oscillator design with a long xtal lead

You might try cutting the coax to a quarter wavelength. Don't forget velocity factor.

Why can't you put the whole oscillator in the vacum?

Apparently yes. What is wrong with having the oscillator in tha vacuum too ?

Rene

Normally the problem is outgassing. With attention to the cooling, you can make the oscillator work in the vacuum--but often you can't make the vacuum work with the oscillator.

This sort of oscillator is used in quartz film thickness gauges, for example. Tweaking the cable length is a good idea, but a half wavelength will work better than a quarter--it will reproduce the crystal impedance, whereas a quarter wave produces Z0**2/ZL. A quarter wave will make the crystal look inductive except between its series and parallel resonances, which will cause the oscillator to not work.

Cheers,

Phil Hobbs

A coil, a cap and a SOT23 together with the XTAL should be sufficient. No ? It the SOT23 producing gasses ?

Rene

The phase delay is about 30 degrees wich if it was just afecting the loop delay it might just still work, however the impedance seen looking into the op amp end of the coax will look nothing like the crystal impedance your circuit needs, it will add a significant amnount of capacitance or inductance depending on the frequency, it might be as well to model it in a simulator, I assume your trying to operate the crystal in its inductive region, however if you can arange to use the crystal in its series resonant mode you might be a better off, especialy if your crystal has an esr of about 50ohms, the cable would then not have any effect on the circuit at all at the frequency of operation, you could then use a butler circuit.

Another thing is to try diferent lenghts of coax to see how long it can be before it stops working.

Colin =^.^=

The easiest way to answer this in a short post is to refer you to "Oscillator Design & Computer Simulation" by Randall W. Rhea. It's currently out of stock at my favorite bookstore, but you can give a try at ordering it anyway:

The book gives a systematic way of designing oscillators with linearized circuit models.

Or you can look at the impedance present at the end of the cable at your desired frequency -- with that cable length it will look like the crystal in parallel with a honking big capacitor. You may be able to fudge it just by putting an equivalent inductive reactance in parallel with the cable -- or you may just get weird mode changes. Rhea's book will tell you how to verify operation one way or another.

Hello Rene,

Only if it had chili con carne for lunch ;-)

You can encapsulate the oscillator and connect the crystal via glass-insulated terminals if needed. Encapsulation is quite common for medical implants where for obvious reasons outgassing would not be cool.

Of course if the medium to be measured is too hot that's another story.

Regards, Joerg

In a high vacuum _everything_ produces gasses. You can't stop it, you can only try to keep out things that persistently outgas. The amount of outgassing you have to be concerned with varies with the level of vacuum you're trying to attain, but things can get pretty nuts.

I suspect that the level of outgassing that would be absolutely unacceptable for a high-vacuum application would be orders of magnitude below the radar screen in many medical applications -- particularly if the gas involved was biologically benign, like oxygen, water or hydrogen.

Hello Tim,

True. But then I wonder how he is going to get a cable or connector in there that won't be outgassing.

Regards, Joerg

How about magnetic coupling through a glass window?

When they put IR imaging arrays into dewars they use wire pins going through glass beads set in a metal disk. I _think_ they fuse the whole thing together, but they may just seal them by clamping (or it may be that different companies do different things).

Metal and glass are pretty good for outgassing -- you only need to keep them hot with the vacuum pump going for a day or five. Some organic materials can be very bad indeed.

High and ultrahigh vacuum is a world to itself, and the rules are really different. I don't know what sort of vacuum the OP is contemplating, but if it's below 10**-6 torr, the cable idea is the right one. You can get low-outgassing cable, usually polyimide-insulated, with no jacket. Cables like that (and water-cooled holders for quartz oscillator crystals) are common throwaway vacuum system parts.

The circuit board is going to be the big killer. Also lots of vacuum systems need to be baked out at > 200C for hours and hours, which would get flux and various FR-4 exhalations all over *everything*. You could probably do this, e.g. on a sintered alumina substrate, but it wouldn't be a slam dunk, and you'd have to make it cleanable--really cleanable.

Cheers,

Phil Hobbs

Hello Tim,

If they can get whole IR iamging sensors in there it shouldn't be a big deal to get a wee oscillator in there. It could be encapsulated in glass. I have some very low frequency crystals that are housed in glass and there is at least 1/4 of a cubic inch of space in there.

The simplest oscillator to run with a crystal would be a tiny unbuffered logic single-inverter and a resistor. Pretty much the size of a few grains of salt.

Regards, Joerg

Hello Michael,

That is no problem, usually. You could feed power on one frequency and the 10MHz comes back. But one has to mind the folks from the FCC.

I always try to remain 100% ISM with magnetic coupling. Makes me sleep better at night ;-)

Regards, Joerg

True, and for a few more grains of salt you could make it so you could supply DC power on the same coax that delivers 10MHz.

Hello Phil,

Ok then, here is another idea: Create a low end impedance analyzer on the outside and instead of oscillating monitor the crystal parameters. With clever software it should be possible to compensate for the cable characteristics and their changes with temperature.

Regards, Joerg

Why can't you move the oscillator out to the crystal and drive the coax from outside?

I once had a gig where I worked around Ultra-High-Vacuum systems, and one day one of my coworkers came into the lab just livid - "These feedthroughs

- they're _brass_!!" It seems zinc outgasses terribly and contaminates the whole system. But, that's "ultra-high" vacuum - the kind where you can manipulate individual atoms and do molecular-beam epitaxy and stuff, so needs to be excruciatingly clean.

Cheers! Rich