[time-nuts] Zero-Crossing Detector Design?

Magnus Danielson magnus at rubidium.dyndns.org
Thu Jul 19 23:17:41 UTC 2012


On 07/20/2012 12:57 AM, Richard (Rick) Karlquist wrote:
> A fast comparator seems like a good idea, and it
> is simple, however it is actually the last thing
> you want to use. High thermal sensitivity and high jitter.

Once your signal has past by a comparator, you can't "treat it" to 
remove the noise-induced jitter. As you gain the signal, the slew-rates 
can be made steeper and steeper prior to the comparator.

The benefit of a comparator is that if you add hysteresis, it stays in 
that position and does not cause transition spikes, which can cause 
false extra triggers, with resulting state-polution. You can see this on 
some counters when you trigger them on a slope with bad slew-rate... and 
the frequency goes unstably up. This is when the experienced trims the 
trigger point for lower jitter (better slew-rate).

Cheers,
Magnus



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