[time-nuts] homebrew counter new board test result

Magnus Danielson magnus at rubidium.dyndns.org
Thu Feb 26 01:46:19 EST 2015


Hi Charles,

On 02/26/2015 02:46 AM, Charles Steinmetz wrote:
>
>>> 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).
>
> Note also that the hysteresis of logic gates with Schmitt inputs is WAY
> too much to be optimal for squaring sine waves (300mV minimum, typically
> 400 to 450mV, for the 74LVC14).  Fast comparators with internal
> hysteresis are optimized for that sort of thing (the LT1719 and LT1720
> have a few mV of hysteresis).

Indeed.

If you think about what large hysteresis does on a sine, it moves the 
trigger points further up and down on the sine from the mid-point, which 
moves them into lower slew-rate areas.

If you are picky, amplitude variations will then also move the phase 
more than mid-point triggers.

A bit of hysteresis can help to avoid flipping back, but considering the 
type of signal, it passes the mid-point (0 V) at highest slew-rate, so 
there is very little risk of flipping back and fourth in the first 
place, so hysteresis may not even be needed.

Cheers,
Magnus



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