Oscillator circuit for 32.768 kHz quartz tuning fork crystal element immersed in water

The technology has changed in the past 20 years. You haven't.

John

There seems to be two types of crystals; the "standard" high frequency and the tuning fork. The latter seems to be available up to at least 2MHz, so a higher frequency crystal would not be a good solution unless it was a "jump" to the "standard" type. OTH, a tuning fork type (in its original case) has the attribute that too much drive will prevent oscillation; naturally too little drive will also prevent oscillation. Look at the circuit i posted concerning an "almost universal" oscillator circuit. A large resistor from a CMOS inverter to the crystal is required for the low erquired drive fot a tuning fork type crystal. So, perhaps a way to have an electronically variable resistor adjusted by a "hunting" circuit might be a possible solution.

I used 680K for in-case tuning fork crystals; a 330K would be overdrive and it very likely would not oscillate.

Nonsense! I do a crystal oscillator designs every few months. Looked to me like the "pi caps" were too small, and the drive resistor too large.

...Jim Thompson

Given an analytical instrument using a crystal in an unusual way, fiddling with the conventional single-stage CMOS oscillator circuit is silly. The first thing to do is to characterize the crystal impedance curve under water, then design a serious oscillator circuit, probably using opamps for gain and active gain control to define drive level.

John

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It doesn't sound as if the technology has changed in any interesting way in the last twenty years. Stanford Scientific Systems may have applied tolerably old-fashioned technology in a way that may interest some new customers, but they don't seem to have set the world on fire.

I'd be appreciably more cheerful if I more closely ressembled the man I was back when I was 46, but back then I was having the time of my life, albeit on a project that was eventually cancelled, despite the fact that it did pretty much exactly what I'd promised it would.

Since you don't know much about what I was like back then, your claim is more than usually empty.

-- Bill Sloman, Nijmegen

SRS is Stanford *Research* Systems.

Did you ever work on a project that wasn't cancelled?

What I know is that you scorn using google to see what has changed in microbalance technology in the last two decades. In fact, you flat claim it hasn't changed "in any interesting way." That last qualification says it all.

John

Those caps were specified by the manufacturers, i did not make up the values. The 680K was the largest value that *reliably* allowed oscillation for 32KHz, 100KHz, 200KHz, etc tuning fork type crystals. Blame the circuit for working contrary to your expectations...

Are you one of those engineers that also drives trains?

disk or square.

We need to measure bulk viscosity changes resulting from solution gelation for a University research project. A crystal that is responding to surface changes might be affected more by precipitate effects than a tuning fork style hopefully would.

We want to monitor viscosity changes vs time hence the oscillator may provide faster measurement intervals. I am not sure what time resolution is needed nor do I know how long it might take to do the frequency sweep if that method were used.

Steve

In this case the crystal in an aqueous solution, the required drive should be much higher.

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Seldom read Newsgroups but saw your post. No flexure (TUNING FORK) or shear mode (at CUT) quartz crystal will self oscillate sensibly in a fluid. Note it is the circuit activity (I like to think of negative resistance) not the dirve level that determine if it will "go" However there is a crystal cut designed to alter frequency in proportion to fluid viscosity, this is a quartz crystal rod with electrodes placed to cause it to twist, a torsional oscillation crystal. I last saw a torsional quartz crystal manufactured by my employers Marconi in the 60s If I was to develop something today I would try to electromagnetically generate twist in a ferrite rod or to get a rod of piezo ceramic poled to generate a twist.

Shear mode crystals oscillate just fine under water, and are the basis for some analytical instruments. Look it up.

I have little doubt a tuning-fork crystal could be persuaded to oscillate in water, but I don't know how useful it would be.

John

....or get Chubby Checkers...

Why not find an ultrasonic transducer such as would be used for a fishfinder and use it as the resonant element of your circuit?

Also, have you considered that the motion of the oscillator element in the solution might inhibit the growth of crystals?

Wouldn't viscosity affect it?

Thanks, Rich

Of course.

John

Well, I'd think that'd be useful, to someone who wants to measure viscosity. ;-)

Cheers! Rich