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  1. The mains frequency is supposed to be tightly controlled. According to the wikipedia article, it goes up and down ~100 PPM throughout the day but they adjust it to achieve the correct number of cycles per day (50 Hz × 60 sec × 60 min × 24 hours = 4,320,000 cycles per day). Supposedly they are accurate down to the cycle.

    The deviations measured are not what I imagined– much larger. The area under the curve should exactly equal the area under the curve but that does not seem to be the case. Have you tried counting the total number of cycles per day? Another thing is that the cycle accounting is supposed to balance at 8:00 AM Swiss time.

    1. I’m in the UK, so we’re not on the European grid, though we probably have similar requirements.

      If I read Wikipedia right, the +/- a few hundred PPM target is for the average frequency over a day, not the frequency at a particular time (which has a much larger variation).

      I could try cycle-counting, if I get a chance to go back the project.

  2. You might want to go with a GPS-disciplined clock for this as there’s many ways to make the RTC chip inaccurate that will mess up your frequency count.

  3. I think it will depend a bit on what country your from (and even what state / locality) as to how accurate your local supply is, but I know they do try to keep it close here.

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