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

Tue Sep 8 20:02:49 UTC 2020

The pickup I use (a Seiko DM-1) is inductive. It picks up the magnetic
field from the motor impulse that drives the hands. It works very similarly
to electric guitar pickups. I have used a piezo microphone (using the
microphone from a commercial mechanical watch timer), but the precision is
far worse - the "tick" sound is only localized to a couple of milliseconds
at best, rather than tens of microseconds from the inductive pickup.

Thanks to everyone for all the guidance so far!  I'm learning a lot very
quickly - I have a long reading list already lined up for tonight.

On Tue, Sep 8, 2020, 3:58 PM Warren Kumari <warren at kumari.net> wrote:

> On Tue, Sep 8, 2020 at 3:15 PM djl <djl at montana.com> wrote:
> >
> > Sort of eavesdropping... you might try a piezoelectric pickup, available
> > cheap on epay for guitars, etc. Looking at the actual motion kick of the
> > hand(s)?
>
> I have one of these for mechanical watches: https://one-of.com/accuracy-2/
>
> It works nicely for tracking / accuracy of my mechanical watches - it
> uses a piezo for pickup, and hooks up to an iPhone / iPad (presents
> itself as a microphone). It reports on "accuracy" and amplitude, and
> allows me to save a "library" of watches so I can monitor them over
> time.
> Of course, this of for mechanical watches, and so "accuracy" is fairly
> relative ( COSC-certified chronometers must accurate to within -4/+6
> seconds a day).
>
> I just tried it with a Citizen Eco-Drive quartz (hoping that it might
> pick up the stepper "tick"), but it doesn't even detect that a watch
> is on the sensor....
>
> W
>
> > Don
> >
> > On 2020-09-08 11:59, Raven L wrote:
> > > 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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> > >>
> > >>
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> >
> > --
> > Dr. Don Latham  AJ7LL
> > PO Box 404, Frenchtown, MT, 59834
> > VOX: 406-626-4304
> >
> >
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>
>
> --
> I don't think the execution is relevant when it was obviously a bad
> idea in the first place.
> This is like putting rabid weasels in your pants, and later expressing
> regret at having chosen those particular rabid weasels and that pair
> of pants.
>    ---maf
>
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