WTF with my computer clock?
 

On Wed, 19 Aug 2009 23:28:19 -0700, isw wrote:

Putting a crystal in a temperature-stabilized "oven" is a well known
technique for generating a stable frequency (the telco folks and the
broadcast folks have been doing that for over 75 years, at least).

Yep. Motorola land mobile radios have had OCXO oscillators since the
1960's. However, in the mid 1970's, most land mobile radios switched
to TCXO (temperature compensated xtal osc), which draw less power, and
are less prone to burning out.

I have thought for a long time that it would be "neat" to glue a
resistor to the crystal case, and use heat to control the frequency.
You'd pulse-width modulate the power going to the resistor...

Yep. Some of the really cheap land mobile radios did that. There was
a metal clip, holding a resistor, sometimes with some silicon grease.
The problem with that scheme is that the lack of thermal insulation
means the resistor is going to burn plenty of excessive power heating
the nearby components and chassis. Same problem with a computah. Some
styrofoam insulation and a plastic can, is usually sufficient
insulation.

Note that there are quite small OCXO's that would work very nicely in
a PC. The small size and internal vacuum insulation means very little
heat loss and fairly fast warm up time.
http://www.vectron.com/products/ocxo/ocxo_index.htm

Incidentally, one problem with using an OCXO is that it sucks quite a
bit of power when the computah is turned OFF. If you kill the power
to the oven, the clock oscillator will drift away merrily, and there
goes your accuracy. I also don't think the EPA or Joe Sixpack will
appreciate the power drain. It certainly won't qualify for an Energy
Star rating.


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