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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