[time-nuts] Temperature sensors and bridge amps

Magnus Danielson magnus at rubidium.dyndns.org
Sun Nov 14 04:06:22 UTC 2010


Bill,

On 11/14/2010 04:41 AM, Bill Hawkins wrote:
> Rick,
>
> Is the double integrator actually a cascade of two controllers,
> where the primary controls the crystal temperature and its
> output sets the setpoint for a heater temperature controller?
>
> That's how industrial control handles the lag between a 5000
> gallon reactor and its steam-heated jacket. There's a sensor and
> primary PID for the reactor contents (stirred) and a sensor and
> PID for the jacket temperature.

If you looked at his paper you would find that the PIC-based controller 
is a PI^2ID controller, with two integrator-staged in series... it has 
three temperature sensors and one stage oven.

> Seems to me that if the only disturbance to the crystal temperature
> is the ambient temperature, then you can lag ambient with enough
> insulation so that the internal controller can be slow enough to
> have adequate gain.

I've found that several commercial OCXOs (not being of the major type) 
can be quite upset by shifts in surrounding air, especially when other 
requirements forces use of forced convection using fans.

In one case I was able to find an instability in the control-loop 
itself. I am not quite sure of the source, but it never quite settled 
but kept oscillating to such degree that it modulated the crystal 
temperature and thus the output frequency.

> There are disturbances within the internal loop that would have
> to be dealt with, such as the heater supply voltage and the offset
> voltage of the first error amplifier.
>
> We are going for stability, not accuracy, right?

Stability is the main goal, putting it at a place where the crystal is 
least sensitive to shifts helps to bring down the effective sensitivity 
to temperature.

Cheers,
Magnus



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