[time-nuts] Improving the stability of crystal oscillators

John Ackermann N8UR jra at febo.com
Wed Oct 10 14:50:32 EDT 2007


SAIDJACK at aol.com said the following on 10/10/2007 02:39 PM:

> BTW: I wonder how much effect thermistor and resistor-bridge aging as well  
> as opamp offset drift has on the long term aging of typical OCXO's since it 
> will  shift the temperature setpoint of the crystal.
>  
> I think this is an issue that's not receiving a lot of attention in the  
> literature..

More anecdotes.  I have a bunch of old Sulzer 2.5 and 5 oscillators.  As
Tom will happily tell you, one of these guys that still works well can
be one of the best pieces of quartz you'll ever find (and a great toilet
flush detector!).  But most of mine are a bit flaky, and the thermistors
seem to be the main problem.  Each unit has at least two -- one is
inside the inner oven and is part of a bridge that drives the "TEMP"
meter position; it's supposed to read 0 when the oven is at temperature.
 Another is part of the bridge that sets the oven temperature; there are
hand-chosen components designed to work with the thermistor to drive the
heater to the right temperature.  I've seen units where one or both was
so far off, you couldn't really tell what the oven was doing.  In at
least one case, there was an open that drove the oven full blast
continuously.

Someday I'd like to build a test fixture that would allow measuring the
crystal turning point and setting the ovens back to optimal temperature...

So, aging of these parts can certainly be an issue.

John



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