Jim Yanik wrote:
"Ian Field" wrote in
:


"gregz" wrote in message
-september
.org...

"N_Cook" wrote:

Bill Proms wrote in message
...

I have 3 Intelli-Time LED alarm clocks around the house, just like
the one
he



http://www.acurite.com/clock/alarm-c...igital-alarm-c
lock-13 027a2.html

I initially bought these due to them keeping time when the power
goes off
and auto resetting for DST. There is a problem, however. Each of
the clocks becomes inaccurate over time. If I set them all
manually to the

same

time, within a few months, each one will be off by 3-5 minutes.

So I ask, what is the problem and is there any way to repair it?

Thanks in advance,
Bill



I've always put this down to hash on the mains being interpreted as
extra cycles by the clock monitoring input. The supply companies
contractually have to correct the mains frequency so an exact number
of cycles per day (50/60)x60x60x24, but at any instant can be above
or below the nominal frequency.

I doubt if any use power line for sync. Most have battery backup.
Crystals jump frequency from time to time.


Even one I bought recently uses mains sync, but it has a battery &
crystal divider backup to cover outages.

The mains frequency varies depending on peak demand/off peak, but long
term its average has less drift than the cheap crystal oscillator
they're going to put in a radio alarm clock.





radio alarm clock probably won't even use an xtal;like Jeff L. says,they
may use a cheap RC osc.

I note my MW oven clock that derives it's clock from line freq. has better
stability than other "digital" clocks.(like my PC clock....)

I wish the most recent MW oven I bought for our office worked like that.
Its clock gains about one minute a day and I doubt that there's any
syncing to the line frequency built in. It keeps far worse time than the
$10 battery powered wall clocks we use around the office.

Could be the problem is that the temperature of the electronics in that
MW oven moves around quite a bit more than those in the clock on the
wall next to it due to the high power loads when the oven is heating
stuff. That could screw up a simple RC or even xtal oscillator, eh?

OTOH the MW oven in our home's kitchen is always dead nuts on time,
barring power failures, after which it needs resetting.

Jeff

Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*10e12 furlongs per fortnight.

(of course,I use an internet program to keep the PC clock fairly close. I
used to use a program(Atomic Clock) that direct-dialed the Naval
Observatory,but the long distance calls cost too much.)



--