[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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