[time-nuts] Improving the stability of crystal oscillators
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Thu Oct 11 05:08:43 UTC 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
>
>
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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