Nikolai Kozlovski

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.

SAW RFID spread spectrum OFC and TDM technology

SAW based RFID sensors can offer wireless, passive operation over harsh environments, and various device embodiments are used for retrieval of the sensed data. SAW has many unique advantages over possible competing technologies, including the following characteristics: passive, radiation hard, operable over wide temperature ranges, small, rugged, inexpensive, and identifiable. In a multi-sensory environment, it is necessary both to identify the sensor and to retrieve the sensed information. The OFC SAW technology approach has been funded by NASA for possible inclusion in ground, space flight, and space exploration sensor applications. The purpose of this paper is to present the concept of encoding SAW-based sensors for use in a multiple-sensor environment. The emphasis will be on orthogonal frequency coded (OFC) devices overlaid with a form of time division multiplexing (TDM). It will be shown that in addition to the benefits of OFC such as enhanced processing gain and lower interrogation power spectral density (PSD), the TDM allows for a decrease in overlapping energy and therefore a decrease in intersensory collisions which are shown to cause peak ambiguity. This approach should be applicable to many different SAW-based sensors (such as temperature, pressure, liquid, gas, etc.), as well as allow for proper IDing. Measured device results are presented and compared with coupling of modes (COM) model predictions to demonstrate performance. This paper will discuss the use of SAW OFC in a temperature sensor application. Devices are then used in computer-simulated transceiver design, and the results of a prototype sensor system are discussed.

SAW passive wireless multi sensor system

Previous work has been presented on passive wireless SAW sensors using the orthogonal frequency coding (OFC) technique [1]. Wireless operation of such sensors has been demonstrated using a network analyzer and only one sensor at a time. A technique to allow multiple sensors to operate at the same time has been developed and demonstrated using network analyzer measurements that were added together to simulate simultaneous operation within the system. NASA is sponsoring research and development of SAW wireless, passive sensors that are environmentally robust and operate in a multi-sensor environment.

Ultra wide band surface acoustic wave (SAW) RF ID tag and sensor

A new and novel spread spectrum approach employing orthogonal frequency coding (OFC) for a SAW sensor/tag is presented. This approach provides ultra wide band operation, multi-device interrogation, and reduced loss as compared to conventional SAW CDMA approaches. A traditional SAW CDMA tag consists of an input transducer that launches a surface wave on the substrate towards an array of single frequency collinear reflectors that reflect the wave back to the input after providing ID coding within the reflections. The OFC embodiment uses an appropriate wideband input transducer that launches a SAW towards a collinear multi-frequency reflector bank which encodes the ID information. The sensor information is encoded by interaction of the SAW with delay path changes due to temperature and other measurands. This novel OFC SAW approach enables multiple coded chips to have high reflectivity since the orthogonal frequency chips appear nearly transparent to others; yielding low reflector loss, minimal inter-chip distortion and both phase and frequency encoding.

A 915 MHz SAW Sensor Correlator System

Previous work has been presented on passive wireless surface-acoustic-wave (SAW) sensors using the orthogonal frequency coding (OFC) technique (Malocha et ah, Proc. Int. Soc. Opt. Eng. (SPIE), Aug. 23-27,2004, vol. 2, pp. 1082-1085), Pavlina et al., Proc. IEEE Int. Conf. RFID, 2009, pp. 110-115). This paper will present a SAW sensor correlator system and its key operational parameters. Description of the radio frequency (RF) transceiver system and the OFC SAW temperature sensors is provided for parameter characterization. The system is based on a software radio approach and an analysis of the analog-to-digital converter (ADC) parameters and receiver noise in temperature extraction is presented. Initial experimental results for a four-sensor system operating over a 280°C range are given.

Ultra wide band communication systems using orthogonal frequency coded SAW correlators

This paper examines ultra-wideband (UWB) communication systems utilizing orthogonal frequency coded surface acoustic wave (SAW) correlators. Orthogonal frequency coding (OFC) and pseudo-noise (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; allowing for improved correlation gain. The transceiver approach is still very similar to that of the CDMA approach but provides greater code diversity. Use of 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. The OFC SAW correlator consists of a dispersive OFC transducer and an apodized output transducer. The dispersive filter was designed using seven contiguous chip frequencies within the transducer. Each chip is weighted in the transducer to account for the varying conductance of the chips and to compensate for the output transducer apodization. Since each chip in the OFC transducer has a different local carrier frequency, inter-electrode reflections are greatly reduced compared to a single carrier frequency. In general, this allows quarter-wavelength electrodes to be used in narrow bandwidth correlator designs; however, for bandwidths above 25%, as in the ultra-wideband case, sixth-wavelength electrodes are used to eliminate bulk mode conversion effects within the transducer. Experimental correlator results of an OFC SAW correlation filter are presented. The dispersive filter was designed using seven contiguous chip frequencies within the transducer. SAW correlators with fractional bandwidth of approximately 29% were fabricated on lithium niobate (LiNbO3) having a center frequency of 250 MHz and the filter has a processing gain of 49. A coupling of modes (COM) model is used to predict the experimental SAW filter response. Good correlation between the predicted COM responses and the measured device data is obtained and presented. Discussion of the filter design, analysis and measurements are presented. Results are shown for operation in a matched filter correlator for use in an UWB communication system and compared to predictions, showing good results. The results demonstrate that OFC SAW devices can be used for UWB communication transceivers.

SAW wireless, passive sensor spread spectrum platforms

SAW technology has been identified as a possible solution for NASAs long term needs for ground, space-flight, and space-exploration sensor requirements. SAW has many unique advantages over possible competing technologies, which include the following properties: passive, radiation hard, operable over wide temperature ranges, small, rugged, inexpensive, and identifiable. The purpose of this paper is a focus on the platform and system constraints; not on any particular sensor. For remote sensing, the device and system platform is the essential element which enables the sensor information to be obtained. This papers emphasis will be on orthogonal frequency coded (OFC), and single frequency CDMA sensor tags. The parameters of interest are device insertion loss, system range, code diversity, center frequency and bandwidth. It will be shown that multi-frequency and OFC tags offer the possibility of much greater ranges and better code collision properties than the more conventional commercial SAW CDMA RFID tags. Reflectors can have less than 1 dB insertion loss, and when using unidirectional transducers, overall SAW device loss can be less than a few dB. Antenna size and gain are key parameters for small foot print and achievable range. By using spread spectrum techniques, processing gain can be encoded in the reflectors, the transducer, or both. The paper will present the fundamental engineering equations which define the system range, receiver dynamic range, minimum detectable signal and processing gain. Theoretical system performance of both the multi-frequency and single frequency SAW RFID sensor will be discussed.

Ultra-wideband communication system prototype using orthogonal frequency coded SAW correlators

This paper presents preliminary ultra-wideband (UWB) communication system results utilizing orthogonal frequency coded SAW correlators. Orthogonal frequency coding (OFC) and pseudo-noise (PN) coding provides a means for spread-spectrum UWB. The use of OFC spectrally spreads a PN sequence beyond that of CDMA; allowing for improved correlation gain. The transceiver approach is still very similar to that of the CDMA approach, but provides greater code diversity. Use of SAW correlators eliminates many of the costly components that are typically needed in the intermediate frequency (IF) section in the transmitter and receiver, and greatly reduces the signal processing requirements. Development and results of an experimental prototype system with center frequency of 250 MHz are presented. The prototype system is configured using modular RF components and benchtop pulse generator and frequency source. The SAW correlation filters used in the test setup were designed using 7 chip frequencies within the transducer. The fractional bandwidth of approximately 29% was implemented to exceed the defined UWB specification. Discussion of the filter design and results are presented and are compared with packaged device measurements. A prototype UWB system using OFC SAW correlators is demonstrated in wired and wireless configurations. OFC-coded SAW filters are used for generation of a transmitted spreadspectrum UWB and matched filter correlated reception. Autocorrelation and cross-correlation system outputs are compared. The results demonstrate the feasibility of UWB SAW correlators for use in UWB communication transceivers.