Car Clock drift - the lowly 32kHz tuning fork crystal specs
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
On Sun, 9 Apr 2017 07:45:23 -0400
Tim Shoppa tshoppa@gmail.com wrote:
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.
Yes, definitely. Although in the 80s it was only the higher class cars.
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.
I'd rather guess that it is some RTC package with crystal, temp sensor
and battery in single package, and they use the internal temp sensor
of the RTC for the dashboard display.
Is there a way to verify my guess at the TCXO method?
Beside opening up the dashboard and looking for the RTC or placing
the car in a climate chamber and measuring the temperature coefficient?
I don't think so.
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.
Most wristwatches do not have any temperature compensation. If worn, the
wristwatch is pretty close at the 25°C (the human body is a quite good
and temperature stable oven). The difference only starts to show when
the watch isn't worn for long periods of time.
Attila Kinali
--
It is upon moral qualities that a society is ultimately founded. All
the prosperity and technological sophistication in the world is of no
use without that foundation.
-- Miss Matheson, The Diamond Age, Neil Stephenson
Good quartz watches are, indeed, temperature compensated. More info here:
http://forums.watchuseek.com/f9/thermocompensation-methods-movements-2087.html#/topics/2087?page=1
Of much greater interest to watch nerds like me is the improvement of
accuracy in mechanical watch movements. Serious watch enthusiasts don't
spend (many would say waste) a lot of time on quartz technology.
Mechanical horology is a corner of time nuttery all to itself.
Bill
On Sunday, April 9, 2017, Tim Shoppa tshoppa@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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--
William H Fite, PhD
Independent Consultant
Statistical Analysis & Research Methods
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@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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and follow the instructions there.
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?
On 9 Apr 2017 2:01 pm, "Tim Shoppa" tshoppa@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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and follow the instructions there.
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@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
time-nuts mailing list -- time-nuts@febo.com
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On Sun, Apr 9, 2017 at 4:45 AM, Tim Shoppa tshoppa@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,
Have you always lived in the same place. What is the average year
round temperature there?
What I live the clocks always gain time, a minute every could mounts.
It seems I always have to set the time back a minute or two. But then
it really never gets cold here, maybe a dip below 50F at night in the
winter
--
Chris Albertson
Redondo Beach, California
In your case, the car sits in an environment that matches their test
setup well. In my case �\200� not so much.
FWIW, mine drifts pretty badly. It's in an aftermarket stereo, and I
don't remember when I bought it (I moved it from my previous car).
I assume that all quartz clocks and watches these days use "inhibition
conpensation".
https://en.wikipedia.org/wiki/Quartz_clock#Inhibition_compensation
Hi
On Apr 9, 2017, at 11:36 AM, Clint Jay cjaysharp@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)
Bob
On 9 Apr 2017 2:01 pm, "Tim Shoppa" tshoppa@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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Interesting that someone would complain their car clock, kept at
temperature controlled 25C, runs fast.
The manufacturer would set the clock calibration, not at 25C, but at 10C
(typical winter temperature) or 40C (average of cool night and baking hot
car interior temperature in summer).
So one half of the year the average temperature is on one side of the 25C
crystal turning point hump.
And the other half of the year the average temperature is on the other side
of the 25C crystal turning point hump.
Someone who put the clock indoors, at a fixed 25C temperature, would indeed
see the clock running fast.
But someone who keeps it in the changing outdoor weather, might find it
running on time (on average) in both winter half and summer half of year.
Still impressive that it's better than 4ppm on average over summer and
winter.
Tim N3QE
On Sun, Apr 9, 2017 at 7:45 AM, Tim Shoppa tshoppa@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