Second-harmonic reduction in CMUTs using unipolar pulsers

In this paper, a CMUT linearization method based on the predistortion of unipolar excitation pulses is proposed. The predistortion is achieved by acting on the amplitude, duty cycle, and slope of the rising and falling edges of the trapezoidal excitation pulses. The pulse-shaping parameters are computed using an open-loop charge control algorithm that uses electrical impedance data for the identification of the nonlinear input-output characteristic of the CMUT. A 0-200V unipolar pulser, provided with a digital control logic for the trapezoidal pulse-shaping, was designed and fabricated using a BCD-SOI technology available at STMicroelectronics. The pulser was connected to a 10MHz CMUT linear array developed for medical imaging applications. Transmit pressure measurements were performed proving a second-harmonic reduction of more than 20 dB, resulting in a final second-harmonic distortion lower than -30 dB, suitable for typical nonlinear imaging operation.