Pre-compensated excitation waveform to suppress second harmonic generation in MEMS electrostatic transducers

MEMS Electrostatic based transducers are finding growing application in the field of diagnostic ultrasound imaging. However, electrostatic transducers inherently produce harmonics since the electrostatic force is a function of the square of the applied voltage. Existing methods for ´linearizing´ the response by using a DC bias voltage have practical limitations. Therefore, we explored the possibility of applying a pre-compensated waveform to electrostatic transducers as a means of controlling this inherent harmonic generation. The first approach assumes linear transducer operation in the vicinity of the second harmonic frequency range. The linear transfer function was calculated in this range and the cancellation signal required to compensate for the nonlinear contribution was (approximately) derived. Iteratively repeating this cancellation approach three times, demonstrated that a 19.7dB reduction on transmitted harmonic can be obtained in finite element analysis (FEA) simulations. The second approach tested involves defining a desired function (including a DC offset) and then taking the square root of this function to determine the shape of the required input function. A combination of FEA and experimental results are presented. An 8 dB reduction in transmitted harmonic was achieved in experimental prototypes.