Gary <[email protected]> wrote:
Zero crossing and frequency measurement are not the same thing. Generally you zero cross detect to switch a load with the minimum glitch. For frequency measurement, I'd filter the signal before counting it.
Grid-nuts are interested in *both* the instantaneous frequency of the grid and also the transients indicative of grid events (grid switching transients, lightning strikes, etc.). So, a data collection system for grid-nuts must capture data sufficient to determine both the instantaneous grid frequency and the time-of-occurrence of grid events.
If you time stamp the zero crossings, you have all of the information you need to compute frequency with any desired windowing, filtering, or averaging function you desire (and much more). So, yes, they are the same thing when the "thing" is frequency measurement, but ZCD gives you the freedom to set the filtering parameters in post-processing rather than at hardware design time.
Of course, in addition to whatever windowing/filtering/averaging algorithm you may apply in post-processing, you can also filter the signal at the data collection stage. This can improve the accuracy of frequency determinations where little post-processing averaging is done (what a time-nut would think of as low-tau measurements).
There has been some lively debate about how much filtering (if any) is acceptable here. On the one hand, the AC line is a very noisy source at frequencies above the fundamental, while the fundamental frequency is determined mainly by massive rotating machinery that cannot change frequency very quickly. On the other hand, if you pass the signal through a narrow filter you could miss the glitches that interest the folks who collect such data (grid switching transients, lightning strikes, etc.), or they could be delayed and smeared out in time so determining when they occurred would be problematic. The filtering in the circuit I posted (two-pole RC lowpass with a -3dB frequency of ~475 Hz) is a good compromise. It filters out the worst of the locally-generated hash without masking grid events. For those who want their data raw, the filter can be omitted as noted in the description sheet that accompanies the schematic. (You did download and read the material before posting about it, right?)
Best regards, Charles _______________________________________________ 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.
