School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA, USA.
IEEE Trans Ultrason Ferroelectr Freq Control. 2012 Jan;59(1):50-9. doi: 10.1109/TUFFC.2012.2155.
The nonlinear relationship between the electrical input signal and electrostatic force acting on the capacitive micromachined ultrasonic transducer (CMUT) membrane limits its harmonic imaging performance. Several input shaping methods were proposed to compensate for the nonlinearity originating from the electrostatic force's dependence on the square of the applied voltage. Here, we analyze harmonic generation in CMUTs with a time-domain model. The model explains the basis of the input shaping methods and suggests that the nonlinearity resulting from gap dependence of the electrostatic force is also significant. It also suggests that the harmonic distortion in the output pressure can be eliminated by subharmonic ac-only excitation of the CMUT in addition to scaling the input voltage with the instantaneous gap. This gap feedback configuration can be approximated by the simple addition of a series impedance to the CMUT capacitance. We analyze several types of series impedance feedback topologies for gap feedback linearization. We show that for subharmonic ac excitation, although resistive and capacitive impedances result in a trade-off between input voltage and harmonic distortion for a desired pressure output, harmonic generation can be suppressed while increasing the Pa/V transmit sensitivity for proper series inductance and resistance feedback. We experimentally demonstrate the feedback method by reducing harmonic generation by 10 dB for the same output pressure at the fundamental frequency by using a simple series resistor feedback with a CMUT operating at a center frequency of 3 MHz. The proposed methods also allow for utilization of the full CMUT gap for transmit operation and, hence, should be useful in high-intensity ultrasonic applications in addition to harmonic imaging.
电输入信号与作用在电容式微机械超声换能器（CMUT）膜上的静电力之间的非线性关系限制了其谐波成像性能。已经提出了几种输入整形方法来补偿源自静电力与所施加电压的平方的依赖性的非线性。在这里，我们使用时域模型分析 CMUT 中的谐波产生。该模型解释了输入整形方法的基础，并表明源自静电力对间隙的依赖性的非线性也是显著的。它还表明，除了按瞬时间隙缩放输入电压之外，通过仅对 CMUT 施加亚谐波交流激励也可以消除输出压力中的谐波失真。这种间隙反馈配置可以通过将串联阻抗简单地添加到 CMUT 电容中来近似。我们分析了几种用于间隙反馈线性化的串联阻抗反馈拓扑结构。我们表明，对于亚谐波交流激励，尽管电阻和电容阻抗在期望的压力输出下在输入电压和谐波失真之间产生了折衷，但是通过适当的串联电感和电阻反馈，可以抑制谐波产生并同时提高 Pa/V 发射灵敏度。我们通过使用简单的 CMUT 中心频率为 3 MHz 的串联电阻反馈，在相同的基频输出压力下将谐波产生降低了 10 dB，从而实验证明了反馈方法。所提出的方法还允许为传输操作利用 CMUT 的整个间隙，因此除了谐波成像之外，在高强度超声应用中也应该是有用的。
