TWI869146B - Demodulation signal generator for air pulse generator - Google Patents

Abstract

A demodulation signal generator, coupled to an air-pulse generator comprising a flap pair, includes a resonance circuit. The resonance circuit produces a first demodulation signal and a second demodulation signal. The resonance circuit and the flap pair co-perform a resonance operation, such that the first demodulation signal and the second demodulation signal are generated via the co-performed resonance operation and have opposite polarity. The flap pair performs a differential movement to form an opening to perform a demodulation operation on a modulated air pressure variation.

Description

Demodulation signal generator for gas pulse generator

The present application relates to a driving circuit or a demodulation signal generator, and more particularly to a driving circuit or a demodulation signal generator capable of driving a pair of petals to perform differential motion and having low power consumption.

Speaker drivers and back enclosures are two of the biggest design challenges in the speaker industry. Existing speakers have difficulty covering the entire audio frequency band (e.g., from 20Hz to 20KHz). In order to produce high-fidelity sound with sufficiently high sound pressure levels, the volume/size of the radiating/moving surface and back enclosure of existing speakers must be large enough.

Ultrasonic pulse generators have been investigated for generating air pulses or sounds to overcome the design challenges faced by conventional speakers. Ultrasonic pulse generators that include capacitive actuators are expected to have high power consumption at ultrasonic frequencies and are therefore not desirable in portable electronic devices or consumer electronic devices.

Therefore, how to design a low-power driving circuit to drive an ultrasonic pulse generator is an important goal in this field.

Therefore, the main purpose of this application is to provide a demodulation signal generator to improve the shortcomings of the prior art.

One embodiment of the present invention discloses a demodulation signal generator coupled to a gas pulse generator. The demodulation signal generator includes: a first node coupled to a first lobe and a second node coupled to a second lobe; and a resonance circuit coupled to the first node and the second node, used to generate a first demodulation signal at the first node and a second demodulation signal at the second node. The gas pulse generator includes a membrane structure, the membrane structure includes a lobe pair, and the lobe pair includes the first lobe and the second lobe. The resonance circuit and the lobe pair jointly perform a resonance operation, so that the first demodulation signal and the second demodulation signal are generated by the jointly performed resonance operation. The first demodulation signal and the second demodulation signal have opposite polarities. The first flap receives the first demodulation signal, and the second flap receives the second demodulation signal, so that the flap pair performs a differential motion. The differential motion is used to form an opening to perform a demodulation operation on a modulated air pressure change generated by the membrane structure.

In the present invention, the term "coupled" may refer to direct or indirect connection. "Component A is coupled to component B" may mean that component A is directly connected to component B, or that component A is connected to component B via component C.

The contents of U.S. Patent Application No. 18/321,757 are incorporated herein by reference.

FIG. 1 shows an air-pulse generator (APG) 1 according to an embodiment of the present application. The air-pulse generator 1 can be used for generating sound or cooling applications, and includes a membrane structure 10. The membrane structure 10 can perform a modulation operation according to an acoustic signal to generate an ultrasonic frequency/air wave (UAW), and perform a demodulation operation according to the ultrasonic frequency/air wave to generate an ultrasonic pulse array (UPA). The modulation operation is performed through a common mode motion of the membrane structure 10, and the demodulation operation is performed through a differential mode motion of the membrane structure 10. After the low-pass filtering effect inherent in the natural/physical environment and the human hearing system, a sound corresponding to the sound signal may be produced.

As taught in U.S. Patent Application 18/321,757, the membrane structure 10 includes a pair of flaps 102. The flap pair 102 is actuated to perform common mode motion to perform a modulation operation, which is used to generate an ultrasonic frequency/air wave. At the same time, the flap pair 102 is also actuated to perform differential mode motion (or differential motion) to perform a demodulation operation, which is to generate an ultrasonic pulse array with a pulse rate (e.g., 192KHz) based on the ultrasonic frequency/air wave.

The flap pair 102 includes a first flap 101 and a second flap 103. Both flaps 101 and 103 are coupled to a modulation signal generator 16 to receive a modulation signal SM so as to be actuated to perform common mode motion and modulation operations. On the other hand, flaps 101 and 103 are coupled to a demodulation signal generator 14 to receive a first demodulation signal +SV and a second demodulation signal -SV, respectively. The demodulation signals +SV and -SV generally have opposite polarities corresponding to a specific level, so that flaps 101 and 103 can perform differential mode motion and demodulation operations. More specifically, the differential mode motion is used to form an opening 112 (as shown in FIG. 2 ) to perform a demodulation operation on the modulated air wave or pressure change generated by the membrane structure 10 or the flap pair 102.

The modulation signal SM has a modulation frequency, which is the pulse rate (such as 192KHz). The demodulation signal +SV/-SV has a demodulation frequency. Due to the differential mode motion of the lobe pair, the demodulation frequency can be half of the pulse rate or half of the modulation frequency (such as 96KHz).

A detailed wiring scheme between the gas pulse generator and the (de)modulated signal generator is shown in FIG. 2, which shows schemes 131 to 133. The gas pulse generator 1 includes a first actuator 101A disposed on the flap 101 and a second actuator 103 disposed on the second flap 103. The actuator 101A/103A includes an upper electrode and a lower electrode. In one embodiment, the actuator 101A/103A also includes a piezoelectric layer, which can be composed of PZT (e.g., lead zirconate titanate, which is capacitive) and is disposed between the upper and lower electrodes. In one embodiment, the demodulation signal generator is coupled to one electrode of the actuator 101A/103A and the modulation signal generator is coupled to another electrode of the actuator 101A/103A. For example, the demodulation signal generator may be coupled to the top electrode of the actuator 101A/103A and the modulation signal generator may be coupled to the bottom electrode of the actuator 101A/103A. In one embodiment, the modulation and demodulation signal generators may be coupled to at least one electrode of the actuator 101A/103A.

FIG. 3 is a schematic diagram of a demodulation signal generator 14 of an embodiment of the present application. The demodulation signal generator can be regarded as a driving circuit for driving the petal pair to perform differential mode motion and demodulation operations. In addition to having nodes N101 and N103 for coupling petals 101 and 103, respectively, the demodulation signal generator 14 generally also includes a resonant circuit 140. The demodulation signal generator 14 generates a demodulation signal +SV which is transmitted to the petal 101 through the node N101, and generates a demodulation signal -SV which is transmitted to the petal 103 through the node N103. The resonant circuit 140 and the petal pair 102 (petals 101 and 103) jointly perform a resonant operation to generate demodulation signals +SV and -SV with opposite polarities.

For example, FIG. 4 shows a schematic diagram of a demodulation signal generator 24 of an embodiment of the present application. The demodulation signal generator 24 includes a resonant circuit 240, nodes N101, N103, and switches _SW1H , _SW1L , _SW2H , _SW2L . The resonant circuit 240 may be or include a switching module 242. The switching module 242 includes an inductor L and a switching unit _SWER . In the embodiment of FIG. 4, the switching unit includes a switch _SWER1 . The switching module 242 is coupled between the nodes N101 and N103.

FIG. 5 shows a timing diagram of the demodulation signal generator 24. The switching module 242 is turned on during a conduction period, such as T12 or T21 in FIG. 5. The demodulation signal +SV switches from a high voltage level _VH to a low voltage level _VL after the conduction period T12, and the demodulation signal -SV switches from the low voltage level _VL to the high voltage level _VH after the conduction period T12. Similarly, the demodulation signal +SV switches from the low voltage level _VL to the high voltage level _VH after the conduction period T21, and the demodulation signal -SV switches from the high voltage level _VH to the low voltage level _VL after the conduction period T21. Therefore, after the conduction period T12/T21, the voltage level of the first demodulation signal +SV and the voltage level of the second demodulation signal -SV are exchanged. At the same time, the demodulation signal +SV and the demodulation signal -SV can be regarded as having opposite polarities.

4 and 5 , switches SW _1H and SW _1L are coupled to node N101, and switches SW _2H and SW _2L are coupled to node N103. Switches SW _1H and SW _2H receive a high voltage V _H , and switches SW _1L and SW _2L receive a low voltage V _L .

During the period T1 (before the conduction period T12), the switches _SW1H and _SW2L are turned on/on, and the switches _SW1L and _SW2H are turned off/off, so that the demodulated signal +SV is at the high voltage _VH and the demodulated signal -SV is at the low voltage _VL . During the period T1, no current flows through the switching module 242, and the switching unit _SWER is turned off.

During period T2 (after conduction period T12), switches _SW1H and _SW2L are closed, and switches _SW1L and _SW2H are opened, so that the demodulated signal +SV is at low voltage _VL and the demodulated signal -SV is at high voltage _VH . During period T2, no current flows through the switching module 242, and the switching unit _SWER is closed.

During the conduction period T12, the switches SW _1H , SW _1L , SW _2H , and SW _2L are closed, and the switching unit SW _ER is turned on. A current is formed from the node N101 to the node N103, causing the demodulated signal +SV to decrease and the demodulated signal -SV to increase. Therefore, the electric energy stored in the capacitor corresponding to the actuator 101A is transferred to the capacitor corresponding to the actuator 103A.

During the conduction period T21, similarly, the switches _SW1H , _SW1L , _SW2H , _SW2L are closed, and the switching unit _SWER is turned on. A current is formed from the node N103 to the node N101, causing the demodulated signal +SV to rise and the demodulated signal -SV to fall. Therefore, the electric energy stored in the capacitor corresponding to the actuator 103A is transferred to the capacitor corresponding to the actuator 101A.

By switching the switches (i.e., SW _1H , SW _1L , SW _2H , SW _2L and SW _ER1 in FIG. 4 ), the demodulation signal generator 24 can generate demodulation signals +SV and -SV having waveforms as shown in the upper part of FIG. 5 . It should be noted that even though the demodulation signals +SV and -SV for the capacitive actuators 101A and 103A include a large number of transitions during the period T12/T21 of the pulse generator operation, since the energy is charged and discharged back and forth between N101 and N103 with the help of the resonant circuit 240, the power consumed by the petal pair 102 and the demodulation signal generator 24 is extremely low.

By applying the demodulated signals +SV and -SV to the petals 101 and 103 , the petal pair 102 can perform differential motion.

In the present application, the flap pair performing differential motion means: 1) in a transient state/period, one flap moves in a first direction and the other flap moves in a second direction opposite to the first direction; or 2) in a stable state/period, one flap is actuated to bend upward and the other flap is actuated to bend downward.

The differential motion of the demodulation signal generator 24 satisfies both 1) and 2) above. More specifically, in the steady-state period T1 shown in FIG. 5 , the flap 101 receives the demodulation signal +SV, which is a high voltage V _H , and is actuated to bend upward; the flap 103 receives the demodulation signal -SV, which is a low voltage V _L , and is actuated to bend downward. In the steady-state period T2 , the flap 101 receives the demodulation signal +SV, which is a low voltage V _L , and is actuated to bend downward; the flap 103 receives the demodulation signal -SV, which is a high voltage V _H , and is actuated to bend upward. During a transient period T12 between T1 and T2 , the flap 101 moves in the −Z direction, and the flap 103 moves in the +Z direction, which is opposite to the −Z direction.

Furthermore, during the conduction/transient/transition period T12/T21, the inductor L in the resonant circuit 240 and the capacitance of the piezoelectric layer in the actuator 101A/103A can (together) perform an LC resonance. The LC resonance can transmit a current from N101 to N103 during the period T12, and transmit a current from N103 to N101 during the period T21.

FIG. 6 is a schematic diagram of a demodulation signal generator 34 according to an embodiment of the present application. The demodulation signal generator 34 is similar to the demodulation signal generator 24, and the same elements are represented by the same symbols. Different from the demodulation signal generator 24, the switching unit SW _ER in a switching module 342 or a resonant circuit 340 of the demodulation signal generator 34 further includes a switch SW _ER2 coupled between the inductor L and the node N103, wherein the switches SW _ER1 and SW _ER2 form the switching unit SW _ER , and the inductor L and the switching unit SW _ER form the switching module 342. In the present application, the switching unit SW _ER is turned on, which means that both switches SW _ER1 and SW _ER2 are turned on, and the switching unit SW _ER is turned off, which means that at least one of switches SW _ER1 and SW _ER2 is turned off.

In one embodiment, the switch SW _ER1 /SW _ER2 can be implemented by a metal-oxide-semiconductor field-effect transistor (MOSFET) having a body diode BD1 / BD2, as shown in the lower half of FIG. 6. The switch SW _ER1 /SW _ER2 is configured so that the body diode BD1 / BD2 can block the current when the switch SW _ER1 /SW _ER2 is turned off. In one embodiment, the anode of the body diode BD1 / BD2 is coupled to the node N101 / N103, and the cathode of the body diode BD1 / BD2 is coupled to the inductor L. In this case, when the switch SW _ER1 /SW _ER2 is turned off, the body diode BD1 / BD2 can prevent the current from flowing from the inductor L to N101 / N103.

For the dual-switch switching module 342, it is not necessary to turn off both switches at the same time at the end of the conduction period. Instead, under the appropriate body diode setting, only one switch is turned off at the end of the conduction period T12/T21. For example, at the end of the conduction period T12, its demodulated signal +SV decreases and the demodulated signal -SV increases (or the voltage of the node N101 is less than the voltage of the node N103 at the end of the conduction period T12), the switch SW _ER1 is turned off and the switch SW _ER2 remains on. In this case, the body diode BD1 with the setting shown in FIG. 6 will block the current from the inductor to the node N101. Likewise, at the end of the conduction period T21, the switch SW _ER2 is turned off while the switch SW _ER1 remains turned on.

The timing of switches SW _ER1 and SW _ER2 can be referred to FIG. 7. Turning on one switch (SW _ER1 or SW _ER2 ) at a time can reduce the switching rate of each switch, thereby further reducing the power consumption of the switches in the switching module 342.

It is worth noting that the switches SW _ER1 and SW _ER2 are not limited to being located on both sides of the inductor L, as shown in FIG. 6 . In one embodiment, the switches SW _ER1 and SW _ER2 may be located on the same side of the inductor L (not shown), wherein the body diode BD1/2 may still block the current when the switches SW _ER1/2 are turned off. As long as the switches SW _ER1 and SW _ER2 are coupled between the nodes N101 and N103, they are within the scope of the present application.

In addition to the inductor-capacitor resonance used in the resonant circuit 240/340, a complementary metal oxide semiconductor micro electro mechanical system (CMOS-MEMS) resonance/oscillation may also be used to generate the demodulation signal ±SV.

FIG. 8 is a schematic diagram of a demodulation signal generator 44 of an embodiment of the present application. The demodulation signal generator 44 includes a resonant circuit 440, which may be or include a starting circuit 442. The resonant circuit 440 or the starting circuit 442 is coupled to the petals 101 and 103, and utilizes the resonant characteristics of the petals 101/103 to generate demodulation signals ±SV, which can be regarded as performing a resonant operation together with the petal pair 102. The starting circuit 442 includes a transimpedance amplifier 444, a detection and control circuit 446, and a variable gain amplifier (VGA) 448.

The transimpedance amplifier 444 includes a first input terminal coupled to the flap 101 to receive a current i+, and includes a second input terminal coupled to the flap 103 to receive a current i-. The transimpedance amplifier 444 can generate an output signal Vo according to the currents i+ and i-. As shown in FIG. 8 , the transimpedance amplifier 444 includes an operational amplifier, and a feedback resistor and a feedback capacitor coupled between the input terminal and the output terminal.

The detection and control circuit 446 is coupled to the output terminal of the transimpedance amplifier 444. The detection and control circuit 446 is configured to perform a detection operation according to the signal Vo, wherein the detection operation may be an amplitude detection operation, a phase detection operation, a frequency detection operation, or a combination of the above-mentioned various detection operations.

The detection and control circuit 446 controls the variable gain amplifier 448 so that the starting circuit 442 in the closed loop 46 can meet the Barkhausen criterion, that is, the loop gain is greater than or equal to 1, and the loop phase is equal to 0 or an integer multiple of 2π. Therefore, the petal pair 102 and the resonant circuit 440 can jointly perform complementary metal oxide semiconductor micro-electromechanical system resonance/oscillation.

In one embodiment, the detection and control circuit 446 may include an amplitude control circuit 4461 (as shown in FIG. 9 ), such that the amplitude control circuit 4461 can control the variable gain amplifier 448 to form an automatic gain control (AGC) loop to satisfy the Barkhausen criterion.

In one embodiment, the detection and control circuit 446 may perform a frequency detection operation to track a resonant frequency of the flap 101/103. The frequency detection operation may be performed by applying a sweep test signal. In one embodiment, the detection and control circuit 446 may include a phase lock loop (PLL) circuit 4462 (as shown in FIG. 9). The phase lock loop circuit 4462 may be used to perform a frequency detection operation to track the resonant frequency of the flap 101/103 or flap pair 102.

In one embodiment, a loop filter (not shown in FIG. 8 ) may be included to avoid unwanted oscillations.

The loop 46 formed by the petal pair 102 and the resonant circuit 440 can perform a self-sustaining oscillation operation. Under self-sustaining oscillation, the displacement of the petals 101/103 can be amplified by Q times. It should be noted that Q is the quality factor of the mechanical resonant mode of 101 and 103. Therefore, the input signal amplitude of +SV and -SV can be reduced by Q times to maintain the required displacement, and the power is greatly reduced by ^Q2 times.

It is worth noting that FIG. 8 only shows an embodiment of the oscillation start circuit. The demodulated signals +SV and -SV can be generated according to any type of oscillation start circuit or frequency detection circuit, but are not limited thereto. As long as the resonance/oscillation between the petal pair and the resonant circuit can be formed, it should fall within the scope of this application.

Furthermore, the demodulation signal generator 44 may include a phase shifter 41 coupled between the flap 101 and the flap 103. The phase shifter 41 sets the phase difference between the demodulation signal +SV and the demodulation signal -SV to 180 degrees, so that the demodulation signals +SV and -SV have opposite polarities. The demodulation signals +SV and -SV are applied to the flaps 101 and 103, so that the flap pair 102 can perform differential motion, wherein the differential motion of the demodulation signal generator 44 satisfies the state 1) described in the above paragraph.

In addition, the demodulation signal generator 44 may include a frequency multiplier 43 coupled to the start circuit 442 and receiving an output signal, denoted as SV_out, of the start circuit 442. The frequency multiplier 43 is used to double the frequency of SV_out to generate an output signal SM_ref, so that the modulation signal SM can be generated according to the signal SM_ref, wherein the frequency of the output signal SM_ref is twice the frequency of the output signal SV_out, so that the demodulation frequency is equal to half of the pulse rate, or when the modulation frequency is the pulse rate, it is equal to half of the modulation frequency.

It is worth noting that the resonance operation performed by the resonance circuit 240 / 340 and the petal pair utilizes the electrical resonance of its internal components; while the resonance operation performed by the resonance circuit 440 and the petal pair utilizes the mechanical resonance of its internal components.

One of the many advantages of a demodulation signal generator that uses resonance to generate a demodulation signal ±SV is reduced power consumption. The resonant operation is not limited to the above-mentioned LC resonance or complementary metal oxide semiconductor micro-electromechanical system resonance. The demodulation signal generator and the resonant circuit can use any resonant method to generate an opposite polarity demodulation signal generator, which all belong to the scope of this application.

In short, the present application utilizes a resonant circuit and a pair of petals to jointly perform a resonant operation to generate a demodulated signal ±SV with opposite polarities, wherein the resonant operation can be an inductor-capacitor resonance or a complementary metal oxide semiconductor micro-electromechanical system resonance that satisfies the Barkhausen criterion. The above is only a preferred embodiment of the present invention, and all equivalent changes and modifications made according to the scope of the patent application of the present invention should be within the scope of the present invention.

1: Pulse generator 10: Membrane structure 102: Petal pair 101, 103: Petals 14, 24, 34, 44: Demodulation signal generator 16: Modulation signal generator SM: Modulation signal +SV, -SV: Demodulation signal N101, N103: Nodes 131-133: Scheme 101A, 103A: Actuator 112: Opening 140, 240, 340, 440: Resonance circuit SW _1H , SW _1L , SW _2H , SW _2L , SW _ER1 , SW _ER2 : Switch 242, 342: Switch module L: Inductor SW _ER : Switching unit V _H : High voltage level V _L : low voltage level T12, T21: conduction period T1, T2: period BD1, BD2: body diode 442: start circuit 444: transimpedance amplifier 446: detection and control circuit 448: variable gain amplifier i+, i-: current Vo, SV_out, SM_ref: output signal 41: phase shifter 43: frequency multiplier 46: loop 4461: amplitude control circuit 4462: phase-locked loop circuit

FIG. 1 is a schematic diagram of a gas pulse generator in an embodiment of the present application. FIG. 2 is a wiring scheme in an embodiment of the present application. FIG. 3 is a schematic diagram of a demodulation signal generator in an embodiment of the present application. FIG. 4 is a schematic diagram of a demodulation signal generator in an embodiment of the present application. FIG. 5 is a timing diagram of the demodulation signal generator in FIG. 4. FIG. 6 is a schematic diagram of a demodulation signal generator in an embodiment of the present application. FIG. 7 is a timing diagram of the demodulation signal generator in FIG. 6. FIG. 8 is a schematic diagram of a demodulation signal generator in an embodiment of the present application. FIG. 9 is a schematic diagram of a detection and control circuit in an embodiment of the present application.

14: Demodulation signal generator

+SV,-SV: demodulated signal

N101, N103: Node

140: Resonance circuit

Claims (25)

A demodulation signal generator is coupled to an air-pulse generator (APG), and the demodulation signal generator includes: A first node coupled to a first petal and a second node coupled to a second petal; and A resonant circuit coupled to the first node and the second node, used to generate a first demodulation signal at the first node and a second demodulation signal at the second node; Wherein, the air-pulse generator includes a membrane structure, the membrane structure includes a petal pair, the petal pair includes the first petal and the second petal; Wherein, the resonant circuit and the petal pair jointly perform a resonant operation, so that the first demodulation signal and the second demodulation signal are generated by the jointly performed resonant operation; wherein the first demodulation signal and the second demodulation signal have opposite polarities; wherein the first flap receives the first demodulation signal and the second flap receives the second demodulation signal, so that the flap pair performs a differential motion; wherein the differential motion is used to form an opening to perform a demodulation operation on a modulated air pressure change generated by the membrane structure. A demodulated signal generator as described in claim 1, wherein the resonant circuit includes a switching module; wherein the switching module is set to be turned on during a conduction period; wherein after the conduction period, a first voltage level of the first demodulated signal is exchanged with a second voltage level of the second demodulated signal. A demodulated signal generator as described in claim 2, wherein the switching module includes an inductor and a switching unit coupled between the first node and the second node. The demodulation signal generator as described in claim 3 comprises: a first switch, a second switch, a third switch and a fourth switch; wherein the first switch and the second switch are coupled to the first node, and the third switch and the fourth switch are coupled to the second node. A demodulated signal generator as described in claim 4, wherein, in a first period before the conduction period, the first switch and the fourth switch are turned on, and the second switch and the third switch are turned off; and, in a second period after the conduction period, the first switch and the fourth switch are turned off, and the second switch and the third switch are turned on. A demodulation signal generator as described in claim 5, wherein the switching unit is disconnected during the first period or the second period. A demodulated signal generator as described in claim 4, wherein the first switch and the third switch receive a first voltage; wherein the second switch and the fourth switch receive a second voltage. A demodulated signal generator as described in claim 3, wherein the switching unit comprises: A fifth switch and a sixth switch, coupled between the first node and the second node. A demodulated signal generator as described in claim 8, wherein, during the conduction period, the fifth switch and the sixth switch are turned on. A demodulated signal generator as described in claim 8, wherein the fifth switch includes a first body diode having a first anode coupled to the first node; wherein the sixth switch includes a second body diode having a second anode coupled to the second node; wherein during the conduction period, a first voltage at the first node decreases and a second voltage at the second node increases; wherein at the end of the conduction period, the first voltage is less than the second voltage and the fifth switch is closed. A demodulated signal generator as described in claim 8, wherein, at the end of the conduction period, the sixth switch remains on. A demodulation signal generator as described in claim 8, wherein, during the first period, the sixth switch is turned off; wherein, during the second period, the fifth switch is turned off. A demodulated signal generator as described in claim 1, wherein the air pulse generator includes a first actuator disposed on the first flap and a second actuator disposed on the second flap; wherein the first actuator includes a first electrode, and the second actuator includes a second electrode; wherein the first node is coupled to the first electrode, and the second node is coupled to the second electrode. A demodulated signal generator as described in claim 13, wherein the first actuator includes a third electrode and the second actuator includes a fourth electrode; wherein the third electrode and the fourth electrode receive a modulation signal so that the flap pair performs a common mode motion to generate the modulated air pressure change. A demodulation signal generator as described in claim 1, wherein the pulse generator generates a plurality of pulses at a pulse rate; wherein the plurality of pulses are generated according to the change of the modulated air pressure; wherein a demodulation frequency of the first demodulation signal is half of the pulse rate. A demodulation signal generator as described in claim 1, wherein the resonant circuit includes a starting circuit; wherein the starting circuit and the petal pair generate an oscillation. A demodulation signal generator as described in claim 16, wherein the starting circuit includes a transimpedance amplifier; wherein the transimpedance amplifier includes a first input terminal coupled to the first flap and a second input terminal coupled to the second flap; wherein the transimpedance amplifier is used to receive a first current corresponding to the first flap and a second current corresponding to the second flap; wherein the transimpedance amplifier outputs a first output signal according to the first current and the second current; wherein the first demodulation signal and the second demodulation signal are generated according to the first output signal of the transimpedance amplifier. A demodulation signal generator as described in claim 17, wherein the starting circuit further comprises a detection and control circuit coupled to an output terminal of the transimpedance amplifier for performing a detection operation according to the first output signal of the transimpedance amplifier; and a variable gain amplifier (VGA) controlled by the detection and control circuit; wherein the first demodulation signal is generated according to a second output signal of the variable gain amplifier. A demodulated signal generator as described in claim 18, wherein the detection and control circuit includes an amplitude control circuit; wherein an automatic gain control (AGC) loop is formed by the amplitude control circuit and the variable gain amplifier. A demodulated signal generator as described in claim 18, wherein the detection and control circuit includes a phase locked loop (PLL) circuit for performing a frequency detection operation to track a resonant frequency of the first lobe. A demodulated signal generator as described in claim 17, wherein the transimpedance amplifier includes an operational amplifier. A demodulated signal generator as described in claim 21, wherein the transimpedance amplifier includes a feedback resistor coupled between an input terminal and an output terminal of the operational amplifier. A demodulated signal generator as described in claim 21, wherein the transimpedance amplifier includes a feedback capacitor coupled between an input terminal and an output terminal of the operational amplifier. The demodulation signal generator as described in claim 16 further comprises: A phase shifter coupled to the first flap and the second flap; Wherein, the phase shifter sets a phase difference between the first demodulation signal and the second demodulation signal to 180 degrees. The demodulation signal generator as described in claim 16 further comprises: A frequency multiplier coupled to the starting circuit and receiving a third output signal of the starting circuit to generate a fourth output signal; Wherein, the frequency of the fourth output signal is twice the frequency of the third output signal; Wherein, a modulation signal is generated based on the fourth output signal; Wherein, the modulation signal is used to drive the membrane structure so that the membrane structure generates the modulated air pressure change.

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