Gap feedback linearization of capacitive micromachined ultrasonic transducers for harmonic imaging

Nonlinear behavior of CMUTs, which is due to the nonlinear relationship between the electrical input signal and electrostatic force acting on the CMUT membrane, results in severe harmonic distortion in the transmit mode. We analyzed CMUTs in the transmit mode using a 1D transient model which we implemented in SIMULINK. The results show that the nonlinear relationship can be compensated by subharmonic AC only excitation of the CMUT and scaling of the voltage acting on the transducer with the instantaneous gap. This gap feedback topology can be approximated with the addition of a series impedance to the CMUT. We show that while resistive and capacitive series impedances result in a trade-off between input voltage and harmonic distortion for a desired pressure output, this trade-off can be avoided with proper series inductance and resistance feedback. To validate this approach we fabricated CMUTs operating at 10 MHz and 3MHz with 100% and 133% FBW respectively. The experiments demonstrated a 10 dB second harmonic suppression for the same output pressure at the fundamental frequency, with both inductive-resistive feedback and resistive only feedback. The proposed method also allows for the utilization of the full CMUT gap swing with suppressed harmonics, which may be desired in HIFU applications.