The Circuit Design For Portable SAW Vapor Sensors

People choose to use SAW (Surface Acoustic Wave) gas sensor, during to its high sensitivity, portability and other unique advantages. Whether a SAW gas sensor is good or bad, we often evaluate it from two aspects:SAW device and detection circuit. Since the center frequency of SAW device determines the sensitivity of detecting gas, the insert loss of SAW device relates to whether the device can start-up; detection circuit relates to the sensor accuracy and stability. So researching both of them has great significance.Firstly how to design a delay line type SAW device is described in this paper, while of which the center frequency is 120MHz. Comparing the various detection methods, we choose the frequency detection method to design the circuit, because the environmental factors have little influence to the frequency detection circuits.For the device design, we use dual delay line structure. Using the coupling of modes (COM) theory to analyze this structure, we derive the mixing matrix (P matrix) of the single-phase unidirectional transducer. And then combining with the device equivalent circuit model, we can finally educe the device's amplitude frequency, phase frequency and so on. The process of aluminum sacrificial layer is chosen to product device.In order to eliminate the environmental impact, we use frequency detection method to design circuit. It is mainly to mix the frequency signal generated by two channels, reference channel and sensitive channel, then educe the kind of gas.Finally, we put the sensitive adsorptive film on the device, and use a method of dynamic gas distribution, and then validate the circuit. The results show that the circuit has high accuracy for detecting trace gas, and meet the needs of gas detection.