On 23/03/2021 11:08, Clive Page wrote:
On 22/03/2021 21:57, Martin Brown wrote:
On 22/03/2021 18:58, Scott wrote:
How does my car clock work?Â* It cannot be quartz or radio controlled
as it runs slightly fast.Â* It cannot be synchronous because the supply
is DC.Â* Is there any form of adjustment to get it to run to time?

It is almost certainly quartz crystal and not quite trimmed right.
Such mistakes were very common. One telescope maker fixed it by adding
GPS functionality to reset their RTC to local time. Mine as built from
a previous generation lost 15s/month reliably. They didn't install the
loading capacitors around the crystal to save money ( about 10c ).

It isn't hard to get a quartz clock good to 1ppm or about 30s/year but
only the better manufacturers actually bother to calibrate them.

It is slightly harder for a clock in a car than a watch on someone's
wrist which tends to be temperature compensated by the wearer.

I've had quite a number of quartz wrist-watches over the years.Â* The
first, bought during a visit to the USA a long time ago, cost under two
dollars.Â* None have cost me more than £20.Â* All of them have been
accurate to a few seconds a month which is good enough for me.

Provided the load capacitors are right and the drive power appropriate
the 32kHz xtals used in watches are very good indeed. Most these days
are digitally tuned by setting a digital fiddle counter so that 1s on
the watch/clock matches a reference second. No variable capacitors.

So it is disappointing to find that car clocks, mine among them, are out
by a minute or two a month.Â* I think it's partly the car environment -
wide temperature variations make it hard to get a quartz crystal to keep
time.Â* The car companies know that it's difficult to get accuracy
without a lot of expense, so they don't even try, they buy the cheapest
crystals then can get and don't bother to trim them.

More likely buy in an electronics module from the cheapest supplier.

This is a nice article by TI on the typical crystal cuts used and their
characteristics.

https://www.ti.com/lit/an/slaa322d/slaa322d.pdf

On the other hand, most cars now have a sat-nav system so it would be
easy to link them to reset the dashboard clock to GPS time at intervals,
but that would cost a few pence more per car during manufacture.Â* Since
few of us decide against buying a new model of car because it has a
poorly designed clock circuit, the problem will persist.

Adding GPS was how the scope makers fixed their dodgy RTC problem!
Also meant the user didn't have to enter their location correctly.

--
Regards,
Martin Brown