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

Scott Stobbe scott.j.stobbe at gmail.com
Sun Apr 9 11:36:51 EDT 2017

The trim method that stands out from memory for generic RTC chips is to
cycle stall or double clock, x cycles every 60 seconds. Yielding 0.5 ppm
trim resolution.

On Sun, 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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