[time-nuts] Improving the stability of crystal oscillators

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Oct 11 01:08:43 EDT 2007

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

A correctly tuned PID control loop should allow even tighter temperature
A "boostrapped" oven like that used by Wenzel should be even better.


Despite Wenzel's claims this type of oven isn't new it was used for
portable standard cell enclosures decades ago.


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