Harmonic reduction in capacitive micromachined ultrasonic transducers by gap feedback linearization

Abstract

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 in order to compensate for the nonlinearity originating from the electrostatic force 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 due to 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 while 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 10dB 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.

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“…The higher frequencies being closer to the center frequency of the transducers could explain this behavior, as already shown by [37]. Furthermore, the addition of the inductance could linearize the CMUT behavior in a similar way to the harmonic compensation proposed by [38]. This aspect needs to be investigated more deeply.…”

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“…Tissue harmonic imaging is a widely used imaging mode [18][19][20][21][22]. The ultrasound system sends out bursts of ultrasound at the fundamental frequency.…”

“…Compared to traditional PZT transducers, CMUT is at a disadvantage in THI mode because of the nonlinear transmit property from its electrostatic actuation mechanism [19,20], where the actuation force (hence the generated acoustic pressure)…”

“…For example, work in [20] focused on pre-distorting the electrical excitation signal's pulse shape, such that the frequency content of the actual transmitted acoustic pulse is HD2 free. The method is heavily dependent on detailed CMUT transmit properties and its bias voltage, requiring complicated and frequent calibration.…”

“…For example, the Harmonic Imaging mode uses the second harmonic of the pulse to provide high resolution images [18][19][20][21][22]; the Power Mode Doppler visualizes the magnitude of Doppler signal, rather than the frequency 1 It is important to point out that in the PW mode, the Doppler effect does not come from the frequency shift of a single received echo pulse, since a short pulse is broadband, and therefore it is difficult to detect the small Doppler frequency shift (typically less than 100KHz). Besides, the frequency-dependent attenuation through the tissue complicates the task even more.…”