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.