I've just been re-reading a twenty-year-old article in SCIENTIFIC AMERICAN
on fitting a pendulum clock with a crystal-controlled device that tweaks
the pendulum period to make the clock super accurate. The author doesn't
use an oven; he uses a "tuning capacitance" that has a special tempco,
along with a selected 3 mHz crystal. Is anyone familiar with this article?
Does anyone know whether the author's crystal recommendations are still the
best available? He claimed an accuracy of about one second per year for his
setup. That seems pretty incredible.

Reg Neale

>>Hmm. The ovens I had to do with had thermistor/transistor oven drive PLUS
>>thermistor/varicap compensator on the rock for startup performance.
>
>That's a new one for me.  I never was too concerned with start-up time, so
>maybe that's why I missed that technique. :-)
>
>BTW, I just checked my 5345 book.  It's got a block diagram of the HP 10544
>OCXO and a schematic of the optional room temp oscillator.  I could scan if
>you're interested, but there's not a lot of detail there (especially the
>OCXO).
>
>
>>I don't think it will be overtone. I'll have to read the relevant things
>
>>From what I've read, overtone rocks are less pullable.  Seems like if
>you're going to use an oven for better stability, might as well use an
>overtone crystal as well.  (Unless you need to be able to pull it, as in a
>PLL...)
>
>
>>I'd like to use an off the shelf crystal and correct for its misbehaviors
>>with (many ?) other standard components, preferrably all SMD, about the
>>size of the HC crystal case. I expect to spend a lot of time on this one
>>and learn a great deal about what can and cannot be done this way.
>
>Can you get off the shelf crystals with turning points of 80¡ C?  If not,
>here's an option that you could explore (I've considered it, but I decided
>that if I was going to all the trouble of building an oven, I wasn't gonna
>stick some low-Q fundamental mode piece of crap in there):
>
>http://www.sonic.net/~shageman/pco.html
>
>It even uses a PIC!
>
>
>newell