shield can

Closed cell foam isn't much worse as an insulator than still air, and it is a lot easier to manage. What you really need is a closed cell foam with roughly 1 cm diameter thin-walled cells.

Bill Sloman, Sydney =================================================================================

Don't they call that bubble wrap? :-) :-) Just add as many layers as necessary.

----- Regards, Carl Ijames

They do, now that you call my attention to it. It's a bit bulky for John Larkin's application, but does suggest that a closed draft shield is close to what he really needs. His problem is that he can't find one off the shelf.

The photo he posted of his first try is un-dimensioned, but looks to be a bit too big to inhibit laminar convention.

Not a lot better and probably a lot more expensive actually. Air won't do the job unless it is trapped. Trapping it in a metal box is almost certainly more expensive than trapping it in a foam box. The "cheap and done" part for this might just be a foam cover. In fact I bet the "better" part is swamped out by the thermal conduction of the PCB. Styrofoam is easy to make in a variety of shapes (the molds are very easy to make) and it is hard to get much cheaper. I bet if you look around you can find a Styrofoam box that will do the job perfectly. Given that they would trap a pocket of air around the OCXO, they are likely a better insulator than air in a metal box.

I haven't seen dimensions of your OCXO, but I can think of at least one Styrofoam box that will almost certainly be big enough, the little boxes they sell fishing worms in. You can get them without the worms.

The easy way to do that is to make it symmetrical so there is *no* wrong way to assemble it.

I did a little searching and these came up as being pretty good candidates, but hard to tell since no sizes have been provided.

Solo makes "extra squat" bowls which should be close to the right size. If any of these are too tall, it is *very* easy to shorten them with a hot wire. String a piece of resistance wire above the bench and pass enough current to warm it up without being so hot it burns the plastic, just melting it. Push the box/bowl under the wire and you now have a ultra squat box or bowl. It would take 10 minutes to run 100 of these. Dart-Solo has sizes from 2 ounces up.

The 5B20 looks very short already.

I remember seeing a local plant that produced styrofoam sheets this way. They made a block of styrofoam maybe 4 x 8 foot and 24 inches thick. It was run through a set of hot wires that cut it into sheets. Very low tech, but effective.

Have you ever used

This is cute:

It's a Digikey stock 50 cent potting shell with one hole drilled into the top. The bypass caps locate and orient the shell on the board, so the hole can be big and sloppy.

There you go!... I never thought of searching Digikey, for thermal stuff like that. (I like to browse the McMaster-Carr catalog, that might be the last paper catalog I still use regularly... OK I've got a ~10lb monster from Thorlabs. and some older ones that have nice reference sections.)

George H.

Last week I dumped my old Digikey and Mouser and Allied and Grainger catalogs. The search engines are finally getting sort of decent.

I still find that googling images is a good way to find parts.