# Finding the Resonance Frequency of a Piezo

Using the circuit outlined in this white paper I have been able to find the anti-resonance of the piezo in question. I am using a Rigol DG1022 to create my signal, a sine wave at 20 V p-p, and a Yokogawa oscilloscope to look at my signal and measure the voltage over the R3 resistor. However, what I am having difficulty with is finding the resonance frequency. I believe what I need to do is use lower value resistors for R1 and R2 as the change in voltage around the resonance is to small for me to see.

It has been a few years since I have messed with electronics at all so I have a few questions.

1) What is it about the piezo being in a low impedance state that makes it so that the oscilloscope is not measuring the changes in voltage?

2)If I in fact do need lower resistance values for R1 and R2 in order to measure a voltage change at the resonance. How would I go about calculating what those values should be?

• How coarsely are you adjusting the frequency? Approximately what is the resonant frequency you expect? – Andy aka May 25 '17 at 9:03
• I expect the resonance to be around 1.3 MHz. I have been stepping my frequency in kHz increments – prov May 26 '17 at 19:21

Drive the piezo with a 50 Ohm sweep generator but only sense current such as a 1 Ohm resistor , this will give you I =V/R and large peak at low series resistance at resonance.

If you know how to measure a resistor by voltage ratios or impedance matching or a current ratio ratio, then it is simple to get min or max resistance.

Just using a 10M scope probe is simply the wrong reference tool for a series piezo application. A log impedance analyzer is the best way.

## my early experience

When lacking test equipment, in my early days as an Eng, I would use the scope x sweep to control an FM gen. then either measure current or voltage with a reference R near what I expected. This could easily display series (or parallel) resonant impedance.

Today there are more clever ways. Here you want high power low Z , so use a low Z current shunt.

Above shows a piezo with a log scale of 10 Ohms at top and 20k at bottom using a log impedance analyzer.

• I want to make sure I understand the beginning of your reply. Are you saying I should have the piezo in series with a low resistance resistor, similar to a voltage divider, and drive this setup with the generator? When you say "sense current" do you mean measure the current through the resistor? Why is a 10M probe the wrong probe to be using for this? I apologize for not knowing much I am just trying to solve this problem with a rudimentary background in electronics. – prov May 31 '17 at 2:58

I expect the resonance to be around 1.3 MHz. I have been stepping my frequency in kHz increments

Here's the likely problem - the resonance will start and be over in a few tens of Hz. You need to step very carefully through a band of frequencies centred around 1.3 MHz. If you don't find it there then build the piezo into a pierce oscillator circuit and note what the resonance is this way.

• Thank you. I will definitely be trying this tomorrow. I have never heard of the Pierce Oscillator circuit and am reading about it. Do you have any suggestions for further reading to understand the Pierce circuit and how to use it to identify the resonance? – prov May 30 '17 at 3:17