[time-nuts] Car Clock drift - the lowly 32kHz tuning fork crystal specs

[Bob kb8tq]

Hi

In my “test environment” car clocks always run fast. That’s been true for many
decades over many manufacturers. 

The idea of putting in an offset on a timekeeping device is an old one. You run the
beast over the “expected” temperature (and other environmental) range. You
observe how fast or slow it is and adjust it. To the extent your test model matches
the real world, the clock runs fine or not so fine. 

In your case, the car sits in an environment that matches their test setup well. In my
case … not so much. 

Indeed a modern watch / clock likely does some basic temperature compensation. The 
gotcha is that the crystals are all over the place. The “25 C” inflection temperature is 
anything from 15 to 35C (or more). The 20 ppm slope is anything from 10 to 30 (or more).
They can’t afford to run the parts over temperature (as you would with a TCXO) so 
you get a “nominal” compensation at best. 

Bob

> On Apr 9, 2017, at 7:45 AM, Tim Shoppa <tshoppa at gmail.com> wrote:
> 
> I've had only a few different cars over the past 25 years but I've been
> impressed with how accurate their mass-market built-in clocks are,
> especially considering the wide and completely uncontrolled temperature
> range. In the winter the interior of the car gets down below freezing most
> mornings, and in the summer the interior gets way above 120F in sunlight.
> 
> (Contrast the above with the time-nuttery here where folks buy double-oven
> OCXO's and then they insist that the OCXO's have to be put in temperature
> controlled environments.)
> 
> I only set the car clock twice a year, at daylight savings time changes.
> Yet between daylight savings time changes, the car clock never drifts by
> more than a minute.
> 
> 60 seconds in half a year is 4ppm. So I went and looked at the specs of a
> stock 32kHz crystal, for example
> http://www.mouser.com/ds/2/77/CFS-CFV-4402.pdf
> 
> 1: The crystal is speced as having a turnover point of 25C. I understand
> that.
> 2: Frequency at the turnover point is speced as being +/-20ppm. OK, that's
> not bad, most of that can be compensated for with a small trimmer cap at
> the factory to the 4ppm range. Or maybe they just program in the clock
> divider at the factory appropriate to the crystal.
> 3: The temperature coefficient of the tuning fork cut around the turnover
> point seems to always be the same: -.034ppm per deg C squared. If the temp
> goes down to 5 deg C, then, the frequency changes by 14ppm. If the temp
> goes down to -5 deg C, the frequency changes by 30ppm.
> 
> With that temperature coefficient, temperatures like -5C or 5C that are
> common every winter would result in a few minutes of drift every winter.
> Yet I never observe that drift.
> 
> So my conclusion, is that all these car clocks must be temperature
> compensated. And they must've been doing this for several decades at this
> point.
> 
> That shouldn't be too surprising - right next to the clock display on the
> dashboard is a digital thermometer. Maybe 30 or more years ago the
> temperature compensation was done by analog circuitry, but today I'm
> guessing there's a digital chip that takes the thermometer reading and
> numerically adjusts the divider word for the 32kHz oscillator to
> temperature compensate the clock digitally.
> 
> Is there a way to verify my guess at the TCXO method?
> 
> I'm guessing that all the better quartz wristwatches use a similar
> technology too. Maybe they have a different crystal cut that is closer to
> body temperature for the turnover point.
> 
> Tim N3QE
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