SAW sensors are ideal for various wireless, passive multi-sensor applications because they are small, rugged, radiation hard, and offer a wide range of material choices for operation over broad temperature ranges. The readable distance of a tag in a multi-sensor environment is dependent on the insertion loss of the device and the processing gain of the system. Single-frequency code division multiple access (CDMA) tags that are used in high-volume commercial applications must have universal coding schemes and large numbers of codes. The use of a large number of bits at the common center frequency to achieve sufficient code diversity in CDMA tags necessitates reflector banks with >30 dB loss. Orthogonal frequency coding is a spread-spectrum approach that employs frequency and time diversity to achieve enhanced tag properties. The use of orthogonal frequency coded (OFC) SAW tags reduces adjacent reflector interactions for low insertion loss, increased range, complex coding, and system processing gain. This work describes a SAW tag-sensor platform that reduces device loss by implementing long reflector banks with optimized spectral coding. This new pseudo-OFC (POFC) coding is defined and contrasted with the previously defined OFC coding scheme. Auto- and cross-correlation properties of the chips and their relation to reflectivity per strip and reflector length are discussed. Results at 250 MHz of 8-chip OFC and POFC SAW tags will be compared. The key parameters of insertion loss, cross-correlation, and autocorrelation 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 results and results predicted by the coupling of modes model are presented for varying reflector designs and codes. A prototype 915-MHz POFC sensor tag is used as a wireless temperature sensor and the results are shown.
The use of ultra-short pulses, producing very wide bandwidths and low spectral power density, are the widely accepted approach for ultra-wideband (UWB) communication systems. This approach is simple and can be implemented with current digital signal processing technologies. However, surface acoustic wave (SAW) devices have the capability of producing complex signals with wide bandwidths and relatively high frequency operation. This approach, using SAW based correlators, eliminates many of the costly components that are needed in the IF block in the transmitter and receiver, and reduces many of the signal processing requirements. This work presents the development of SAW correlators using orthogonal frequency coding (OFC) for use in UWB spread spectrum communication systems. OFC and pseudonoise (PN) coding provide a means for UWB spreading of data. The use of OFC spectrally spreads a PN sequence beyond that of code division multiple access (CDMA) because of the increased bandwidth providing an improvement in processing gain. The transceiver approach is still very similar to that of a CDMA but provides greater code diversity. Experimental results of a SAW filter designed with OFC transducers are presented. The SAW correlation filter was designed using seven contiguous chip frequencies within the transducer. SAW correlators with a 29% fractional bandwidth were fabricated on lithium niobate (LiNbO3) having a center frequency of 250 MHz. A coupling-of-modes (COM) model is used to predict the SAW filter response experimentally and is compared to the measured data. Good correlation between the predicted COM responses and the measured device data is obtained. Discussion of the design, analysis, and measurements are presented. The experimental matched filter results are shown for the OFC device and are compared to the ideal correlation. The results demonstrate the OFC SAW device concept for UWB communication transceivers.
Recently, SAW material parameters have been reported for CNGS, CTGS, SNGS and STGS [1,2]. These materials are in the same family as LGS, LGN and LGT but have a lower density, differing coupling coefficients, higher velocity, and differing temperature coefficients. As compared with quartz, these materials have a higher density, lower velocity, higher electromechanical coupling coefficient, and higher temperature of operation without degradation of piezoelectric properties. Initial efforts have been made on investigating the various SAW parameters of CNGS and CTGS. Very little material is available, but very interesting properties have been found while investigating the Y-cut CNGS and CTGS substrates. It has been observed that there is a temperature compensated propagation angle near room temperature which is both promising in itself, as well as suggesting other compensated cuts may be possible at differing cut angles [2]. Electromechanical coupling, velocity, temperature coefficient of frequency, power flow angle, and fractional frequency curvature results are provided and compared with those of quartz. In addition, a comparison of previously published results for CNGS, CTGS, SNGS, and STGS Y-cut substrates are given. These results include coupling coefficient, velocity, capacitance, and reflectivity measurements for various propagation angles. These results are also compared with previously reported measurement for LGT and LGN and a discussion is provided.Based on reflectivity measurements and parameter extraction, a COM model is used to predict SAW resonator performance and fabricated SAW resonator results are provided and compared with the COM model. 0-7803-7922-5/03/$17.00 (c)
Orthogonal frequency coded (OFC) SAW reflectors and transducers have been recently introduced for use in communication, sensor and RFID tag applications. [1,2] The OFC SAW technology approach has been funded by NASA for possible inclusion in ground, space flight and space exploration sensor applications. In general, SAW technology has advantages over possible competing technologies: passive, wireless, radiation hard, operation from cryogenic to furnace temperature ranges, small, rugged, variable frequency and bandwidth operation, encoding and commercially available. SAW sensor embodiments can provide onboard device sensor integration, or can provide integration with an external sensor that uses the SAW device for encoding the sensor information and transmission to the receiver. SAW OFC device technology can provide RFID tags and sensors with low loss, large operating temperatures and a multi-use sensor platform. This paper will discuss the key parameters for OFC device design, which include reflector and transducer design, coding diversity approaches, and insertion loss considerations. Examples of several OFC device sensors and RFID tags will be presented to show the current state-of-the-art performance for several NASA applications, as well as projections for future sensor and RFID tag platform performance.I.
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