[time-nuts] MTI 260-0624-D OCXO

Bob Camp kb8tq at n1k.org
Sat Feb 20 22:10:47 UTC 2016


Hi

Anything electrical (duty cycle/ harmonics / output level / etc) 
is designed in on a modern part done in high volume. 

The only candidates for a pot on top of the oven are:

Inner oven temperature
Outer oven temperature
EFC

You have already ruled out EFC (it would be a coarse set so
visible on a normal counter). 

That leaves inner oven and outer oven. 

Why inner oven - to adjust for the data from your temperature run. The parts
are at best coarse set based on crystal temperature. 

Why outer oven - same as above or to set for inner / outer separation
and clearance after the thermal stabilization in the sealed can. 

If there is no apparent change in frequency (and from the location of
the pot) that suggests the outer oven. Regardless of which oven is being set, the
adjustment is likely a “move it 1.25 turns" sort of thing.  You then 
re-run the temperature test to see if it is in spec yet. 

Bob



> On Feb 20, 2016, at 2:56 PM, timenut at metachaos.net wrote:
> 
> Bob,
> 
> I still have a problem with the various theories. Again, how is it measured?
> If you have a production line, you don't want a "tweak" that requires hours or
> days to determine.
> 
> As far as setting the crystal set point (presumably the inner over set point),
> that makes even less sense. Consider that they clearly knew the turn over
> point of the crystal BEFORE it was packaged. They wrote it on the sticker inside.
> So, they would have no need to adjust the inner oven. That could easily have
> been done on the bench during assembly. It could also be done via a selection
> from a standardized set of components. That temperature is "sort of" critical.
> The curve is very flat, to it could be a little off. Since they knew the
> crystal turn over point in advance, there wouldn't be any need to "hunt" for
> it by an elaborate procedure after assembly.
> 
> However, the temperature of the outer oven is much less critical and I would
> suspect is simply set by the choice of components. And, if it needed tweaking
> then it could also have been done on the bench. Since they would know the
> thermal transfer characteristics, I suppose that they could measure the outer
> case's temperature to deduce the correct temperature of the outer oven. But,
> again, that doesn't make sense from a production standpoint. It would simply
> take too long.
> 
> Perhaps, they would be willing to eat the time needed for that type of
> adjustment, but it doesn't seem likely. All they would need to do would be to
> add some monitoring circuits that they could access. I've seen that type of
> thing a lot. It would be cheaper to add a few circuits for a quick adjustment
> than it would be to spend the time on the adjustment. People's time is worth a
> lot more than a few components.
> 
> On the other hand, tweaking the output voltage is something that is quickly
> measured as part of testing. Put it in a jig, make a quick check for
> functionality, tweak the output voltage and you are done. Or, perhaps it
> adjusts the duty cycle, the bias point or some other characteristic of the
> output signal. Perhaps it adjusts the reference voltage. I think I showed that
> it doesn't make a large change in the frequency. But, it doesn't make sense
> for it to make a small change (although I would be unable to measure a small
> change).
> 
> I think that there are a lot of unexplored possibilities, but I definitely am
> not convinced that it sets either the inner or outer oven temperature. There
> are simply better ways to do that in production.
> 
> I really, really want to know, though! I am going to try to repair mine one
> more time and see what I can measure. I didn't try very hard the first time,
> but I think its worth doing. If I can get it repaired again (I have to fix
> some traces, especially for the ground pin) it will probably take a couple of
> days. I have the parts to fix my fan circuit in my oscilloscope and I will do
> that first. And possibly tweak the power supply capacitors to reduce noise.
> 
> 
> Mike
> 
>> If this is a double oven, there are two oven circuits and two oven controllers.
>> Both would have a set point and both would be adjusted somehow. The pot
>> may be the adjustment on the outer oven. Get things all set up and packaged,
>> then adjust the outer oven to make things do what the should do.
> 
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