[time-nuts] Re: Should a double oven XO be thermally isolated or just draft protected?

[Bob kb8tq]

Hi

Unless you measure the change of the device over a controlled temperature
range ( like 0 to 70C ) at a controlled rate ( like < 0.1C / minute ) it’s hard to 
know if this or that restriction / insulation on an OCXO has “upset” its temperature
compensation. If you “make the heater work half as hard” you may have doubled 
the thermal gain. That’s big change ….. 

Bob

> On Jul 5, 2022, at 1:48 PM, ed breya via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> This may give some idea of how fast things can happen when the OCXO is subject to drafts. I have this dual GPSDO box that usually is open for experimenting, and have a setup comparing one of the 10 MHz outs to my portable Rb reference. The 10 GHz multiplied output from the Rb is indicated on a microwave counter, using the GPSDO as reference. This gives 1 mHz resolution on the 10 Mhz signals at the 1 Hz counter resolution limit. It normally reads 10 GHz "exact" +/- 1 Hz when things are stable, or up to maybe up to 2 Hz when garage ambient is changing. I just turn the counter on whenever I'm in the mood to take a look.
> 
> The upper GPSDO board is exposed, so I can just put a finger on the case of the small (about 1" x 1.5") OCXO for a few seconds. Almost immediately, the counter shows several Hz change, which gradually recovers, with some over- and under-shoot. During all this, the OCXO is changing, and the GPSDO is trying to fix it.
> 
> Having a bigger OCXO with more thermal mass and insulation, and having more protection from fast ambient changes can help a lot. As others have said, you don't want to overdo it - the oven heating system must be kept working under all conditions, but it's OK to make it not have to work too hard.
> 
> An extreme example of a bad thermal situation is in the beloved HP8566. I have often lamented about the poor placement of its internal OCXO, which is right in the main air plenum that feeds the fan cooling air to the whole instrument. The OCXO is subject immediately to any change in ambient, and its heater has to work very hard. I'm convinced that this is the cause of most OCXO failures in the 8566. I've had to refurbish a number of these. The typical failure I've encountered is that the foam insulation deteriorates from the high heat flux needed, and the chemicals from the foam cause the oven setpoint adjustment pot wiper contact to fail. An easy way to spot this problem is to gently shake the OCXO - if you can hear and feel the guts clunking around inside, then it's due for repair.
> 
> At an opposite extreme, in my "Z3801A in a HP5065A carcass" project, I substantially isolate the OCXO from ambient. It's already a double-oven style, and I further enclosed it in a mu-metal box (made from a CRT shield). The OCXO is suspended on rubber vibration mounts, inside the box, and has a thin (~1/4") layer of non-woven fiber insulation on all sides between it and the box. The insulation has very little R-value, but suppresses turbulence and convection flow inside. The Z3801A guts are arranged specially to fit and occupy about two thirds of the cabinet volume, and this section is largely sealed off from the outside and from the right side battery compartment. A small fan runs at very low speed to gently circulate the air inside the compartment, and the plentiful amount of cabinet skin easily dissipates the total power. The same type of insulation is also placed under and atop the main board in the DAC/EFC circuit area, to slow down thermal changes there. The EFC's SMB connector set will also be shrouded with an insulating tube, to reduce thermal voltage. I even changed the nearest board mounting post to plastic, to reduce effects of thermal conduction and ground current in the vicinity.
> 
> All of this does not protect from ambient, but only the rate of change. It's more or less a constant temperature rise type deal, assuming constant power dissipation when everything's stable - and not too much wind or draftiness on the whole cabinet.
> 
> Ed
> 
> 
> 
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