mechanical excitation of tuning fork and electrical readout

Question

For my experiment, I am trying to mechanically excite a quartz crystal tuning fork and measure the voltage across the tuning fork. When the TF is driven at its resonance frequency, there should be more charge accumulation and hence greater voltage.

I have just encountered a mystery though. A quartz crystal tuning fork has a dip and a peak: series and parallel resonances. When I was electrically driving the tuning fork, I was able to perfectly understand them and depending on the measuring scheme I was employing, either the series or parallel resonance peaked and the other one dipped.

Yet, for the mechanical excitation scheme, when measuring the voltage across the tuning fork, I wasn't sure which of the series and parallel resonances would peak. When I started measuring the voltage value, I ended up seeing the series resonance peaking (left plot of 2nd image).

Strangely, when I just swapped TF lead 1 and TF lead 2, the amplitude profile was basically turned upside down: the series resonance dipped and the parallel resonance peaked (right plot of 2nd image).

Also, when I swapped shear piezo lead 1 and shear piezo lead 2, I saw that the amplitude plot's peaks reversed.

I wasn't able to find anything related to my observations in the existing literature.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

Looks you are using one of the HP DSA's . What you are seeing is probably some stray leaking of the driving signal into your pickup. Ideally you should be using a differential input pre-amplifier, if the piezo is just plugged straight into the DSA, one end of the piezo will be grounded, so the other piezo lead will pick up stray signals. You should also be using a differential driver to the actuator, this will also minimise spurious signal generation.

Instead of seeing a transfer function of (output/input) , you are seeing (k.input + output/input) at some point in the sweep, now k.input and output/input have similar magnitude but opposite phases, creating the dip in the transfer function; at some other point they are in phase, and add to the real peak, shifting it to the side. Swapping the leads around just changes the + above to a - , so the In-phase and out of phase artifacts swap places.