Surface Acoustic Wave Sensor Signal Processing Circuit Design

Surface acoustic wave (SAW) sensor has been widely used in many domains such as environmental monitoring, chemical disposal controlling, and clinical analyzing because of its small size, high sensitivity and low price. Now the sensors based on SAW device have became more and more popular.SAW sensing signal is mainly detected via measuring the changes of frequency, phase or amplitude of SAW sensors. In this thesis, we detect the gas environment by the way of the change of frequency. At first, we introduce the SAW devices and the principle of SAW sensor, and then provide a signal processing circuit.In order to detect the frequency of SAW, two SAW oscillators are first designed. We analysis the basic principle of oscillator and the model of SAW device, and then design a circuit. In this circuit, the SAW device is in parallel-resonance region. The signal in time domain, harmonic and noise is simulated. The simulation results prove that this SAW oscillator is a high purity and low noise oscillator. The frequency detecting circuit is constructed based on dual channel structure. The frequencies signals of the two channels are inputted into the mixer to get the frequency difference, which illustrates the information of environmental gas.For frequency difference, we analysis and design a Signal Transform circuit. In order to select the frequency, we adopt a Band Pass and Low Pass Filter, and then put tow signal into a mixer, which is a Gilbert Cell Mixer. The waveform reshape circuit is a comparator using hysteresis and transforms the sine signal into pulse signal.Finally, we design a circuit of frequency measurement. This circuit is based on FPGA and realized by VHDL in QuartusⅡ. We also test this digital circuit and give the test result.As a part of the research of SAW gas sensor, this research of signal processing circuit will contribute to the further study of the SAW gas sensor.