Hi
> On Apr 9, 2017, at 11:36 AM, Clint Jay <[email protected]> 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) Bob > > On 9 Apr 2017 2:01 pm, "Tim Shoppa" <[email protected]> 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 >> _______________________________________________ >> time-nuts mailing list -- [email protected] >> To unsubscribe, go to https://www.febo.com/cgi-bin/ >> mailman/listinfo/time-nuts >> and follow the instructions there. >> > _______________________________________________ > time-nuts mailing list -- [email protected] > To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
