Analysis on the impedance of ultrasonic probes

Analysis on the impedance of ultrasonic probes

We know that normal people can hear the sound at a frequency of 20Hz ~ 20 KHz, ultrasound is the sound signal with a sound frequency of more than 20 KHz, avoiding audible signals that human ears can hear, so that ultrasound has a strong anti-interference ability.

The simplest method is to use the network analyzer. The two ports are displayed on one pair. | Z | ~ F. However, the network analyzer is too expensive. It is generally unavailable to large companies. Another method is to build a circuit and use a sine wave signal generator for measurement. The following is the measurement method of the circuit (this part of the method and test data comes from the network article "Ultrasonic Ranging module").

Finally, the matching impedance values at different frequencies are given. The nonlinear circuit also satisfies Ohm's law, but the resistance is replaced by the impedance. "Ohm's law is a great discovery ",

The ultrasonic transmission distance is related to the energy P = U ^ 2/| Z | emitted by the ultrasonic probe. What we hope for remote Ultrasonic Ranging is: at the same voltage, the smaller the impedance, the better.

Because 40 KHz and 51 KHz are the two low peak values of impedance matching resonance and correspond to the minimum value of impedance, this is why the ultrasound in the market is usually 40 KHz (the variation at the 40 KHz resonance point is gentle than that at 51 KHz, when the drive frequency has a small deviation, the impact on the output is smaller than 50 kHz ). From the above curve, we can see that if the circuit permits, the distance between Ultrasonic Ranging with 51 kHz will be farther (the impedance is lower than 40 kHz ).

Due to the production of ultrasonic emission headers, the resonant frequency is not necessarily accurate to 40 KHz and 51 KHz, and there will inevitably be errors. The best method is to use a Vector Network Analyzer for Direct Testing | Z | ~ F curve, use | Z | ~ F indicates the resonance frequency. I purchased a 51.48 KHz resonant frequency point for an integrated ultrasonic probe.