[time-nuts] Improving the stability of crystal oscillators

Neville Michie namichie at gmail.com
Wed Oct 10 20:04:13 EDT 2007

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  
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

On 11/10/2007, at 9:24 AM, Didier Juges wrote:

> I bought 10kohm nominal NTC thermistors  from Digikey, P/N 490-4653- 
> ND. They
> are very small, 1/10th of an inch long or so.
> I have used those (or similar parts) in projects both at home and  
> at work
> (in military equipment...) for about 15 years.
> Didier KO4BB
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com
>> [mailto:time-nuts-bounces at febo.com] On Behalf Of SAIDJACK at aol.com
>> Sent: Wednesday, October 10, 2007 1:17 PM
>> To: time-nuts at febo.com
>> Subject: Re: [time-nuts] Improving the stability of crystal
>> oscillators
>> In a message dated 10/10/2007 11:13:21 Pacific Daylight Time,
>> jra at febo.com
>> writes:
>>> We  first built a small proportional heater circuit that we
>> could shove
>>> in  next to the crystal, and that worked OK.  We finally found a
>>> source for thermistors spot welded to a spring clip that
>> would mount
>>> snugly directly to the crystal case.  With 12 volts applied, these
>>> heated the crystals very nicely.  With the heaters, we were able  to
>>> keep the systems running year round with about 1/10 the
>> service  calls.
>> Hi John,
>> interesting anecdote!
>> Would you know if the thermistors are still available? Who made   
>> them?
>> Thanks
>> Said
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