[time-nuts] homebrew counter new board test result

Magnus Danielson magnus at rubidium.dyndns.org
Wed Feb 25 16:02:12 EST 2015


Hi Hal,

On 02/25/2015 10:44 AM, Hal Murray wrote:
>
> attila at kinali.ch said:
>> Yes. You should not use a logic gates with analog input signals. Using a
>> 74LVC14 helps due to its Schmitt-Trigger input. I think the proper solution
>> here would be to use a high speed comparator instead (with hysteresis).
>
> The Schmitt trigger mostly avoids glitches on the output.  Does it do
> anything to reduce timing noise if the input signal is clean enough that it
> doesn't make any glitches?

No, it just avoids flipping state at the transition point(s).

The trigger jitter problem remains the same, regardless if it is at one 
voltage (comparator with no hysteresis) or two voltages (comparator with 
hysteresis aka Schmitt trigger), the slew-rate at the comparator voltage 
and the noise will interact to create trigger jitter. If you want to 
improve on that the main solution is to improve slew rate, but naturally 
careful filtering can help.

It is all to often that I have encountered people to confuse the 
Schmitt-trigger for improving the timing jitter. It's movement in two 
different domains, voltage (schmitt trigger) and time (trigger jitter).

Cheers,
Magnus


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