That's a bad plan, generally. You would have to put the piezo and its fixture into some kind of model, including the cable that connects to that big box of a 'preamp'.
Putting an op amp and a feedback resistor into the fixture, is no big deal.
It doesn't save any of the really intimidating parts of measuring your device, just a few probably-don't-care items. Do you need an RS-232 interface, and is it a benefit to have adjustable bias voltage, and just how low does the noise HAVE to be, anyhow?
If you can clamp things, an old camera with telephoto lens (hey, the
35mm optics are kinda... cheap... at thrift stores!) onto a screen, with a silicon photocell behind that. Either use a venetian-blinds arrangement in front of the lens, or focus carefully and mask the light by scribbling with magic marker on the screen...Or, lens and pinhole. mounted in a cardboard tube with the inside painted black. Either way, you're sculpting the active solid angle acceptance of the detector.
For extra points, find the lamp's ballast power frequency (probably 120 Hz or 5 kHz) with an NE567 tone decoder.
Why not just amplify the voltage across the tuning fork, with a good opamp or a jfet?
Since a crystal looks inductive one side of resonance, you might even resonate it with a capacitor and get more voltage into a hi-z amp. You'd need to gat an approximate motional-equivalent Spice model for the crystal, and play with that in simulation maybe.
I wanted to use a vacuum-sealed 32K tuning-fork crystal as a cryogenic temperature sensor, for the CEBAF/Jefferson Labs accelerator, but we couldn't get them interested.
Sounds like the safe way to go.
Have you looked at the electrical model I dug out, and the L and C values in the Spice sub-circuit that I found?
If might be difficult to find real components big enough to be useful, and even more difficult to get them to couple to any useful part of the tuning fork.
Electronic reactive components - made with gyrators or whatever - might do better, but you may be stretching the OP's competence.
If you didn't know what the electrical Spice model of the part looked like, that might explain their indifference. How well did the vacuum seal cope with cryogenic temperatures?
The trouble with physicists is that while they can be shaky on electronic details, they tend to have a reasonable gasp of physical reality.
"Bill Sloman"
** Thinking a tiny clock crystal makes a good ultrasonic microphone is not very realistic.Quote from the OP:
" I am using the quartz tuning fork as a very sensitive force detector to pick up sound wave at its resonance frequency. "
... Phil
Is this crystal operated close to a resonant frequency ?
If so, you would need quite different circuits, if you are interested at frequencies near the series or parallel resonance frequency.
They certainly do GASP >:-} ...Jim Thompson
-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at
c details, they tend to have a reasonable gasp of physical reality.
"Physical reality" is the reality defined by physicists. As a chemist by tr aining, I'm attuned to the fact that "chemical reality" and "physical reali ty" aren't quite the same thing - though physicists will argue that since S chroedinger chemistry has merely been a branch of physics, just as - since Watson and Crick - biology has merely been a branch of chemistry.
Not the kind of joke that Jim could be expected to get.
I'd think that external vibration would couple very poorly into a tuning-fork crystal. Reciprocity, COE, high-Q, stuff like that.
If you could couple into just one tine, it would be very sensitive. You could probably detect light pressure or something.
And other such buzz words >:-}
...Jim Thompson
Ambient vibration, quite so. However, if the OP is using the crystal in a poor's scanned probe microscope, the coupling could be quite a bit stronger locally.
Given enough integration time. ;)
Cheers
Phil Hobbs
The meaning should be obvious to any reasonable engineer. Do I need to spell it out for you?
Do you think that external vibration will couple well into a tuning fork crystal? Say why or why not.
nt to do:
35 kOhm as a sensitive sound detector at its resonance frequency from its p iezoelectric signal.g-
ell
You could probably provide a certain amount of amusement by trying. You did n't pay enough attention to your lectures at Tulane to be able to produce t he routine answers. Your intuitive grasp of what's going on is probably fin e, but you aren't somebody I'd turn to for a helpful exposition of the theo ry
The right answer is probably that we don't know, since we don't know exactl y which tuning fork crystal he is trying to use. We do know that he's looki ng for acoustic vibrations, which is a sub-set of all the possible external vibrations.
Well? Can't remember any of that MIT physics? I took two great physics courses at Tulane, had fun, learned a lot. [1]
TCA = Thompson Chokes Again.
[1] in those days, the Tulane physics department had a subcritical nuclear reactor, a big stainless tank full of water and uranium rods. They let us handle the fuel rods. They had a high-power unshielded x-ray source too, with tape on the floor to define the keepout region. The old Physics building was brick and wood construction with copper nails, for turn-of-the century magnetics experiments.Hi, I've been trying with an instrumentation amplifier. so far, the noise is still large.
just to provide more information. The idea here is that the source of vibration is at the resonance frequency of the tuning fork. So high Q actually helps to tuning fork to pick up the sound.
I don't think that external vibration will couple very well into a tuning-fork crystal. If it did, the Q would be low, and it's not.
Tuning forks achieve low coupling to the outside world by balancing the torques exerted on the mount.
It sounds like the OP is using it by putting one side up against the vibrating object, though, rather than detecting sound in air. That coupling is mechanically unbalanced, so he might be able to detect it. Lots of scanned-probe microscopes do that sort of thing.
Cheers
Phil Hobbs
Why the instrument amp? They are much less flexible than a single opamp.... you can't even roll of the gain with an R/C in the feed back. Maybe you could post a circuit diagram.
You could try a Q ~10 bandpass filter, centered on the 32kHz of the tuning fork. That should cut your noise down. (no sense looking at frequencies where there is no signal.)
George H.
If he can couple asymmetrically into the tines, it could be very sensitive, but the crystal would have to be removed from its can.
A jfet would be a good first gain stage.
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