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

Tue Sep 8 18:58:01 UTC 2020

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)?
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