[time-nuts] Zero-Crossing Detector Design?

Azelio Boriani azelio.boriani at screen.it
Fri Jul 20 22:09:02 UTC 2012


OK, very interesting. Now is it possible to measure/verify this? I think
that using any test equipment, the comparator-style approach is
unavoidable: the trigger of the scope or the counter cannot be an
amplifier/limiter. How to tell what is up to my design under test and what
is the trigger contribution? Maybe only by comparison: test design A then
design B and see which is better...

On Fri, Jul 20, 2012 at 2:31 PM, Magnus Danielson <
magnus at rubidium.dyndns.org> wrote:

> 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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