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Low jitter trigger circuit for a frequency counter
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TmaxElectronics:
Hello,

I have a project that will log the mains frequency deviation over time and i need a reliable trigger circuit.

I have already tried to just use a comparator (general purpose and a fast prop. delay one) but i observed about 100us of jitter across one cycle time of the waveform
(yes that is only a fraction of the actual waveform and easily enough for measuring 50Hz but since i want this to be really acurate i would need one that has at most 1us of jitter, with lower of course being better).

What am i doing wrong/which part should i use?

BTW i did google this and try out the first few results but they all had the same issue and i didn't quite know what exactly to google for after that.
Rerouter:
If you ever look at mains with a scope, generally it is extremely noisy, your idea for a trigger would likely work if you had an aggressive low pass or band pass filter.

as your only after drift in frequency, measure the zero crossing with some hysteresis, and have a dead time to ignore any secondary pulses.
TmaxElectronics:
So far i was only running it from my signal generator so there probably wasn't too much noise on the input signal ;) .

And i also had some hysteresis (220k and 47k as the comparator wouldn't sink a lot of current).

I also just tested the setup using other waveforms and it appears that the jitter was always at ~1/500 of the rise time so using a square wave completely eliminated any jitter, not sure if that is in any way important though.
RoGeorge:

--- Quote from: TmaxElectronics on March 24, 2019, 11:31:19 am ---What am i doing wrong

--- End quote ---

You are looking at each trigger point instead of averaging for a very long number of periods.
Benta:
Timebase is the word, not trigger.
Measuring an AC sine wave precisely is close to impossible, the tiniest drift in DC levels will offset the measuring point significantly. This also goes for squaring up the sine wave with a Schmitt trigger, comparator or whatever.
Your best bet is averaging as RoGeorge says.
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