Design Of CUMT’s Signal Extracting Circuit

Ultrasonic imaging technology has been played a huge role in medical diagnosis,non-destructive detection and seafloor topography detection,and ultrasonic sensors as the corecomponent has also become a hot research topic. Capacitive ultrasonic transducer(CMUT)has emerged as the main research direction of the high-frequency ultrasonic sensor offeringadvantages such as without matching layer,wide bandwidth, easy to fabricate array, goodstability.However, the capacitance of the sensors is so small, generally a magnitude of0.1-1fF,the capacitance value change of the capacitor also in a magnitude of0.1-1fF, or even smaller,while the operating frequency is relatively high. Therefore to design a circuit to detect thehigh-frequency weak dynamic value has become a urgent problem.To solve the above problem, we carried out the research. Firstly, systematic analysis ofbasic structure and working principles of capacitive ultrasonic sensor, capacitive ultrasonicsensor corresponding circuit model is created, and vibration membrane displacement formulais derived when sensor is under pressure, by the current formula of the sensor as well as filmvibration velocity formula, and so on, then performance simulation analysis is done by usingANSYS finite element simulation software on the sensor that the parameters has been setted,displacement map and modal diagram is drawn, and resonant frequency and change value isalso been determined, which provide the basis for the subsequent design of detection circuit.Then, two weak dynamic capacitance detection circuits of the capacitive ultrasonic sensorboth applicable to our group are designed: One is based on the capacitance-voltage conversionprinciple of the capacitance detection circuit of the AC bridge, the other is based on thecapacitor-current conversion principle of the transimpedance amplifier capacitance detectioncircuit, then simulation analysis of the circuit is done by the Multisim Simulation Software. Inaddition, since the CMUT in the ultrasound imaging system need to transmit ultrasonic todetect front target and receive the reflected sound waves, which lead to the capacitance valuechange, so the chirp signal generating circuit is designed. Since the capacitive ultrasonic sensor need to switch back and forth between the emission state and the reception state,timing problems of each module in the overall capacitance detection system is analyzed andtiming control programs is written. Finally, a test platform is builded to test the capacitancedetection circuit and the chirp transmitter circuit, sensitivity, linearity, and stability of thecapacitance detection circuit as well as amplitude and bandwidth of the pulse transmittingcircuit transmitting signals are also tested.The experimental results show that the chirp generating circuit can generate a betterpulse signal waveform, and the frequency and duty cycle of the signal and the transmissioninterval can be convenient for the control, the pulse compression circuit can make the widthof the pulse signal obtained considerable compression, which can improve the resolution ofthe system and increase the intensity of the acoustic signal. The sensitivity of the capacitancedetection circuit is0.908mv/fF; the linearity of the circuit is good, theoretical linearcorrelation coefficient of the curve and the actual curve reached0.9895; good stability, in the1-hour course of the experiment, the circuit output value can be stabilized within0.7mV.