Pulse-echo and Pitch-catch Measurements in Immersion

For many applications, such as medical imaging, the CMUT is operated in immersion or in contact with tissue. For these applications, some of the key characteristics for the transducer are output pressure, receive sensitivity, center frequency and fractional bandwidth. The output pressure and the receive sensitivity determine the penetration depth; center frequency is determined by the application; the frequency response determines the resolution of the system.

The transmit/receive behavior of a CMUT can be characterized in the pulse-echo measurement. Usually, the transducer is biased at a certain DC voltage, and a broad band electrical excitation pulse is superimposed on top of the DC voltage. A plane reflector is placed in the far field of the transducer element. The acoustic wave that bounces back from the reflector will change the CMUT capacitance, causing a voltage change across the CMUT. This electrical signal is amplified and read out onto an oscilloscope. A Fourier Transform can be performed on the received waveform to determine the band shape. This band shape can be corrected for the medium attenuation and diffraction as well as the excitation pulse shape to yield the true transducer frequency response of the transducer [1, 2].

The absolute value of the total output pressure can be measured with a needle type hydrophone. Instead of a plane reflector, a hydrophone is placed perpendicular to the transducer surface in far field. The hydrophone gives a pressure reading. The pressure at the surface of the transducer can be inferred from this pressure level, after taking into consideration of the medium attenuation and diffraction.

The pitch-catch measurement can also be performed with two CMUTs in air for applications such as flow measurement and non-destruction evaluation.

 

FIGURE 1. Experimental setup for pulse-echo measurements.

 

 

FIGURE 2. Typical pulse-echo wave-form and band shape for a 250-um by 250-um CMUT element in a 2D array.
Measurement was perfomed with a custom designed font-end IC.

 

 

References

 

[1] Ö. Oralkan, X. C. Jin, F. L. Degertekin and B. T. Khuri-Yakub, "Simulation and experimental characterization of a 2-D capacitive micromachined ultrasonic transducer (cMUT) array element," IEEE Trans. Ultrason. Ferroelect. Freq. Contr., vol. 46, no. 6, pp. 1337-1340, Nov. 1999.

[2] X. C. Jin, Ö. Oralkan, F. L. Degertekin, and B. T. Khuri-Yakub, "Characterization of one-dimensional capacitive micromachined ultrasonic immersion transducer arrays," IEEE Trans. on Ultrason. Ferroelect. Freq. Contr., vol. 48, no. 3, pp. 750-760, May 2001.