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@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 > > > > _______________________________________________ > time-nuts mailing list -- time-nuts@lists.febo.com > To unsubscribe send an email to time-nuts-le...@lists.febo.com _______________________________________________ time-nuts mailing list -- time-nuts@lists.febo.com To unsubscribe send an email to time-nuts-le...@lists.febo.com
