Saw Gas Sensor Circuit Optimization Design

The surface acoustic wave (SAW) gas sensor is one kind of novel sensors, which has already been paid much attention because of its unique advantages. The precision and the stability of the sensor directly depend on the sensor circuit. Therefore, the optimized design of the circuit for the SAW gas sensor is of great significance.Based on the analysis of the SAW device and the present SAW circuits, frequency detection circuits as well as phase detection circuits are designed in this paper. A 433.92MHz one-port SAW resonator device is used as sensing device. The sensor consists of two SAW devices to form the two channel structure which aims at reducing the influence of environmental temperature variation.For the frequency detection circuit, the frequency signals of the two channels are inputted into the SA602A mixer to get the frequency difference, and subsequently read out by the SS7200 frequency counter directly. This paper focuses on some problems in circuit design such as oscillation blocking, frequency hopping and testing blind zone. An offset technology is used to avoid the testing blind zone.For the phase detection circuit, the phase detection capability is realized by the AD8302 phase detector which compares the phase difference between the two channels. Then the voltage corresponding to the phase difference is obtained out by the UP8035 digital panel meter. This paper also analyzes how to determine the dynamic test range by this method.To some extent, the limit of detection for the SAW gas sensor depends on the stability of the circuit. Thus, the stability of the circuits has been tested. The stability of the SAW oscillator is 8×10^-8 and achieves the level reported in literatures (10^-8). The stability of the frequency detection circuit is 7×10^-6. The voltage drift of the phase detection circuit does not surpass 2mV (0.2°) in 12 hours.Finally, accuracy of the circuits has been verified by using the SAW sensors with different sensitive films prepared by spin coating to absorb corresponding gas. The error of the frequency detection circuit is approximately 30Hz. Furthermore, the frequency detection circuit is much better than the phase detection circuit in the sensitivity and the linearity.