Daniel R. Gallagher

Orthogonal frequency coding for SAW device applications

This paper presents the concept of orthogonal frequency coding (OFC) for applications to SAW device technology. OFC is the use of orthogonal frequencies to encode a signal. which spreads the signal bandwidth in a manner similar to a fixed M-ary frequency shift signal. Also, a pseudo noise (PN) sequence can be added for additional coding. The OFC technique provides a wide bandwidth spread spectrum signal with all the inherent advantages obtained from the time-bandwidth product increase over the data bandwidth. The theory of OFC is presented and discussed, defining the fundamental equations and showing the time and frequency domain relationships. The application of OFC to SAW devices for tagging is introduced.

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.

SAW sensors using orthogonal frequency coding

This paper presents a spread spectrum approach using orthogonal frequency coding (OFC) for encoding SAW sensors. The encoding technique is similar to M-ary FSK in terms of its implementation, where a transducer or reflector is built with the desired code. It is shown that the time ambiguity in the autocorrelation due to the OFC is significantly reduced as compared to a single frequency tag having the same code length. The OFC approach is general and could be applicable to many differing SAW sensors for temperature, pressure, liquids, gases, etc. Device embodiments are shown and a discussion is provided for device design considerations such as the number of chips used, chip length, transducer fractional band-width, and chosen piezoelectric material. Measured device results are presented and compared with COM model predictions to demonstrate performance. Devices are then used in computer simulations of multiple transceiver designs and the results 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.

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.

A Passive Wireless Multi-Sensor SAW Technology Device and System Perspectives

This paper will discuss a SAW passive, wireless multi-sensor system under development by our group for the past several years. The device focus is on orthogonal frequency coded (OFC) SAW sensors, which use both frequency diversity and pulse position reflectors to encode the device ID and will be briefly contrasted to other embodiments. A synchronous correlator transceiver is used for the hardware and post processing and correlation techniques of the received signal to extract the sensor information will be presented. Critical device and system parameters addressed include encoding, operational range, SAW device parameters, post-processing, and antenna-SAW device integration. A fully developed 915 MHz OFC SAW multi-sensor system is used to show experimental results. The system is based on a software radio approach that provides great flexibility for future enhancements and diverse sensor applications. Several different sensor types using the OFC SAW platform are shown.

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.

Orthogonal Frequency Coding for Use in Ultra Wide Band Communications and Correlators

This paper presents the development of surface acoustic wave (SAW) correlators using orthogonal frequency coding (OFC) for use in ultra-wideband (UWB) spread spectrum communication systems. The use of ultra-short pulses, producing very wide bandwidths and low spectral power density, are the widely accepted approach for UWB communication systems. This approach is simple and can be implemented with current DSP technologies. However, SAW devices have the capability of producing wide bandwidths and relatively high frequency operation. As an example, Brocato, et al., has recently shown the numerous advantages of utilizing SAW correlators in UWB communication transceivers at center frequencies up to 5.6 GHz and fractional bandwidths up to 50% (Brocato, 2004). Their approach was to use a PN coded SAW transducer which produces a PN code sequence on an RF carrier. Information is encoded by pulse-phase-modulation used to excite the SAW which can be amplified and then transmitted. Reception is achieved by correlation of the matched filter received response and base-band envelope detection to extract the pulse phase. This approach, using SAW correlators, eliminates many of the costly components that are needed in the IF block in the transmitter and receiver, and reduces much of the signal processing requirements. This paper introduces OFC SAW transducers which can be used in UWB communication transceivers. OFC and PN coding provides a means for UWB spreading of data. The use of OFC spectrally spreads a PN sequence beyond that of CDMA because of the increased bandwidth. The transceiver approach is still very similar to that of the CDMA approach 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 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 results show that OFC SAW devices can be used for UWB communication transceivers

Wireless SAW Sensor Temperature Extraction Precision

Surface acoustic wave (SAW) devices are suitable for use in harsh environments because of their ruggedness under radiation and extreme temperatures and ability to be configured as passive wireless sensors. This orthogonal frequency coded (OFC) sensor system consists of a transceiver that wirelessly interrogates the passive SAW sensor, which can be used in several types of environments. A modified radar equation is presented that can be used to estimate the signal power from the returned sensor signal as a function of practical parameters. The precision performance of the OFC passive wireless sensor, not previously investigated, is shown to be a function of signal-to-noise ratio as expected from an analog sensor; extracted temperature precision better than 0.001 °C is reported. Signal integration is an effective means to increase the precision and/or range of the wireless temperature extraction scheme when deciding if the OFC passive wireless sensing scheme will fit a particular application.

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.