“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).