N. Saldanha

Orthogonal frequency coding for SAW tagging and sensors

Surface acoustic wave (SAW)-based sensors can offer wireless, passive operation in numerous environments, and various device embodiments are used for retrieval of the sensed data information. Single sensor systems typically can use a single carrier frequency and a simple device embodiment because tagging is riot required. In a multisensor environment, it is necessary to both identify the sensor and retrieve the sensed information. This paper presents the concept of orthogonal frequency coding (OFC) for implications to SAW sensor technology. The OFC offers all advantages inherent to spread spectrum communications, including enhanced processing gain and lower interrogation power spectral density (PSD). It is shown that the time ambiguity in the OFC compressed pulse is significantly reduced as compared with a single frequency tag having the same code length, and additional coding can be added using a pseudo-noise (PN) sequence. The OFC approach is general and should be applicable to many differing SAW sensors for temperature, pressure, liquid, gases, etc. Device embodiments are shown, and a potential transceiver is described. Measured device results are presented and compared with coupling of modes (COM) model predictions to demonstrate performance. Devices then are used in computer simulations of the proposed transceiver design, and the results of an OFC sensor system are discussed.

Orthogonal Frequency Coded SAW Sensors for Aerospace SHM Applications

National Aeronautics and Space Administration (NASA) aeronautical programs require structural health monitoring (SHM) to ensure the safety of the crew and the vehicles. Future SHM sensors need to be small, lightweight, inexpensive, and wireless. Orthogonal frequency coded (OFC) surface acoustic wave (SAW) reflectors and transducers have been recently introduced for use in communication, as well as in sensor and radio-frequency identification (RFID) tag applications (Malocha , 2004, Puccio , 2004). The OFC SAW technology approach has been investigated by NASA for possible inclusion in ground, space flight, and space exploration sensor applications. In general, SAW technology has advantages over other potentially competitive technologies, because the devices can operate in ranges from cryogenic to furnace temperature. SAW devices can also be small, rugged, passive, wireless, and radiation hard and can operate with variable frequency and bandwidth. 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 multiuse sensor platform. This paper will discuss the key parameters for OFC device design, which includes reflector and transducer design, coding diversity approaches, and insertion loss considerations. Examples of several OFC device sensors and RFID tags are presented to show the current state-of-the-art performance for several NASA applications. Projections for future sensor and RFID tag platform performance are discussed, along with some of the current challenges and issues of the technology.

Pseudo-orthogonal frequency coded wireless SAW RFID temperature sensor tags

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.

Orthogonal frequency coded SAW sensors and RFID design principles

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.

SAW Parameters on Y-cut Langasite Structured Materials

This paper presents results and investigations of several new, man-made piezoelectric single crystal, Czochralski-grown substrate materials for surface acoustic waves (SAW) applications. These materials, langanite (LGN), langatate (LGT), Sr₃TaGa₃Si₂O₁₄ (STGS), Sr₃NbGa₃Si₂O₁₄ (SNGS), Ca₃TaGa₃Si₂O₁₄ (CTGS), and Ca₃NbGa₃Si₂O₁₄ (CNGS), have the same structure as langasite (LGS) and are of the same crystal class as quartz. These compounds are denser than quartz, resulting in lower phase velocities. They also have higher coupling. Unlike quartz and lithium niobate, there is no degradation of material properties below the material melting points resulting in the possibility of extreme high-temperature operation (> 1000degC). This paper gives a summary of extracted SAW material parameters for various propagation angles on Y-cut substrates of the six materials. Parameters included are electromechanical coupling, phase velocity, transducer capacitance, metal strip reflectivity, and temperature coefficient of frequency. Using previously published fundamental material constants, extracted parameters are compared with predictions for LGT and LGN. In addition, power flow angle and fractional frequency curvature data are reported for propagation angles on CTGS and CNGS Y-cut substrates that exhibit temperature compensation near room temperature. Detailed descriptions of the SAW parameter extraction techniques are given. A discussion of the results is provided, including a comparison of extracted parameters and an overview of possible SAW applications.

Orthogonal frequency coded filters for use in ultra-wideband communication systems

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 pseudo-noise (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.

Multiple access SAW sensors using orthogonal frequency coding

Orthogonal frequency coding (OFC) provides a novel method of coding SAW sensors for use in multi-sensor environments. The OFC approach is general and should be applicable to physical, chemical, and biological measurands. OFC sensors offer increased range due to enhanced processing gain and reduced power spectral density. In addition, a reduction in compressed pulse time ambiguity results in increased sensitivity when compared with single frequency signals of similar duration. Successful implementation of an OFC SAW sensor system requires optimization of parameters related to sensitivity and range for the measurand and environment of interest. Several device embodiments are shown, and a discussion of design parameters such as SAW coupling, strip reflectivity, attenuation, and temperature coefficient, and optimal materials for various applications are given. Experimental sensor data are shown and compared with coupling-of-modes (COM) model predictions

Spread spectrum orthogonal frequency coded SAW Tags and sensors using harmonic operation

Wideband SAW devices with Orthogonal Frequency Coding (OFC) are expected to be highly advantageous for various applications in NASAs exploration effort. Surface acoustic wave (SAW)-based sensors can offer wireless, passive operation in numerous environments, with various device embodiments used for retrieval of the sensed data information. Single sensor systems typically can use a single carrier frequency and a simple device embodiment because tagging is not required. In a multi-sensor environment, orthogonal frequency coding (OFC) permits the system to both identify the sensor and retrieve the sensed information discriminately. Previous research efforts have concentrated on a relatively lower operational frequency in the 250 MHz range due to available fabrication technology limitations. Using harmonic frequency operation, as shown in this paper, higher frequency devices are possible while continuing to work within the same limitations of contact lithography resolution. This paper presents a preliminary investigation of OFC SAW tags and sensors at a 900 MHz operational frequency range using conventional contact lithography techniques. The higher frequency devices are achieved using second harmonic operation. The objective of this effort is to increase the technology readiness level (TRL) of wideband OFC tags at higher frequencies. Measured device results are presented and compared with coupling of modes (COM) model predictions to demonstrate performance.

Investigations of new materials, CTGS and CNGS, for SAW applications

Recently, SAW material parameters have been reported for CNGS, CTGS, SNGS and STGS. 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. 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.

SAW reflectivity and resonator results for LGT and LGN

SAW properties are currently being investigated for the use of LGN and LGT materials. Current efforts are focused on the measurement of SAW reflectivity and resonator performance. This paper will present recent results of SAW reflectivity for various cuts and propagation directions on LGT and LGN. The measurement procedure and the parameter extraction method for the reflectivity will be discussed. Theoretical predictions of the reflectivity for aluminum electrodes, based on fundamental material parameters, will be compared to experimental measurements for various film thicknesses on differing cut angles and propagation directions. Recently fabricated SAW resonator results will be provided.