If you go the PWM route then given the negligible hysteresis wouldn't it make more sense to digitally compensate for temperature than ovenise?
Hi Marco,
The thought did cross my mind and I've done similar software based temperature compensation for things before, but I had access to a temperature chamber then and I don't now
Maybe in the future, but for the initial version, I'll probably ovenise the critical sections, possibly the whole lot. I'll then perturb the oven temperature (say +/-5 deg C) and use that to determine and set the overall system 0 TC point as I should be able to (at least to first order) use the JFET to offset any other system TC.
I haven't yet put much thought into the 10V generation, and it may prove too much trouble, but I'd like to be able to generate an arbitrary voltage in a range (0-12V?), basically like a calibrator. Will have to see how that plays out also with the overall temperature compensation as well. However first I have to get some confidence and understanding of the reference itself. I'll also have to think about the noise implications as well...
I'm also going for something that can be built and calibrated with minimum of equipment (basically a good DMM and/or null against a +10v reference). I'm not making any quantity so I'm happy to spend the time tweaking the TC of each unit. I should be able to do this by setting up PWM for approximately 10V, then monitoring the output voltage as I slew the oven temperature. Once the TC is tamed, then I would allow the over the settle mid-range for a while and then calibrate the 10V PWM against a suitable reference.
http://shop.kuhne-electronic.de/kuhne/en/shop/amateur-radio/accessoires/crystal-heater/Precision+crystal+heater+40%C2%B0+QH40A/?card=724
That might help stabilise the JVR, as it is quite small.
Hi MK, thanks for the link. Interesting. I'll probably end up with a bit larger of an oven than that is for. But saved for future reference - thanks!
Ash.