Low loss SAW RF ID tags for space applications

NASA has identified the need for wireless, passive sensors for ground, in-flight, and space exploration. In general, devices must simultaneously sense and tag measurands, such as temperature or pressure, in a multi-sensor environment. SAW sensor devices are ideally suited since they are passive, small, rugged, operate over broad temperatures, and are radiation hard. In a passive, wireless multi-sensor environment, the communication distance of the sensor-tag is related to its insertion loss and the processing gain of the system. To maximize the tag readable distance, the reduction of tag insertion loss is critical. Single frequency CDMA tags for high volume commercial applications must have universal coding schemes and large numbers of codes. The large number of bits for coding and CDMA operation necessitates reflector banks with > 30 dB loss. The relaxation of these parameters allows the design emphasis to be shifted to low-loss reflectors. The use of multiple frequency coded tags reduces adjacent reflector interactions for low insertion loss (< 4dB), increased system operation distance, complex coding, and system processing gain. The semi-custom SAW sensor devices provide low loss platforms for multi-sensor integration and diverse embodiments are possible. The goal of the current research is to develop a very low loss SAW tag-sensor platform for use in a broad range of sensing applications. The paper will discuss the reflector design aspects with respect to a communication platform, without reference to any particular sensor. The goal is to reduce the coded reflector bank insertion loss to less than 4 dB by investigating long reflector banks that use a pseudo-orthogonal frequency coding (P-OFC) definition that reduces intra-chip reflections and optimizes spectral bandwidth of the device. The new coding approach is defined and contrasted with the previously defined orthogonal frequency coded (OFC) technique. Auto- and cross- correlation properties of the ch- - ips and their relation to reflectivity per strip and reflector length are discussed. Frequency and time-domain inter-symbol interference will be compared and quantified with respect to system performance. Results at 250 MHz of a single frequency CDMA tag will be compared to 8 chip OFC, and pseudo-orthogonal frequency coded P-OFC SAW tags. The key parameters of insertion loss, cross-correlation and auto-correlation of the two types of frequency coded tags will be analyzed, contrasted and discussed. It is shown that coded reflector banks can be achieved with near zero loss and still maintain good coding properties. Experimental and COM model predicted device results with varying reflector designs and codes are presented.