[time-nuts] Improving the stability of crystal oscillators

Neville Michie namichie at gmail.com
Thu Oct 11 05:01:40 EDT 2007

Hi All,
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.
For a system that has a coal truck that must dump its coal down a  
hole, the system has mass, velocity
and distance, all the qualities to get a perfectly damped system with  
PID control.
For thermal control, the function is more likely to be a Bessel  
Function, and a Z transform filter
is more likely to find a match.
In any case, PID controllers are often to be found in totally  
unsuitable situations giving worse control
than even a bang-bang controller.
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-
Because the block is well insulated it soon becomes very close to  
cheers, Neville Michie

On 11/10/2007, at 3:08 PM, Bruce Griffiths wrote:

> Neville Michie wrote:
>> I have made several ovens for oscillators over the years.
>> The recipe is:
>> get a piece of aluminium big enough to contain the oscillator,
>> voltage regulator and first stage amplifier.
>> With a mill remove the shapes of each component.
>> Bolt a large power transistor, large power fets are best, to the
>> outside of the block as a heater, and it is run
>> off the unregulated input power. Judicious selection of a component
>> decides the start-up current.
>> Make a plate to cover the excavation for the components, and bolt it
>> down.
>> The circuit can be made with discrete transistors in the most
>> unstable looking amplifier ever seen,
>> alternating NPN and PNP transistors, connected directly to each other
>> with load resistors.
>> The main temperature sensor is a resistor bridge with a high value
>> glass encapsulated thermistor.
>> These are available a several trade houses. The amplifier is also
>> temperature sensitive, but is within the thermal loop.
>> The thermistor bridge gives a very large signal ~ 50mV per degree.
>> Gain may have to be backed off if thermal
>> oscillations do not die down, but the metal block acts as an
>> integrator and the circuits are very easy to get
>> high gain and sensitivity.
>> The whole block is packed in two inches of foam insulation, my 1MHz
>> oscillator only draws about 80 mA at 12 volts.
>> The temperature is set to 40 C.
>> The stability of the oscillator is very good, but as I have not yet
>> got a disciplined oscillator going I dont know which is drifting,
>> the HP 10811 in my frequency counter or the 1MHz oscillator. After a
>> year, the difference is currently 0.3 ppm.
>> cheers Neville Michie
> Neville
> A correctly tuned PID control loop should allow even tighter  
> temperature
> control.
> A "boostrapped" oven like that used by Wenzel should be even better.
> (http://www.wenzel.com/documents/Sub-pico%20Multiplier.pdf).
> Despite Wenzel's claims this type of oven isn't new it was used for
> portable standard cell enclosures decades ago.
> Bruce
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