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

[Tim Shoppa]

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