[time-nuts] Zero-Crossing Detector Design?

Magnus Danielson magnus at rubidium.dyndns.org
Fri Jul 20 12:31:04 UTC 2012


On 07/20/2012 07:42 AM, Chris Albertson wrote:
> On Thu, Jul 19, 2012 at 5:47 PM, Rick Karlquist<richard at karlquist.com>  wrote:
>
>>
>> Hysteresis does nothing to eliminate jitter or temperature
>
> Maybe, but it is absolutely needed if there is any noise on the
> signal.   A perfect comparator with zero hysteresis would dither on
> every zero crossing.

Yes, and this dither is due to the additive noise on the signal. The 
slew-rate at and about the trigger point will determine how much of that 
additive noise is converted into time-noise. The schmitt trigger is 
there to make sure that you surpress the dither around each transition, 
but it will not help you to remove the time polution, as the first time 
the dither occurs, is bound to be early and bound to be controlled by 
the noise. Those, the noise will shift the trigger point.

You can view the schmitt trigger detector as having a state, and when in 
proximity of the trigger point, you let the noise control when the 
trigger point occurs.

If you noise is pure gaussian noise, this is not so bad, since the 
trigger point will be shifted by the noise RMS, but it will be noisy.
If you have say a sine signal, then the non-linearity of the trigger 
point will act like a mixer and it will cause the time jitter to be 
spread out, and the peak-to-peak amplitude of the signal will when 
divided by the slew-rate of the trigger point convert to the 
peak-to-peak time modulation at that frequency. The distribution has a 
very steep bath-tub look, since the sine spend most of it times at its 
extremes (where it's slew-rates are low) but very little time in the 
middle (where it's slew-rate are high). The sine signal would modulate 
the trigger point up and down on the slope it's at. The schmitt trigger 
action doesn't help to protect this behaviour.

Schmitt trigger is a nice tool, but it can do you great harm if you do 
not understand what it does help you with and what it doesn't help you with.

You need to gain yourself to slew-rates where a schmitt trigger would do 
no harm, and when you are there it will do essentially no good either, 
as you are looking at a high slew-rate square signal.

So, you *can* do better than a Schmitt trigger. A schmitt trigger can be 
sufficiently good. A schmitt trigger can work well if you have filtering 
in front of it to significantly reduce unwanted systematic noise.

Cheers,
Magnus



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