Re: OCXO Kit - Are These Ideas OTT? #ocxo #mods

Alan de G1FXB

Hi Brian,
just my thoughts you are free to do as you choose.

a/.??? I guess your comment on using multiturn & vernier control is a result of the headline "the adjustment point is critical" ?
It's the Xtal oven target temperature that is critical, in maintaining the final goal of minimising Xtal frequency variation.
Less so the actual heater current. It is whatever it is, to heat it up to temperature.

It's really a set & forget adjustment, not ambient dependant.
Once done you don't want the temptation to give it just a little tickle,
You may get lost, very lost and be glad of the vernier scale...........

Be careful not to induce noise on the remote front panel wiring.

b/.??? Current limiting just the heater feed is highly desirable on first test, and initial setting.
I would caution too much day to day manual intervention, at best it just delays the oven temperature reaching equilibrium.
At worst it may result in overshoots which were not envisaged, and result in the very thing you are looking to avoid.
99.9% of builders do nothing special and the design works just fine.

(For a none QRP-Labs application I thought about adding a simple constant current circuit (Not variable voltage) from a 78xx or LM317 data sheet.
Else a L200 device, not gone beyond the brainstorm what devices I'm most familiar with to see which, if any is most suited for a circa 300mA absolute max figure.
More than likely there are more modern devices available now.
Alternatively add a series current limiting resistor, even better use that inpart to contribute to the heater function or just over engineer the heater with a power FET, IRF510 anyone.? :-[
And none of which will fit into this kit.....)

If I was going to have the patience to play,
is to trial a suggestion from Dick W7WKR, and simply short C1 and see if it does all it promises with little or no compromise.
It "breaks" Hans choice of oven control operation, so will not be acceptable to all.
It would be interesting if it provides a little more positive control during the setup process, and then note if it affects the adjustment point, and real world short / long term control if only as an experiment.
(the main reason of lag will still be the heat needs to peculate from the heater through the xtal can to the temperature "sensor" and reach equilibrium,
If you don't wait long enough before adjustment for it all to stabilise it will overshoot.)
Gilding the lilly would be to have a LED to show oven heat in operation and allow several heat / cool cycles before tickling the adjustment each time.
If it was still touchy then maybe a pcb mounted multiturn pot is of value, it will likely not fit the existing footprint and in doing so interfere with the oven enclosures......

OTT is your choice, whatever you find works....
Just don't loose sight that Hans wrote the book
on the OCXO kit. (both the assembly manual &
And perhaps Keep that vernier drive for another project, something nice like a ATU or VFO......


On 25/07/2019 10:45, Brian George via Groups.Io wrote:
Just starting to build the OCXO kit for my own homebrew project. There is plenty of room in the project case so I wondering whether to incorporate two possible modifications :

a) use a front panel-mounted control for R21 (temp adjust) with a vernier slow-motion drive. Could this be useful or is the setting of R21 truly "set and forget" regardless of the ambient room temperature?

b) take the heater supply via a separate LM317 (with heat-sink) and use a front panel-mounted potentiometer to vary the regulated output voltage from "low" to +5v. It would probably be necessary to feed the LM317 with around +6v to compensate for the drop-out. This would allow control of the oven start-up current, manually cranking up to the full +5v as the oven approaches a stable operating temperature.

Any thoughts on whether or not all this is completely unnecessary / OTT - particularly from those who have built and used this kit?



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