Max XBee ADC sample rate spec >1mS?
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Max XBee ADC sample rate spec >1mS?

by greyf350 on Thu Oct 24, 2013 2:24 pm

I was looking at the AC voltage/current waveforms in the TweeAWatt tutorials/guides and comparing with some of my own measurements. It appears to me the published 1mS XBee sampling rate spec is a bit optimistic.

17 samples @ 1mS sample rate should return a single 16.7mS 60Hz cycle, but it appears to me 12 samples is more accurate. That could only mean the best case sample rate is closer to 1.3-1.4mS??

I'm running the latest 10ED firmware if that is of any consequence.
Last edited by greyf350 on Mon Nov 25, 2013 7:00 pm, edited 1 time in total.
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Re: Max XBee ADC sample rate spec >1uS?

by adafruit_support_bill on Thu Oct 24, 2013 2:32 pm

We can only speculate on the firmware internals. The engineers on the Digi forum can probably give you a more definitive answer: http://www.digi.com/support/forum/
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Re: Max XBee ADC sample rate spec >1uS?

by thefatmoop on Sat Oct 26, 2013 2:37 pm

Nah that's not right. For S1 I think the RF link is only 2*115.2kb/sec, and a lot of that BW is loss to retransmissions
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Re: Max XBee ADC sample rate spec >1mS?

by greyf350 on Mon Nov 25, 2013 6:37 pm

The way the TAW works is, it's intended to measure voltage and current for exactly one 60Hz period. It assumes a sample rate of exactly 1mS per sample, and therefore 16 or 17 samples will cover a 16.7mS period. Makes perfect sense. However, looking at the documented waveforms and examples, 17 samples always returns 1 and a fraction of the next 60Hz cycle which, if all samples are used for RMS calculations, will result in erratic readings depending on where the first sample starts especially with low power factor loads.

I'm seeing 12 samples per 60Hz period is sufficient and acceptably accurate since the 1mS sample rate setting is nowhere close to 1mS.

Anything more or less results in wildly erratic measurements just like these college kids were seeing, but they simply dismissed the errors as being caused by a 'low sample rate.'
http://www.ee.duke.edu/files/ece/GWDD2010_Hamner.pdf
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