Research On Testing Circuit Of Surface Acoustic Wave Sensor

The SAW sensor is a kind of new sensors which is sensitive to the various tested objects andadapt to the development trend of the micro sensor and MEMS. It has a wide application, but theexisting practices can not bring out the dominance in sensor. In order to realize the onlinemeasurement, two kinds of circuits of the active sensor and the passive sensor are studied based onSAW sensor construction features.This paper illustrates the theories relate to SAW sensor tested circuits, including the model ofdelay-line type SAW sensor, characteristics and measurements; the model of resonator type SAWsensor and echo characteristic; DDS and harmonic analysis; PLL circuit model; mixers’principle andthe origin of interference; the impedance match theory and smith chart.Base on the structural feature of delay-line type SAW sensor, phase difference measurementcircuit and amplitude-frequency measurement circuit are studied. The chip AD8302can convert thephase difference and the amplitude-frequency to the analog voltage, so two kinds of circuits are ableto integrate together. The signal sources, consist of DDS and PLL, generates fixed-frequency andsweep signal. The chip AD9850and LTC6946are chose as DDS and PLL. The filters are designed tofilter the output signal harmonic of the DDS and PLL. The measurements of the circuits areaccordance with results of the network analyzers.Base on the structural feature of resonator type SAW sensor, the passive sensor circuit is studied,including transduction circuit, receiver circuit and transceiver isolation circuit. The carrier signal isgenerated by DDS and PLL up-conversion, modulated by RF switch, transmitted by antenna. Thesensor will return an echo signal, which includes sensing information. Then, the echo signal isamplified and down converted by receiving circuit to obtain sensor information. RF swith achievestransceiver isolation and transmit signal modulation. The conversion efficiency of the antenna isdirectly related to the performance of the passive sensor test circuit. Using ADS software as asimulation tool, network analyzer as a measuring tool, the antenna’s impedance is matched. Theresonator type SAW sensor is measured by the test circuit, and measurement results prove theeffectiveness of the test circuit.