Hi Vortexico,

Thank you so much for hooking up to the calibrator! This is really helpful information.

Regarding AC calculation: By default the AC calculation is done by sampling at 4kHz with a sample buffer 256 samples long. The calculation is done on board the meter itself between the two zero crossings nearest the edges of the buffer.

The 4kHz rate performs well for 60Hz waveforms, which is what the vast majority of users do. But you’re right to point out that it doesn’t behave well at higher frequencies, and when I was validating the numbers before the meter went to production I was using 8kHz sample rate. I don’t want to make more trouble for you, but if it’s not hard I would love to see what the calibrator numbers look like when measuring at 8kHz and 256 sample depth.

So to try to directly answer your questions:

– what did you do to verify the meters?
With the pre-production prototypes, I hooked a few up to a signal generator in parallel with a known good meter – I think a Fluke 115 – and swept through a range of amplitudes and frequencies while checking the outputs matched within 1%.

The final calibration is all DC though, so not all the meters that go in to the wild have their bandwidth checked.

– how did you come up with 1kHz bandwith?

By design this is where the low pass filters on the inputs cut off by 1% (or 0.5%… I would need to dig through my notebooks to remember).

I tested it experimentally as described above.

– Another thing: I think the VAC/IAC ranges are expressed in Vpp and Ipp
But the measurements are in Vrms and Irms.

I understand why you would think that because I’m reusing the DC labels, but no the range is actually expressed as RMS. So the 600V range can actually measure DC up to around 1000V

Thank you for using the calibrator, it’s a level of hardware I haven’t had access to when designing this meter. Those AC numbers >60Hz are disappointing so I’d like to get to the bottom of this. If you can I’d really like to know how it behaves with the sample rate pumped to 8K.

Best
~James

Hi Vortexico,

“You mean you measure at the 2 zero crossings of the 4kHz sample frequency right?
Because at the zero crossings of the signal would be strange.”

The RMS is calculated between two zero crossings of the signal. Catching a partial cycle throws off the RMS reading.

“So you actually measure with 8kS/s then?”
Not sure I understand. Right now the only way to measure with 8ksps is to set it manually through the app. I did early testing at 8ksps as well.

Actually now that you’ve got me thinking about it, there’s a filter that was in early versions of the firmware that didn’t get added back in later that might account for some of the error at higher signal frequencies… in early versions of the firmware the zero crossings were fractional – the firmware was clever enough to say the zero crossing happened 5/16ths of the way between samples 4 and 5, for example. This got taken out somewhere along the line, I think because processing time became an issue and because the effect was minimal at 50/60Hz, but that might be having a larger effect at higher frequencies. Let me take a look.

“Americans… :P Most of the world uses 220V-240V @ 50Hz :P”
Don’t worry! I know! I just didn’t think it was an important distinction to make in this discussion :)

“about the 1kHz:
The spec website states:
Frequency: Better than 1% accuracy up to 1kHz
Sampling : 4kHz analog bandwidth for most measurements”

Yup. If I can’t get this figured out and there’s a mistake, I’ll downgrade the frequency spec. Obviously I’d prefer not to do that, I think this is a firmware issue that can be hunted down.

“…Since the bandwith of the meter is only 1kHz you have to cross reference it to know what is does at 1kHz.
So I think you measured with 3% tolerance…”

Yes, the setup I described is 3% tolerance. I did some other tests too which had much better tolerance. Sorry, it’s been almost a year since I was doing this validation so it’s not all on the top of my head.

I checked my lab meter and oscilloscope against a traceable 0.01% DC reference immediately beforehand (voltagestandard.com sells a couple of nice options). I think once I got above 100Hz I switched over to calculating RMS on my oscope. I don’t think I calibrated for signal attenuation on my oscope because I wasn’t really worried about it, it should be flat up way past 10MHz so 1kHz didn’t worry me. So yes, my initial description was of a 3% check but I did some more that were much better. I wasn’t really expecting this depth of review :)

Anyway, I’m very interested to know what you find at 8ksps. You’ve got me thinking about AC measurement past 60Hz again. I am not in a place where I can check right now but I have a strong suspicion that the AC error you’re seeing is numerical in nature and patchable. I’ll let you know what I find when I can get to it (probably early next week).

Thanks again
~James