On Wed, 19 Aug 2009 18:14:14 +0100, "Dave Plowman (News)"
wrote:

But is there any real difference between a 'quartz' watch and a PC clock?
They both rely on a low cost crystal?

Oh yes. The original Accutron was a steel tuning fork osillator. No
crystal of any kind to drive it. It depended totally on mechanical
stability.

Watch crystals come in a few flavors. The original version used
Statek type quartz tuning forks. They're really a mechanical tuning
fork made out of quartz:
http://www.statek.com/products.php
They work nicely at low frequencies and do not require a large divider
chain to drive the gears. 32.768Khz was the most common.

As IC technology progressed, it was more economical to use a big
divider chain and a higher frequency crystal such as 3.57945Mhz.
Meanwhile, someone figured out how to shrink the 32.768Mhz crystal, so
the next generation went back to those. (This is a gross over
simplification). The problem is that these relatively low frequency
and small physical size crystals have a terrible temperature
coeficient. Here's a typical data sheet:
www.abracon.com/Resonators/AB26T.pdf

The original IBM PC used a 14.31818MHz AT cut crystal. It was much
more stable, but there was no mechanism for adjusting the exact
frequency. There was also no temperature compensation or even the use
of temperature stable capacitors. This sorta explains how it works
and includes at series of curves for AT and SC cut crystals.
http://www.4timing.com/techcrystal.htm
The IBM PC oscillator was somewhat of an improvement in stability over
the typical watch crystal, but without an adjustment, it was nearly
useless.

Since 1981, I've looked inside literally hundreds of computahs and
SBC's. Not a single one has a tunable clock oscillator. One or two
used replaceable modular oscillators, which could pre purchased as a
TCXO, but which were usually supplied as a commodity clock oscillator.

These daze, the way to stabilize a TCXO is to first pre-age (beat-up)
the crystal to reduce long term drift. The crystal oscillator is then
characterized over the required temperature range. A table of
frequency versus temperature is generated and saved in a PROM. A PIC
controller on the oscillator takes the measured temperature, reads the
table, and applies the necessary correcting voltage to a varactor to
stabilize the oscillator over a very wide temp range. With this
method, you can take a really awful crystal, and compensate it to
impressive accuracies.

gotta run...


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558