[time-nuts] Improving ocxo temp control

Bob kb8tq kb8tq at n1k.org
Fri May 18 22:46:14 EDT 2018


There are a number of papers out and about about the limits on OCXO performance.
The bottom line is that coming up with a high resolution control circuit is the easy part
of the task.

Simple answer to the question: 

Set up a thermistor bridge and feed the difference into an op amp. Crank up the gain on
the op amp to whatever you feel comfortable running. 

Some simple numbers: 

Thermistor changes 3% / C 
Single thermistor bridge changes 1.5% / C
Output of the circuit will change the oven by 150C from power off to full on
Neglecting the scale factors for simplicity 
Put in a gain of 100 on the op amp

So, the bridge moves 1% and the controller goes from full off to full on. 
Crank in more gain “as required”. The op amp isn’t bothered until you get
into the millions.  

With the simplified numbers above, the circuit has a thermal gain of 150. Getting 
much past 300 with a single oven is unusual. 


> On May 18, 2018, at 2:03 PM, Gilles Clement <clemgill at club-internet.fr> wrote:
> Hi,
> I am trying to improve performance of an OCXO.
> Could you point me at a good design of a high resolution oven temperature controler please ? Preferably analog.
> Thx much,
> Gilles.
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