[time-nuts] Zero-Crossing Detector Design?

[Azelio Boriani]

I see that from one way or the other, we always end up in a TimePod. OK,
then the TimePod has no comparator, no trigger but has A to D conversions.
Is the A/D conversion process supposed to be threshold-free? Maybe, in this
case, the DTMD is the only analog and threshold-free way.

On Sat, Jul 21, 2012 at 12:37 AM, Bob Camp <lists at rtty.us> wrote:

> Hi
>
> Simple test:
>
> 1) Run sine wave into a power splitter
>
> 2) Run one port to your limiter / zero crossing detector / what ever
>
> 3) Run other port from the power splitter into the "reference" port on a
> DTMD, 5125, or (better yet) TimePod.
>
> 4) Route the output of the limiter to the "input" port on the instrument.
> You may need to convert it back to a sine wave for some of the above gear.
>
> What you read on the instrument will be the jitter added by the detector.
>  None of them use a comparator on the input.
>
> Bob
>
> On Jul 20, 2012, at 6:09 PM, Azelio Boriani wrote:
>
> > 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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