[time-nuts] Zero-Crossing Detector Design?

[Bob Camp]

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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