[time-nuts] -hp- 10811 repair (Thermistor)

[Bruce Griffiths]

Rick Karlquist wrote:
> Bruce Griffiths wrote:
>   
>> Dan
>>
>> The real reasons that this part is labelled non replaceable are
>>
>> 1) its epoxied to the oven.
>>
>> 2) Setting R20 to the correct value actually requires plotting the OCXO
>> frequency vs R20 and selecting R20 so that the OCXO frequency is located
>> at the stationary point on the frequency vs R20 value plot. This
>> requires a high resolution frequency measurement setup with a highly
>> stable reference and takes considerable time as one has to allow the
>> oven temperature to settle between adjustments to R20. Drift due to
>> aging and ambient temperature changes limit the accuracy with which the
>> OCXO turnover temperature can be achieved.
>>
>> Bruce
>>     
>
> That's news to me.  AFAIK, the individual crystal has a known oven
> set point that is determined in the crystal fab.  R20 is selected
> to achieve this set point.  What Dan did is exactly right, IMHO.
> BTW, many 10811 crystals did not have a turnover temperature as
> you imply.  They simply had a point of minimum (not zero) tempco
> where there was an inflection point (2nd derivative is zero)
> but 1st derivative is not zero.  This is well established SC
> cut theory ("true" SC cuts, that is).  The stability of the oven
> set point was quite adequate in terms of the overall 10811 error
> budget.  Most the tempco is due to the electronics pulling the
> crystal, as detailed in my paper on the E1938 oscillator.
>
> When you install the new thermistor, try to duplicate the lead
> dress of the old one.  That was supposed to mitigate against
> heat running up the leads and corrupting the thermistor.
>
> Rick Karlquist, N6RK
> R&D engineer, HP Santa Clara Division 1979-1998
>
>
>
>   
Rick

Sorry, as soon as I posted the above, I realised I was actually
referring to the older 10544A/B, etc., OCXOs which did require such an
involved oven tuning procedure.
Meanwhile, before posting a correction I was trying to find data on the
oven setpoint temperature tolerance.

The relevant HP Journal with an article on the HP10811A is:
http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1981-03.pdf

One concern is:
How accurate does one' thermometer which one uses to calibrate the
thermistor bridge setpoint have to be?

Bruce