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 _______________________________________________ time-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
