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

Bob kb8tq kb8tq at n1k.org
Sun Apr 9 15:07:49 EDT 2017


> On Apr 9, 2017, at 11:36 AM, Clint Jay <cjaysharp at gmail.com> wrote:
> The clocks in my car have been set by the RDS data, DAB data or GPS in the
> last five or six I've had. Drift is a thing of the past as long as i listen
> to digital radio or the BBC on analogue FM, if i listen to neither then the
> clock drifts a couple of seconds a month but it syncs right up withing a
> minute or two of DAB or BBC FM.
> The GPS set clocks never noticeably change.
> I have a vague memory of at least one of the crystal controlled clocks
> having a 4.194304MHz crystal which, i think, so a divide by 2^22 if memory
> serves which would make for lower drift in the 1HZ?

The advantage of the 4 MHz frequency is that it gets you in range for an AT cut 
crystal. That gives you a third order temperature coefficient rather than the parabola 
you get with the various bar cuts at 32 KHz. For a modest amount of money you
*could* cut an AT so it will hold 5 ppm over the 0 to 50C range (sort of but not really
0.5 ppm/ C) . That compares to the 20 ppm / C previously quoted for the 32 KHz 
parts (which is also a “sort of” number since the parabola gets steeper as you 
get further from the inflection) 


> On 9 Apr 2017 2:01 pm, "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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