[time-nuts] Making a HP 10811 and similar types better in the real world Part I

J. Forster jfor at quikus.com
Wed Sep 21 23:02:44 UTC 2011

That is not always as easy as it sounds.

The thermal equivalent of a "rigid body" does not exist. If you apply heat
to a block of metal at one end, it takes a while for it to propagate to
the other end. In fact, a long thin rod looks a lot more like a
transmission line than an isothermal block.

This matters because if you try and increase the loop gain, the wrap-up of
the phase shift soon reaches 180 degrees, and the thing becomes unstable
as negative FB at low frequencies becomes positive FB at higher



> Hi
> Actually, overshoot is pretty easy to eliminate on a conventional OCXO by
> picking a good location for the thermistor. The heater will always run
> "hot", but the rest of the stuff does not have to.
> Bob
> On Sep 21, 2011, at 6:37 PM, Magnus Danielson wrote:
>> On 09/22/2011 12:30 AM, Chuck Harris wrote:
>>> Perry Sandeen wrote:
>>>> GM List,
>>>> ..............................................................................The
>>>> ovens are proportionally controlled. On start-up all ovens,
>>>> proportional or not,
>>>> will have over-shoot. Some more, some less. An inescapable fact of
>>>> life.
>>> Imagine that the set point is variable, and can be set below the
>>> desired
>>> temperature. Then imagine that the set point can approach the desired
>>> temperature more closely as it gets closer to the desired temperature.
>>> ... And you will have discovered (100 years late) the PID controller.
>>> PID controllers do not have to overshoot the desired temperature. It is
>>> not an inescapable fact of life. 30 years ago I was designing PID
>>> controllers,
>>> with a little microprocessor magic, that could quickly arrive at the
>>> set
>>> point
>>> temperature and never, I repeat, never, exceed that temperature.
>>> Someone's
>>> internal organs would have become toast if it did.
>> Overshot is fairly easy to avoid for a well controlled PID loop simply
>> by setting the damping factor properly.
>> PIDs is nice in that you can control loop bandwidth and damping factor
>> fairly well. Overshot properties vs. damping factor is a well researched
>> field and already tabulated before I was born.
>> Cheers,
>> Magnus

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