Attila,
On 04/11/2016 12:19 AM, Attila Kinali wrote:
Why all these complicated filtering systems? As Jim wrote, we live in
a digital world. One can easily sample the 60Hz with an ADC, 200sps is more
than enough, the resolution doesn't need to be good either, 8bit would be
sufficient. Do some filtering in the digital domain with some narrow band
FIR or IIR filter. No need to worry about temperature stability or whether
there will be any spikes. Time stamping is also easy as the zero crossing
"detection" is just a simple bit of math.
All this can be done in a 16bit uC.. or use one of the many cortex-M0/M3 out
there... or if you want to use floats for simplicity, use an M4.
If you want to go all out and do a luxury solution, how about sampling
at 100ksps (something which most modern 32bit uC support with their
internal ADC), so a simple 200Hz or so RC filter would be enough to get
rid of all harmonics, spikes and other stuff that would cause aliasing.
Filter in the digital domain, decimate, filter again, decimate, filter
until you're down at a couple of Hz of bandwidth. Then mix the signal
down to DC with an 60Hz generated from an NCO and detect the phase.
Send phase value to PC using USB or ethernet interface.
The professional equipment does it this way. It samples, filters and
decimate the data. For the professional use the absolute phase is
relevant, so group delay needs to be flat, known an
calibrated/compensated. It also produces frequency and ROCOF (Rate of
Change of Frequency) according to standardized estimators. The phasor is
normalized to that of the reference time and the frequency (50 or 60 Hz)
setting. Phasor measurements up to 120 Hz is seen in installations,
typically on voltages of all three phases, but also currents and
sometimes on both.
NIST in Gaithersburg work on calibration of these PMUs. It was
interesting when I could show them some of the T&F tricks to measure
phase of limited slew-rate, proposing calibrator designs etc. so promote
absolute phase measurements etc.
Cheers,
Magnus
_______________________________________________
time-nuts mailing list -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
