Signal Control in Micromachined Ultrasonic Transducer

Abstract

A capacitive micromachined ultrasonic transducers (cMUT) uses signal control methods to reduce harmonic distortion of the output signal. The method uses an AC transmission input signal characterized with a frequency ω and takes the second-order frequency component with frequency 2ω, rather than the first-order frequency component with the base frequency ω, as the desired output pressure signal. A frequency ω is preferably equal to ω0/2, where ω is the desired cMUT output frequency. Various examples of AC transmission input signals, in combination with or without a DC bias signal, that are suitable for producing a large second-order frequency component and small (ideally zero) first-order frequency component are disclosed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a conventional flexible membrane cMUT.

FIG. 2 is a schematic cross-sectional view of embedded-spring cMUT (EScMUT).

FIG. 3A shows a simplified schematic cMUT model

FIG. 3B shows a further simplified circuit model having a variable capacitor representing a cMUT.

FIG. 4 shows an exemplary cMUT with an unmixed AC transmission input signal without a DC bias signal in the transmission mode.

FIG. 5 shows an example of using a shifted AC transmission input signal combined with a DC bias.

FIG. 6 shows another example of using a shifted AC transmission input signal combined with a DC bias.

FIG. 7 shows another example of using a shifted AC transmission input signal combined with a DC bias.

FIG. 8 is a diagram of an example of a suitable transmission input signal having a sine wave.

FIG. 9 is a diagram of an AC component of the output pressure signal generated by the transmission input signal of FIG. 8.

FIG. 10 is a diagram an example of Gaussian-shaped signal as transmission input signal.

FIG. 11 is a diagram of an exemplary absolute-value signal used as transmission input signal.

FIG. 12 is a diagram of an AC component of the output pressure signal generated by the transmission input signal of FIG. 11.

FIG. 13 is a diagram of an absolute-value Gaussian-shaped voltage signal that can be used as the transmission input signal.

FIG. 14 shows an exemplary cMUT implementation of the peak-to-peak voltage extension technique.

Claims

1. A method for operating a capacitive micromachined ultrasonic transducer (cMUT) system including a cMUT having a first electrode and a second electrode, at least one of the first electrode and the second electrode being movable and interfacing with a medium, the method comprising: applying a transmission input signal Vtx(t) having a base frequency ω to one of the first electrode and the second electrode of the cMUT, wherein Vtx(t) defines an output signal function Vtx(t)2 which has a dominating second-order frequency component having an output signal frequency 2ω; andallowing the movable electrode of the cMUT to move in response to the applied transmission input signal to actuate the medium.

2. The method as recited in claim 1, wherein the base frequency ω is about half of a desired operating frequency ω0 of the cMUT, such that the output signal frequency 2ω is close to the desired operating frequency ω0.

3. The method as recited in claim 1, wherein the transmission input signal Vtx(t) is solely contributed by an AC signal source without applying a separate DC bias voltage in a transmission mode.

4. The method as recited in claim 1, wherein applying a transmission input signal Vtx(t) is enabled by switching the cMUT system to a transmission mode.

5. The method as recited in claim 1, wherein the cMUT system has a DC signal source providing a DC bias voltage which is switchably connected to one of the first electrode and the second electrode of the cMUT, the method further comprising: disconnecting the DC bias voltage from the cMUT before applying the transmission input signal Vtx(t).

6. The method as recited in claim 1, further comprising: before applying the transmission input signal Vtx(t) to the cMUT, shifting the transmission input signal to result in a shifted transmission input signal Vtx(t)−Vdc; andapplying a DC bias voltage Vdc to the same electrode of the cMUT such that the net transmission input signal applied to the cMUT is Vtx(t).

7. The method as recited in claim 1, further comprising: before applying the transmission input signal Vtx(t) to the cMUT, shifting the transmission input signal to result in a shifted transmission input signal Vtx(t)−Vdc; andapplying a DC bias voltage −Vdc to the other electrode of the cMUT such that the net transmission input signal applied to the cMUT is Vtx(t).

8. The method as recited in claim 1, wherein the output signal function Vtx(t)2 has a negligible first-order frequency component of the base frequency ω.

9. The method as recited in claim 1, wherein the transmission input signal Vtx(t) comprises a component W(t)×Vtx sin(ωt), where W(t) is a gated time window function.

10. The method as recited in claim 1, wherein the transmission input signal Vtx(t) comprises a Gaussian-shaped sine signal defined as

11. The method as recited in claim 1, wherein the transmission input signal Vtx(t) comprises an absolute-value signal.

12. The method as recited in claim 1, wherein the transmission input signal Vtx(t) comprises an absolute-value signal component W(t)×2Vp-p×abs(sin(ωt)), where W(t) is a gated time window function, and Vp-p is a peak-to-peak amplitude.

13. The method as recited in claim 1, wherein the transmission input signal Vtx(t) comprises an absolute-value signal defined as

14. A method for operating a capacitive micromachined ultrasonic transducer (cMUT) system including a cMUT having a first electrode and a second electrode, at least one of the first electrode and the second electrode being movable and interfacing with a medium, the method comprising: generating a voltage signal V(t);shifting the initial signal V(t) to obtain a shifted transmission input signal V(t)−Vsh;applying the shifted transmission input signal V(t)−Vsh to one of the first electrode and the second electrode of the cMUT;applying a DC bias voltage Vdc to one of the first electrode and the second electrode of the cMUT such that the net transmission input signal applied on the cMUT is Vtx(t)=V(t)−Vsh+Vdc, or Vtx(t)=V(t)−Vsh−Vdc, wherein Vtx(t) defines an output signal function Vtx(t)2 which has a dominating second-order frequency component having an output signal frequency 2ω; andallowing the movable electrode of the cMUT to move in response to the applied transmission input signal to actuate the medium.

15. The method as recited in claim 14, wherein the DC bias voltage Vdc and the shifted transmission input signal V(t)−Vsh are applied to the same electrode of the cMUT, Vdc is substantially the same as Vsh to cancel each other, such that Vtx(t)=V(t)−Vsh+Vdc is approximately the same as the V(t).

16. The method as recited in claim 14, wherein the DC bias voltage Vdc and the shifted transmission input signal V(t)−Vsh are applied to the opposite electrodes of the cMUT, Vdc is substantially the same as −Vsh to cancel each other, such that Vtx(t)=V(t)−Vsh−Vdc is approximately the same as the V(t).

17. The method as recited in claim 14, wherein generating the voltage signal V(t) comprises amplifying an interim voltage signal Vint(t).

18. The method as recited in claim 14, further comprising: generating an initial voltage signal Vi(t) having a voltage swing from −Vi to +Vi;converting the initial voltage signal Vi(t) to an absolute-value voltage signal |Vi(t)| having a voltage swing from 0 to Vi;generating an interim voltage signal Vint(t) by shifting the absolute-value voltage signal |Vi(t)| to obtain the interim voltage signal Vint(t) as defined by |Vi(t)|−Vs; andamplifying the interim voltage signal Vint(t) to obtain the voltage signal V(t).

19. A capacitive micromachined ultrasonic transducer (cMUT) system comprising: a cMUT having a first electrode and a second electrode;at least one of a transmission input signal port and a reception signal port connected to one of the first electrode and the second electrode, wherein the transmission input signal port is adapted for applying a transmission input signal to the cMUT in a transmission mode, and the reception signal port is adapted for receiving an output signal from the cMUT in a reception mode;an AC signal source for generating the transmission input signal to be applied to one of the first electrode and the second electrode of the cMUT; anda DC signal source for providing a DC bias voltage to be applied to one of the first electrode and the second electrode of the cMUT, wherein, when applied, the transmission input signal and the DC bias voltage together result in a total transmission input signal Vtx(t) having a base frequency c), and wherein Vtx(t) defines an output signal function Vtx(t)2 which has a dominating second-order frequency component having an output signal frequency 2ω.

20. The cMUT as recited in claim 19, wherein the DC signal source and the DC bias voltage is switchably connected to the cMUT, the DC bias voltage being disconnected to the cMUT in the transmission mode.