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

[Bob kb8tq]

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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