[time-nuts] Time Interval Counter(?) for high-precision watch measurement

[Raven L]

Hi Bob, thanks very much for all the info!

You're right that my input signal is pretty crummy - if anything you
overestimate it. Eyeballing it on the scope, the edge is not particularly
defined at all - resolution better than a couple of microseconds just isn't
possible. I've typically used gate times in the 10^4 to 10^5 seconds range
(though typically collecting a measurement each second and doing the gating
myself in software, as modern watches have multi-millisecond phenomenon
that are interesting to study in the 1 to 100 second range).  I have tried
picking up electrical signals from the actual contacts going from the IC to
the motor, but even then the cleaner edge I got wasn't worth the trouble.

I took an initial pass through the manuals of the models you mentioned, I
really appreciate the guidance. (and the well-deserved correction on 2g
tipover for AT crystals - I'm not sure what I was thinking when I wrote
that but my notes confirm I was way off). I do hope to move up to a basic
home lab running off CSAC and Rb standards in the next few years once I've
seen all there is to see in wristwatches, but it looks like I have a lot to
digest.

One question if you know the answer on these 53131 and family models - the
manual mentions that RS232 is talk-only to a printer. Are there reasonable
tools out there for emulating a printer on a computer to pick up the data?
Otherwise it looks like I would need to set up GPIB.

On Tue, Sep 8, 2020, 1:12 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> First off, 0.001 seconds per year is ~ 3x10^-11. If you are talking about
> the 2G tip effect on a typical AT cut crystal that’s up around 2 ppb.
>
> Next up, low frequency / small package crystals are (inevitably) relatively
> low Q devices. Low Q degrades ADEV performance / increases noise. If
> you *could* measure a high Q device to 3x10^-11 in one second, it’s a
> good bet that a low Q device will take 10X to 100X that amount of time.
>
> The stepper motor in a watch is a low frequency inductive device. The
> waveform out of it has a (very) limited bandwidth. Again another factor
> that
> will stretch out the time involved in the measurement. Your pickup coil
> likely
> also has some issues. ( That assumes the watch is still closed up. I would
> not
> recommend opening one up for testing …).
>
> So far, none of this is looking at the frequency counter. We’re just
> looking
> at the device you are trying to measure.
>
> Since there does not appear to be a need to get the data really quickly,
> none of this is a show stopper. It simply suggests that something out in
> the hundreds of seconds is likely to be the sort of gate time involved.
>
> If you are after 3x10^-11 on a 100 second gate, that comes out to a rather
> convenient 3 ns resolution. Better than that would be fine, but that’s
> roughly
> what you “need” to have.
>
> There are lots of low cost counters out there that will hit that sort of
> number.
> The HP 5334 and HP 5335 both come to mind. They should be available for
> < $200 (delivered). The TAPPR TIC would easily do the job for slightly
> more.
> The TIC probably would be easier to automate compared to running GPIB
> on the 5334 or 5335.
>
> Next step up would be something like a 53181 or 53131. They seem to start
> out around $300 (delivered). You now have an RS-232 serial I/O and a device
> that is about 10X better than your “need”.
>
> These are only a very small sample of the vast number of counters out
> there.
> The only reason for picking them is that they all are devices I have used
> a lot.
> They all (with some effort) can be used to do what you are trying to do.
>
> Fun !!!
>
> Bob
>
> > On Sep 7, 2020, at 6:23 PM, Raven L <cuervamellori at gmail.com> wrote:
> >
> > Hello time nuts and greetings from the 10^-10 world of high precision
> > wristwatches.
> >
> > I'm trying to set up a lab for automated watch measurement. I have a
> basic
> > GPSDO with a PPS and a 10MHz output. I use an inductive sensor to pick up
> > the signal from the motor inside the watch.
> >
> > I've been using a basic digital oscilloscope to measure the interval
> > between the PPS and the watch signal.  The signal from the watch has a
> > total rise time of about 10us to 40us, varying by watch, and rises a few
> > volts above a noise floor of about 50mV. The oscilloscope does a
> > serviceable job but doesn't allow automated measurements and can't be
> > driven by an external timebase. My goal is to make measurements with a
> > precision of about 10us, with a goal of ultimately pinning down a rate to
> > better than 0.001 seconds per year (initial testing shows this is what I
> > need to resolve the effect of tipover on AT-cut MHz-range quartz
> crystals).
> >
> > Are there specific time interval counters or frequency counters that
> would
> > make this easier?  I've been looking at the SR620 as a candidate, which
> > appears to be available used for around 2k USD, but wouldn't be surprised
> > to learn if there was something that was a meaningfully better fit.
> Thanks!
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