[time-nuts] HP 106B quartz frequency standard...the story so far

Bob Camp lists at cq.nu
Thu Aug 6 22:45:38 UTC 2009


Hi

Regardless of weather it's a single or double oven, you want the  
crystal to be at it's "turn temperature". If you have a BT cut  
crystal, that's going to be the highest frequency you find as you move  
the oven around. With either a double or single oven, you should be  
able to "map out" a parabolic plot of frequency versus temperature.  
That's only close to "right" since the electronics contribute  
something to the temperature performance. To really get it *right* you  
need to vary the external temperature and see what happens ....

On a double oven, the outer oven is set for some "nominal" amount of  
inner oven current. The idea is to keep the inner oven controller from  
either cutting off or saturating as the external temperature is  
varied. Often the inner oven is set to the "low side" of it's power  
range.

Bob

On Aug 6, 2009, at 6:24 PM, Jim Palfreyman wrote:

> Hi All,
>
> Thanks very much for your interesting replies.
>
> I'm beginning to think I'm not letting it settle between adjustments.
> And besides, The thing has been powered off for at least a decade I
> would suspect and maybe I'm just watching the crystal adjust to micro
> changes in temperature as it warms up. It's been on for about 10 days
> with only about 30 min off time in between. I shall leave it on the
> bench for a month or two before changing anything again.
>
> Now my next question is to those with double oven experience. The
> service manual says the oven is "not field repairable". And there is
> minimal description on how to get the oven settings right.
>
> The container has 61.3 C on the side - obviously the point of
> inflection for the temperature curve of the crystal. There are two
> test points which connect to either side of a thermistor buried deep
> inside the inner oven. The label next to it say 432 ohms. The
> resistance it should be at the correct temperature. I'm currently
> running at 422 ohms (hotter) - that's the temperature that causes
> minimal drift in frequency (at the moment - maybe this will change
> after further burning in).
>
> The outer oven has no such thermistor. How do I know that is set
> correctly? If it is too high, I'd assume it would heat the inner oven
> too much and not allow you to drop the temperature in the inner oven.
> If it is too low I'd guess the inner heater would have to use more
> power and do more work to keep it ok. What is the ideal outer oven
> temperature and how is this worked out?
>
> In all this, keep in mind my power rails are running a few volts high
> and I'm not sure the effect this is having in the oven.
>
> Regards,
>
> Jim Palfreyman
>
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