I thought I might do a project or two (for fun) with an ultrasound transducer. I have several to work with, but don't have a data sheet. I thought I'd measure their Resonant Frequency to help with the rest of the design.
The one (web) image shows a "typical" piezo ultrasonic transducer's impedance vs frequency, with a nice dip in the impedance at its resonant frequency.
I didn't get the results I thought I would get, and thought I'd seek some input from others. Its just been too long since I did much work with complex loads and j-omega this that and the other.
I was under the impression that a piezo ultrasonic transducer had a primary resonant frequency, and that its impedance was minimal at its resonant frequency. I thought that if I swept the input frequency I could simply measure the transducer current and find the frequency at which the current peaked. (I=V/R, ought to get a maximal current at a minimal resistance (impedance)). Easy. I put a 10 ohm resistor in series with the transducer and feed the signal across it to an op-amp difference amp, to measure the signal across the resistor. This is the transducer's current monitor.
So I fired up a sine wave to an op-amp driver for the transducer, and also fed an inverted sine wave to the transducer's other terminal. With the two Sine wave signals there is no DC bias across the piezo. Sweep the frequency and there is a clear current peak at about 60 KHz. Cool. So its a 60 kHz transducer.
Next I add a (wide band) signal receiver, and watch the output of the receiver as I sweep the transmitter. There is a very clear received signal peak at about 25 KHz. So, it must be a 25 KHz transducer. Clearly the transmitting and receiving transducers put out a large amplitude signal at 25 KHz.
So, why don't the two match, or even come close? Even if the 60 KHz current peak was either a secondary resonant frequency, or a harmonic of a lower resonant frequency, why isn't there a current peak at 25 KHz?
The rather noisy scope image shows two signals as a function of the sweep frequency. The horizontal axis corresponds to the transducer's drive frequency, which sweeps from 5 KHz to 65 KHz. The Top Center marker with a "T" is the scope's trigger point, and is at 25 KHz.
The top, blue, trace is the ultrasonic transducer receiver's signal. There is clearly a peak at about 25 KHz, plus or minus a couple of kHz.
There is a small signal "peak" at about 60 KHz. A little bit stronger than the rest of the sweep, but nowhere near as strong as at 25 KHz.
The 3 spikes just below the 5 KHz label are actually the 60 KHz mini-peak, just before the sweep starts again at 5 KHz.
The lower, yellow, trace is the current monitor for the transmitting transducer. It shows a clear peak at about 60 KHz. The current actually appears to slightly increase as the sweep frequency increases, and then has a nice peak at 60 KHz.
The traces are very noisy, the circuit is laid out on a bread board, and the scope leads are long... But the findings seem consistent.
Clearly, my original concept of measuring the piezo's current, by monitoring the voltage across a small series sense resistor, and using that as a indicator for the piezo's resonant frequency isn't working as expected.
I'd be glad to hear some thoughts on this. Clearly I'm missing some basic concepts...