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Calibration for Pierce Oscillator ciruit in openQCM

Hi all
I just wonder about calibration for Pierce oscillator circuit in openQCM. Please link to this hyperlink to view schematic circuit
http://openqcm.com/openqcm-community-develops-and-shares-the-new-electronic-design-using-kicad.html
In basically, R3 & C2 make a "Low pass filter" with frequency cut-off Fc=1/2*pi*RC, when I try this circuit with my QCM sensor (5MHz) from MATEX, in air the frequency output is OK, but in liquid the frequency was not proper because my frequency counter ( Aligent 53230 ) can not detect the signal from this circuit. I try to replace 2 capacitors C1&C2 in circuit. as consequenlty, the circuit is proper operation with value of 130pF.

This is my situation and my solution for this case.

And I think I have a interesting question about the rule of R3 and C2 in this circuit. That is:

" Are the function of R3 and C2 same in calibration of output frequency ? " what happen if we change the value of R3 and remain the value of C2 ?

As far as I know the R3 & C2 make a "Low pass filter" with frequency cut-off Fc=1/2*pi*RC, change in value of R3 or C2 will be change the Gain and Phase of output frequency. So when we increase C2 the Gain will also increase so the level of voltage driving QCM will be increased.

Anyway my question is only a different thinking about aspect of design QCM circuit.

Best regards

Nhannd from UNIST, Korea

Comments

  • edited April 5
    Hi Nhannd,
    thank you very much for your evaluations.

    In this first version of openQCM shield we preferred to choice a basic approach. In this way we decided to make an electronic for general applications. But your evidences are absolutely correct.

    For example we started with C1 and C2 at 33pf, but after few weeks and thank to first feedbacks from scientific community we changed these values at 100pF capacitors.

    As matter of fact, in order to reach a good compromise for different environments and different experimental conditions, despite the shield oscillator can drive frequencies between few KHz to 28 MHz, we decided to made a setup specifically for 10 MHZ.
    In this way we made a calibration campaign and found that 100pF was a good value.

    This implies that for use with different resonance frequencies you should change C1, C2 and evenctually R3, too. So your approach is correct.

    In order to indifferently use openQCM with several resonance frequencies an improved shield should consider all these things, as you suggested.

    As you know we are working around an openQCM for dissipation measurements. So we are designing a new model. The same design will be also adopted for the new version of the "basic" openQCM. So could be a very good idea to modify the shield in this way!
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