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The comparators feed the flop, of course. As I said, it's sort of like a 555.

That I believe. A bit of Styrofoam is the right medicine for a lot of ills. Do you do anything to the ground plane round the oscillator to reduce external gradients?

Cheers

Phil Hobbs

Haven't done that for xo's. We have occasionally slit ground planes to reduce conductivity, to reduce thermoelectric and electrical gradients inside a sensitive patch.

TCXOs often have good compensation for slow temperature changes but bad transient correction. They really benefit from a little foam.

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from Hamlet

"Hamlet: Do you see yonder cloud that's almost in shape of a camel? Polonius: By th' Mass, and 'tis like a camel, indeed. Hamlet: Methinks it is like a weasel. Polonius: It is backed like a weasel. Hamlet: Or like a whale. Polonius: Very like a whale. "

It's a pity that Shakespeare couldn't anticipate the 555.

-- Bill Sloman, Nijmegen

That's getting to be a lot of parts. I could use a bandgap reference and an opamp-based closed-loop thing. Lots of parts, and it wouldn't glow in the dark.

A depletion mosfet would sink about 1.5 mA and have a more constant LED drive than the 2K resistor. But I don't have room for that!

I don't think noise in the 2K resistor matters. It divides against the low dynamic resistance of the LED, roughly 100:1. My power supply drift, and thermals, probably dominate the error.

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I mostly only like the comedies. We just saw "Two Gentlemen" at the Tahoe Shakespeare Festival, on the Nevada side of Lake Tahoe, on a peninsula jutting out into the lake, at dusk. Stunning, recommended to anyone who can make it. We had Adirondack chairs (and blankets!) near the stage and our own waiter for food and drinks. Things like that are fun, now and then.

We sort of adopted the single guy sitting next to us, bought him wine and loaned him a blanket. Turns out he teaches English and Shakespeare at UofN in Reno and knows the cast and the touring gossip. Fun.

I think my favorite movie of all time is 12th Night, the one with Imogen Suggs and Ben Kingsley and what's-her-name.

Yup, thermal diffusion sure is slow. Compensated thermal structures such as the gridiron pendulum also have horrible transient responses too, for the same reason.

Cheers

Phil Hobbs

Have you ever tried the foam in insulation, like 'Great Stuff', or would the chemicals it releases cause problems?

I haven't used it for insulation except around pipes. My present house had the main heating feed pipe to the bedroom end of the house cemented into a concrete block wall for 20 feet, so I had to chisel it out and add a combination of neoprene wrap and Great Stuff. (Now the inside of the house gets warm instead of the outside.) ;)

My main worries would be (a) it's ugly, (b) it's hard to control, and (c) it's slow. I suppose you could squirt a bit into the bottom of a greased tube, stick the whole works over the reference or XO, and clamp it down to the board so the foam couldn't leak out the bottom. After a day, remove the tube and trim the foam to size.

I'd much sooner use styrofoam attached with RTV or hot melt.

Cheers

Phil Hobbs

I was thinking about using it inside a shield, with a small hole in the lid to let the excess escape as it swells up. I am working on a GPS derived frequency standard, and looking into building a 10 MHz TCXO from scratch. I do have a couple open VCXO modules, without crystals that I could use as a starting point.

We made our own OCXO once. It was a thin PCB stuck to the top of an anodized aluminum block, with a mosfet heating the block. The crystal was embedded in the block. We cover it with a deep-drawn aluminum cover, 1.75" square and .625 high.

We buy glass-packaged, vacuum-sealed crystals for this. They have a specified turning-point temperature of 60C, so the TC is almost zero there.

We tried putting foam inside the cover and it was no better than, maybe worse than, still air.

Want to try some really nice foam? It's about 5x better than urethane.

We tried a slab of styrafoam. Is your stuff better?

--

John Larkin Highland Technology, Inc

jlarkin at highlandtechnology dot com

Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom laser drivers and controllers Photonics and fiberoptic TTL data links VME thermocouple, LVDT, synchro acquisition and simulation

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When we lived in England I was signed up for the Royal Shakespeare Company's subscription list. We'd see a couple of plays a year, mostly at the Barbican, though we went to Stratford at least once. It's more comfortable indoors.

-- Bill Sloman, Nijmegen

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Don't edit my text. HBC was OK, but I meant Imelda Staunton.

Before I moved to CA, I visited SF and decided to have a look at the UC Berkeley campus. There was a line outside a building, so I got into line. It was the Oxford Shakespeare Company doing Midsummer Night's. The stage was a bare wooden oval, nothing else but a couple of ropes hanging down from above, and the actors were all clad in fetish leathers. They were awfully good. I decided to move here.

Why? Cheaper? Better? Strange packaging?

Way better.

Best regards, Spehro Pefhany

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The point about foam is that it is mostly air, but the cell structure stops the air from moving.

Double-glazing works the same way - a 6mm gap between the panes is enough to suppress convection currents. Bigger gaps are better for sound insulation, but degrade thermal insulation because the convection currents can circulate fast enough to shift a significant amount of heat.

-- Bill Sloman, Nijmegen

Aerogel? Zillion layer mylar superinsulator in vacuum?

The real way to do a good OCXO is a double oven.