Impedance curve doesn't show resonance Answered

I'm simulating a CMUT cell where I excite it using a ricker wavelet to check the resonance frequency.

When I press FFT for the displacement of the center of the membrane, I get the following curve which shows a resonance frequency of nearly 5.7 MHz.

However, when checking the impedance curve (by pressing the impedance option while selecting the charge of the top electrode), I got the following curve which doesn't show any resonance. I expected to see a dip and a peak as evidence of the resonance frequency but I didn't. So, I wonder what's wrong with the impedance curve.

I also tried the review script code explained in the following tutorial's link and I got the same result.

1 comment

  • You have to be careful when using built in functions for evaluating impedance. Impedance is not a simulation output, rather it is something obtained in post processing using charge and voltage to estimate the impedance with a discrete fourier transform. In addition the procedure uses windowing by default and zero padding. This is not appropriate for the type of signals such as those from CMUTs which have a large DC bias. 

    The procedure assumes the DC component is small and much smaller than the transient component. If it is not the case the result can be distorted. You can implement your own routine to avoid these issues.

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