On 6/26/2011 11:56 AM, daestrom wrote:
On 6/25/2011 22:28 PM, Salmon Egg wrote:
In article
,
wrote:

Anyone who has a device with a crystal oscillator timebase knows that
the device has to be reset to the correct time occasionally. On the
other hand, plug-in devices that use the 60 Hz signal for the timebase
have been (up until now) essentially perfect since the signal has been
manually twiddled to make it so. (There is actually a 10 second
tolerance in the East, less in the West.) And implementation is dirt
cheap: you capacitively couple to the power line, use a Schmitt
trigger, and some divide-by-60 counters. With better performance at
less cost, using the power-line frequency as a reference is the
preferred method.

Thing become ambiguous in plug-in devices with "battery backup."
These devices do have crystal oscillators to ride out the power
outage. When the power is on they can either use the crystal
oscillator as reference, or the power line as reference, however the
device was designed.

I must have lucked out. For about $30 I bought a Pulsar wrist watch at
least 15 years ago. I estimate its stability as about one or two parts
in 10^7 as long as the temperature does not have extreme deviations. I
only reset it when I need to change for daylight savings time or for a
new cell. It is about four seconds fast this now. That may be because of
the cell having been in the watch for over two years now and it is
probably going to fail soon.


An interesting story, some of the early crystal watches had their
accuracy tied to maintaining the crystal temperature. By accounting for
the watch being on a human's wrist, the designers were able to design
for a specific temperature at the back of the watch.

In some old power plants, we had ovens to maintain the crystal
temperature of our frequency base. But curiously, the frequency base was
not used for controlling the generator (base-load plant), only the
oscillograph for transient plots.

daestrom


A lot of the test equipment from the big old name brand manufacturers
had crystal ovens inside for the time base circuitry when I worked a
bench tech in a repair depot 30 years ago. Like gold boat anchors and
just as expensive. :-)

TDD