Re: Watch Crystal dissipation

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.


--
Thanks,
- Win
.

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