I have been using quarz based oscillators for most of my professional life, as well as making a first 1 transistor  Collpits osc  as a kid for RF down convertor to receive VHF on an HF reciever..  I may be able to advise ;-)

So is there anything unusual about this system ? What package is the crystal in, what is its crystal cut, I assume it is fundamental resonant not overtone, and how accurate do you need the frequency will determine the temp control you will need.
Phase noise will determine the drive level and topology.  DO you need any pulling ability ?

Mike

Hey Mike, that's great to hear!

It's in the creation of a microbalance system, it's going to be used under vacuum, in terms of package they're 1" research crystals that look like this (1 inch (25.4mm) Research Crystals (testbourne.com))

We're driving this at 9Mhz, and monitoring the amount of molecular deposits that land on it over time, and comparing this to a reference crystal which is the same but not exposed to the deposits (so you can see the difference over time). 

the way we'd been interacting with the crystal currently was using off the shelf pogo pins but they're a bit heavy handed and it meant that assembly of the system tended to result in the potential to crack the crystal if you weren't ultra careful putting everything together. 

I was looking towards potentially a spring contact this time to lessen the impact it has mechanically on it's oscillation. 

Yeah we're looking at resonant frequency, accuracy isn't ultra critical as we have the reference crystal to compare to which is in the same environment, just not open to the deposits (so it should always give a reference at 9Mhz), 

in terms of monitoring we're in the realm of days and weeks, but accuracy, my understanding is we're only going to be looking at incredibly small drops in frequency even when the crystal is fully saturated, so it's not going to drop from 9 to 5 mhz, it's going to be something like 9mhz and a drop of tens of Hz. 

ah yes.

Interesting.

I have done something similar for a measuring metal deposition in an evaporator jig we had in a clean room  - about 20 years ago,  when I worked at a place with a clean room on-site.  Actually we used to buy cheap crystals made for TV in a  metal can and then drill, well mill actually,  the case to expose the actual crystal  of about 10mm dia and  250um thick.
. As we were mainly measuring aluminium thickness we used to etch back and re-use the crystals sometimes. From memory we used to have a mixer and counter arrangement  looked at the difference in frequency between the one being plated and the other not, and with metal at the vibration antinode we got something like few  100Hz per micron on the test slide so I reckon some tens of nanograms per Hz on the crystal sweet splot of about 1cm area. My memory of the numbers may be a bit off.

Being canned the connection  to crystal problem was replaced by a connection to wire problem and we had berylium copper or phosphor bronze springs, not sure which now, that were rather like a PCB edge connector.

 Yours look rather  more up-market - are you using them with liquid contact as that datasheet suggests ? I presume that you do not clamp the crystal wafer edges, or do you?  Can you sketch or photo your arrangement? Also how far away is the oscillator electronics? If you are not careful you will end up measuring things that do not just depend on the crystal - is your oscillator usinf the crystal as a series (transmission) element or parallel (so one wire and gnd to the Xtal.)

To temp control the reference outside the vacuum we cheated and soldered a power transistor to the can of the reference rock, and had a circuit that put more  or less current into it to maintain a steady voltage on a diode also in good thermal contact used as a thermometer and tracking a similar diode in the vacuum. Nowadays I'd probably buy something.

M.

Yeah i was thinking something like spring contacts to drive the crystal to replace the pogo pins as they were a royal pain to fit anyway. 

No liquid contact no, at present the entire crystal sits on an o-ring, but I did see other parties using clamps on the crystal edge, they almost looked like keyrings you prise apart slightly and gently put over the sides of the crystal. 

In terms of distance to the oscillator electronics, currently the design we had (which we're trying to revise), has the electronics on the other end of the pogo pins, so if you think there's the crystal, then two sets of pogo pins, and they're mounted to a pcb so it all sits quite compact, so distance wise you're looking at a centimetre or two at most. 

I believe the crystals in parallel, on X1 and X2 of a 74LVC1GX04 chip.

The reference crystal is also under vacuum. 

Yeah we've been thinking of using a TEC, or power resistor cooling / heating wise.

That is an interesting chip, our circuits were more discrete  datasheet for your chip. So you are using the crystal  as a series mode transmission structure,  now to get the best phase noise you'd normally match the Zin and Zout of the amplifier to the Rs of the crystal, (as per the oscillator models of Leeson and later Everard ) but the optimum match is pretty soft and it will work very well quite a long way from that equal impedance condition. However this chip seems to be high Z input low Z output and has more in common with the sort of oscillator built into digital watches and PC clocks. For what  you need it may well be more than adequate. In vacuum you may need to keep the dissipation in the crystal lower than you would in the fresh air - I'd aim for tens of microwatts max. (appnote about measuring that) Even with no deposition a fresh crystal ages in two ways, it moves up  in freq as it shakes any dust from the grinding process free, but down as the metal plating of the contacts beds into any voids. To pre-age frequency standards  we would drive hard and then drop the levels to  a few tens of microwatts dissipated in the ESR to give many years of stability,

In your case long term age is not so critical as you zero at the start of the run, but jitter may be,

I'm not sure I am the best placed to answer questions about mounts in your situation as it will have more to do with the constraints of the rest of the experiment around it,  though it may be worth sawing a few commercial crystals open to see how they have done it.  The sort of thing I'd expect is quite gently springy - almost at the compliance level of an O ring with wire wrap so the acoustic impedance is way lower than the quartz ,and it is free to flap.

~I see these chaps are using loops of wire and conductive dag - this is similar size  to what I recall when cutting open HCU49 style ones all those years ago

Image from https://www.researchgate.net/figure/An-image-of-a-quartz-crystal-used-in-the-quartz-crystal-microbalance-when-mounted-in-a_fig4_266038341

Mike.

That was the chip we were using previously, but now I'm thinking of using one of these, STM32H747XI

DS12930_STM32H747xIG.book (arduino.cc)

Purely because it looks like it has the control circuitry for the crystal and can also do the frequency sampling, counting and reporting in a single package, whereas at present that's split out into:

XTAL Driver > AND Gate > Binary Counter > Multiplexer. 

I did see the image you mentioned a few times online, I'm looking into that. One of the worries I had was being able to not just drive the crystal, but heat and cool it too! 

So what you were saying about driving the crystal, presumably you're doing that to standardise the response of the crystal over a production run to ensure they're all as close to each other as they can be?