 0.86

What it feels like is the ADC sampling clock is flicking between two incorrect values.

[VirtinsTech]

If that is the case, please contact support@virtins.com if you need to send it back for repairing.