[time-nuts] HP 10544A Repair
kb8tq at n1k.org
Sun Feb 28 09:49:29 EST 2016
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.
> 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
> These are my opinions. I hate spam.
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