Read Tim's paper!
The thing about an electric meter is that you're not trying to reconstruct the waveform, you're only gathering statistics on it. The
27.xxx Hz sample rate was chosen so that its harmonics would dance between the line harmonics up to some highish harmonic of 60 Hz, so as to not create any slow-wobble aliases in the reported values (trms volts, amps, power, PF) that would uglify the local realtime display or the archived time-series records.From a signal-theory standpoint, the bandwidth of the signal is in fact narrow, so the sample rate can be low. The "signal bandwidth" is actually the sum of the bandwidths of the various spectral harmonic lines, multiples of 60 Hz, mostly of the ugly current waveforms, which is pretty weird when you think of it. The sample-hold is simultaneously undersampling a bunch of narrow but disjoint spectral zones, still following the Shannon rules for each one.
Given that, it was a considerable nuisance to come up with that 27.xxx Hz sample rate. Using available crystals.
I also used a 7-bit single-slope ADC, which I didn't reveal to the customers because they would have argued over that, too.
I did waveform acquisition on demand, in a burst of samples, at some other goofy sample rate, some hundreds of Hz. I sampled over many line cycles, stuck the samples into RAM, and then reordered them to make them equivalent-time sequential. That was fun.
12K lines of MC6803 code!John