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

[ed breya]

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