I am using a piezo transducer to launch an ultrasonic pulse in air, at a frequency of 125kHz. A picture of the received waveform is attached. My question concerns how I can (electronically) limit the long resonance tail in the emitter pulse which I assume is because of the relatively high Q of the device.
Yes, these transducers tend to have high Q, which makes them "ring" like a bell.
In the example below, a 40kHz piezo was measured and fitted to an electronic model that substitutes L-C-R components for the mechanically-resonant components. For this transducer, the electrical equivalents were:
- Lm = 92.1 millihenries
- Cm = 175.384 picofarads
- Rm = 634 ohms
The value of parallel capacitance is real, measured with a capacitance meter: 1.3 nanofarads.
The current through the series arm comprising Lm, Cm, Rm is proportional to physical motion of the transducer. When driven directly from a voltage source, current takes a long time to rise to full amplitude (purple trace, I(R3)).
An external inductor (L2=12.42 mH) along with a damping resistor (R1=5400 ohms) gives a significantly faster, critically damped rise time (green trace). The penalty paid is far less current - making the transducer less sensitive.
The simple approach of damping with a resistor (R1=5400 ohms) is wasteful of available transducer signal. An impedance-matching transformer would be a better, more costly alternative.