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 <li...@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 < > > mag...@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<rich...@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 > >> > >> _______________________________________________ > >> time-nuts mailing list -- time-nuts@febo.com > >> To unsubscribe, go to > >> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > >> and follow the instructions there. > >> > > _______________________________________________ > > time-nuts mailing list -- time-nuts@febo.com > > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > > and follow the instructions there. > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
