Re: Watch Crystal dissipation
- From: dougfgd@xxxxxxxxxxxx
- Date: Mon, 17 Oct 2005 19:37:41 +0100
In message <BY-dnVQPScAVRc7enZ2dnUVZ_smdnZ2d@xxxxxxxxxxxx>, Tim Wescott <tim@xxxxxxxxxxxxxxxx> writes
Intrigued by the application.Winfield Hill wrote:
Hey! That's a cool idea! It's potentially cooler than my circuit, but only if you need to detect _really_ small changes in the resonator Q. As described such a circuit would depend on the effective oscillator regeneration changing with signal level, and the slope of Q change vs. amplitude change would depend on this dependency. It seems like it'd be very sensitive to changes in temperature and just about everything else, though. It sounds like it'd be fussy and finicky to adjust, and that it'd require it as part of the operation of the thing.Steve Roberts wrote...
For a microscope system here at the university I need to monitor the energy used by a oscillating 32Khz watch crystal, anybody have a clue how to do that and keep the waveform sinusoidal? I'm looking for papers or sample schematics on leveled crystal oscillators with some form of AGC volatge that can be monitored. I need to measure around 800 microwatts at a 10 microwatt resolution without disturbing the oscillation frequency, and no a scope probe across the cystal wont do, I need a voltage to a A/D for feedback.
Better yet, anybody know how to build a bridge circuit with crystals?
I'm sure this has been published in like IEEE Journal of Ultrasonics or something, but I'm having a hard time finding it.
For those who wonder why, tuning fork watch crystals make remarkable sensors when you cut the outer can off, in our application we glue a atomic force microscopy tip to one of the forks and watch the FM as it contacts the surface in the microscope, but now we need to run AM and we think our megabuck commercial DSP and PLL based system is not acting correctly, so I need to mimic it on the work bench.I don't know how the megabucks commercial DSP and PLL guys do it (they aren't saying?), but I'd use a "marginal oscillator," e.g., http://www.auditory.org/asamtgs/asa94mit/1pPA/1pPA3.html This is a design technique created for NMR probes, and not a "marginal" oscillator circuit, e.g. as referred to in Microchip's excellent appnote http://ww1.microchip.com/downloads/en/AppNotes/00849a.pdf
:>) I have a nice marginal oscillator design we used for semiconductor lifetime measurements, and thought I'd once discussed it and posted an ASCII schematic here on s.e.d., but couldn't find it using Google.
Since that reminds me of a regenerative receiver for shortwave, I wonder if you could do something like that using a superregenerative oscillator, where you quench the oscillations periodically then measure the amount of time it takes for them to come back as an estimate of the circuit Q. You'd have to make sure to start it with a bang or otherwise feed it some pilot tone, but it would make a simple circuit and because you could guarantee enough regeneration for oscillation it shouldn't be as fussy.
Various thoughts arise from the fact that the fork flexure crystal is not the lowest loss resonator though the cheapest and quite convenient.
Remember that the oscillator exactly equals the crystal loss in negative resistance.
The uncontrolled oscillator does this by varying negative resistance with time/phase through the cycle.
The agc oscillator varies negative resistance with a time constant of many cycles to give a linear region so the multiplier should be the best approach but complex.
Most oscillators start off with too much activity and then back off a long way.
You will encounter a problem with contamination where the crystal activity reduces due to particles , surface films.
A convenient means of achieving small gain variation for agc is to vary the current through a bipolar transistor oscillator transistor after a precise level is reached. Works well if you don't start with too high gain.
Finally the flexure fork may not be the best resonator config.
The lower the resonator loss (the higher the Q) the smaller the tip force that can be detected.
Fork flexure Q 30000? (In air)
Fundamental Shear mode Q 200000
Overtone Shear mode Q 2000000
An extensional resonator not normally encountered could be a practical compromise.
Change "gr" for "h"
-- dd .
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