Re: Watch Crystal dissipation

Tim Wescott wrote: 
osr@xxxxxxxxxx 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.

So far I've tried this, but cant see much change in the agc:

http://www.zen22142.zen.co.uk/Circuits/rf/cco.htm

Any help would really be appreciated.

Thanks,

Steve Roberts

Research Associate,
Raman Scattering and Nanofiber labs
Maurice Morton Institute of Polymer Science,
The University of Akron.  Akron, Ohio

So the issue is not so much that you want to build any old oscillator and then measure the crystal dissipation. It sounds like what you want to do is build an instrument, possibly an oscillator, that measures crystal dissipation?

Usually when you set about to build an RF oscillator circuit you want to aim for the fewest number of active components you can, to reduce the amount that they'll screw you up. You also want to reduce phase noise. Both of these goals can be met if you design your bias networks so that your active element squirts about the same amount of charge into your resonant network (your crystal in this case) on each cycle. This is good for getting nice stable oscillation, but not for getting information about the crystal's load impedance.

Building a bridge wouldn't be a bad idea except that you would need to keep the excitation frequency just so. Another approach would be to build an op-amp oscillator with very explicit AGC action:

                     ___
 .------------------|___|-------------------.
 |      _                                   |
 |     | |                                  |
 o----|| ||----.                      ___   |
 |     |_|     |                  .--|___|--o
 |             |  Multiply        |         |
 |    |\       |     _            |         |
 '----|-\      |    / \    ___    |  |\     |
      |  >--o--o-->| X |--|___|---o--|-\    |
   .--|+/   |       \_/              |  >---'
   |  |/    |        ^         .-----|+/
  ===       |        |         |     |/
  GND       |        |        ===
            |        |        GND
            V        '--------.
            -                 |
   Level    |     .------.    |
   Detect   |     |      |    |            Output
         .--o-o-->| H(s) |----o------------------
         |    |   |      |
        .-.   |   '------'
        | |  ---
        | |  ---
        '-'   |
         |    |
         |    |
        ===  ===
        GND  GND
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

This circuit depends on the fact that with a 32kHz crystal you can find opamps with relatively low phase shift -- so make sure you do, and consider putting a bit of lead into the oscillator's loop somewhere to make it nominally zero.

The idea behind the circuit is that it keeps everything linear, so you should get a very clean sinusoid out of the crystal. The level out of the oscillator should be steady when you find a damping proportional to the crystal load resistance. Since the line marked "output" will be proportional to the damping as long as you have an AGC loop filter that achieves stability and servos the level to the same value every time you'll always have a good reading out of the thing.

If this were me I'd implement the loop shaping in a microcontroller and the multiplier in a multiplying DAC; this would not only allow me to play games with the inherently nonlinear effect of regulating the oscillator output by changing it's damping but it would also let me easily communicate the output level directly, since it would be available internally to the processor. It has the added advantage that 1 quadrant multiplying DAC's are easier to get these days than good analog multipliers.

You may feel more comfortable implementing the compensation in analog with a good analog multiplier. Since the oscillator itself will have a more or less integrating effect as you change it's damping you should be able to do this with just a proportional gain, or perhaps a PD. You may get faster settling overall with a PID controller; you'll have to experiment to find out.


This sort of thing is done in optical spectroscopy using a "ring-down" approach. There's an electronic version which is a very useful method of calibrating log amps, and is also good here, I think:

1. Build a garden-variety Colpitts crystal oscillator, tuned to the series resonance of the crystal (which will require a series inductance). This maximizes the actual mechanical oscillation amplitude for a given electrical swing. Put a small resistance in series with the grounded end, and attach a buffer amp there.

2. Add a diode switch that ac-grounds the hot end of the crystal when a current is applied. This will stop the oscillator and allow the crystal to ring down at its natural frequency.

3. Measure the oscillation amplitude as a function of time. You can do this by sampling with an ADC, or with a DLVA. You get about Q cycles per decay time, so you can do this quite conveniently and accurately.

4. The excess loss goes as 1/Q-1/Q_0, where Q_0 is measured before whatever guck you're using gets put down.

This trick makes a great on-line calibrator for DLVAs, since the decay is very accurately exponential with time after the switch transient dies away, so a good DLVA will produce a straight line graph.

Cheers,

Phil Hobbs
.

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