Small temperature controlled oven for housing an ocxo

As the owner of fifty 14.4 MHz tcxo's, it's quite hard to find a use for all of them. However, a slight modification to bring the frequency up to 14.402 MHz or so make them useful for multiplying up to the beacon section of the 10 GHz band. Ageing of the quartz unit hasn't been a problem in that application, but better frequency stability is really a must and ovening the modified tcxo (and it's accompanying buffer) suffices - hence this oven project.

The TCXO in question is a leaded module with pinning based on a 14 pin DIL format and a cut-down cavity extrusion from a Sigma Wireless (Pye) UHF duplexer section is ideal for forming the oven housing and results in a nice compact assembly, as shown opposite with the TO-220 transistor added for size comparison.
oven box

A number of small ovens based on the circuit shown have been used successfully before and the arrangement seems fairly docile. Delay in the heating elements (two TO-220 power FETs) impacting on the NTC thermistor sensor will produce an oscillating heating cycle if the feedback op-amp gain is set too high, but that's about all that needs to be checked. All the assemblies produced to date have been for UK use, so rather than setting the oven temperature to the usual 60C, they have all been set to a more friendly 45C.

The aluminium extrusion has corner slots to allow end cheeks to be attached using the appropriate size self-tappers, and the slot proves a useful location with good thermal coupling for the thermistor, which is in an 0805 smd format. This is held securely in place using two part resin (thin enameled wires having first been soldered to the end terminations).

Two power fets were used on opposing faces to reduce the thermal path length that a single fet would have resulted in. The thermistor is located within a few mm of one of the fets. TO-220 format load sharing source resistors were used so that what little heat they generated at least went into the extrusion.

oven and control pcb

For final application use, the 5v regulator soldered to the control pcb would be located inside the oven on the same pcb that the tcxo and buffer stage is assembled on.

oven circuit

thermistor characteristics
At 45C, the thermistor will have a resistance of about 4k3, so this should be the value also chosen for the lower half of the potential divider on the reference side of the comparator.
The upper half was also arbitrarily set to this value. It follows that the upper half resister (ie, the one feeding the thermistor) also needs to be this value to balance the comparator at 45C.

Current limit circuitry is required on the control pcb to set the maximum current that can be drawn by the oven. This is particularly important at switch on, when the comparator will be into hard limiting at all but the highest ambient temperatures. Thus the oven only operates proportionally when close to it's nominal working temperature. This saturation feature speeds up initial warm up time.




--- to be continued ---