[time-nuts] HP 10544A Repair

[Bob Camp]

Hi

Thermistors are often spec’d in terms of a tolerance on a value at a calibration temperature and a
tolerance on a value for “beta” that goes into the standard thermistor R/T calculation. Typical parts
are calibrated at something like 25C and you might get 1 or 2% at that point if you spend enough 
money. With a normal tolerance on beta, that 1 or 2% becomes a small part of the tolerance by the time 
you get past 40C. 

Can you get a large batch calibrated at a higher temperature? Sure you can. They already are expensive
parts and when you put an adder on top of that … the finance guys math goes tilt. If you need a very 
accurate number, the old style approaches were to turn hunt the oven or to measure the value of the part
in a “local standard temperature of 95C” oil bath.

Pretty much nobody does an OCXO either of those ways anymore. It’s all automated and uses a couple
of measurements to set the part to best measured temperature performance. At some point in the performance 
equation, you don’t do anything more than take a guess at oven temperature and things work “good enough”
for that spec. 

Bob  



> On Feb 27, 2016, at 11:21 PM, Hal Murray <hmurray at megapathdsl.net> wrote:
> 
> 
> richard at karlquist.com said:
>> If you replaced the thermistor with an exact replacement, then you shouldn't
>> need to change the pot.  ...
> 
> How repeatable are thermistors?  How close do you need/want to get the 
> temperature?
> 
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
> 
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