[time-nuts] Improving the stability of crystal oscillators

[Richard (Rick) Karlquist]

Neville Michie wrote:

> I am not trying to start an argument, but I would like to point out  
> that PID controllers are only
> good at controlling a certain class of system.

> The thermal block controllers work well because of the dominant  
> integrating effect of the block,
> the time delay for a heat front to propagate through the block is the  
> only concern for instability.
> When instability is a problem I relocate the thermistor closer to the  
> heater, giving a marginal degree of under-
> control.
> Because the block is well insulated it soon becomes very close to  
> isothermal.
> cheers, Neville Michie

I would like to point out that the E1938A uses a PID controller and
has a *transient* thermal gain of many 1000's not to mention a static
gain that has in some cases exceeded 1,000,000 for a single oven.

Your last statement is not generally true.  The block cannot be well
insulated because of the thermal overhead of the oven circuitry (the
heat has to escape).  I explained in my 1997 FCS paper how to achieve
the isothermal condition, which is achieved by symmetry rather than
high amounts of insulation.  The E1938A oven works quite well if the
insulation is omitted or replaced by poor insulation, except that it
consumes more power.

Rick Karlquist N6RK