Mark W. Gallagher

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.

Mixed orthogonal frequency coded SAW RFID tags

Orthogonal frequency coded (OFC) SAW radio-frequency identification (RFID) tags are currently being explored as a multi-sensor platform because of their passive spread-spectrum operation, low loss, and resilience in harsh environments. Ongoing research continues to search for robust device embodiments that increase the number of identifiable codes, in the presence of intersymbol interference, while maintaining reasonable device lengths. This paper presents a technique that shortens the SAW response length while preserving code diversity and bandwidth by utilizing a multi-track SAW configuration. These new devices allow the time response of multiple OFC chips to overlap and yield a mixed-frequency chip having the sum of the chip bandwidths but shorter overall time response. The theoretical development is presented and examples are discussed for these new mixed orthogonal frequency coded (MOFC) SAW devices. Experimental results for MOFC sensors, fabricated on YZ-LiNbO3, with a 7% fractional bandwidth and five chip frequencies in three cells, provide a good contrast to similar OFC designs. Experimental results are presented for the simultaneous operation of eight wireless temperature sensors-four OFC and four MOFC-in a 915-MHz wireless correlator receiver system, highlighting the ability of these devices to operate in the same system.

Wireless wideband SAW sensor - antenna design

Wireless SAW RFID sensors offer several advantages over similar silicon technology that include passive operation, radiation hardness, and the ability to operate in extreme temperatures. Due to these unique material and device properties, NASA has shown considerable interest in passive, wireless SAW sensors for ground and space flight operations. Several embodiments of SAW sensors have been well established in literature, but often the limiting factor on device size and performance is the tag antenna. Therefore, in order to develop a unified sensor target, a discussion of design principles and tradeoffs for both the SAW device and antenna will be presented in this paper. Antennas designed and fabricated will be presented, including a simple disk monopole, a planar open-sleeve dipole, and an on-wafer meandered dipole, with a discussion on gain, size, and bandwidth. The evolution of the antenna design is toward smaller antennas that minimize tradeoffs. The eventual wafer level integration of the antenna allows the sensor application to exploit some of the known SAW substrate advantages and has application in high temperature or strain sensors. Example orthogonal frequency coded (OFC) temperature sensors on YZ-LiNbO3 for use at 250 and 915MHz will be used as device examples. By properly designing the combined antenna/SAW target matching, insertion loss can be minimized and range increased. Experimental results on the integration of several sensor targets will be presented. Work presented is a foundation for a realizable wireless, multi-sensor platform.

Coherent correlator multi-sensor receiver

The focus of this paper is to discuss theoretical and practical applications of a coherent correlator transceiver (CCT) system approach for SAW RFID sensors, with the vision toward low cost sensing. For high volume applications, it appears achievable to meet the ultimate system goals and costs with RF device integration. As with most technologies, the sensor device and system volumes will drive the cost down, and it seems reasonable to expect a similar product cycle as with SAW filters. Theoretical predictions of range, SNR, and correlation properties will be shown for differing system parameters, such as bandwidth, power, coherent integration, and ADC parameters. A comparison of SAW device resonant, CDMA and OFC type signal formats will be given and an approach for signal detection and extraction discussed. Recent measurement results for a 915 MHZ, 64 MHz bandwidth, CCT system using OFC SAW temperature sensors will be given and compared to predictions, which demonstrates the practicality and implementation of the system architecture.

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.

SAW sensor correlator system performance parameters

There has been little published data on spread spectrum SAW RFID correlator receiver performance, since most approaches published have used an FMCW system. The purpose of this paper is to discuss issues related specifically to correlator receiver performance parameters with respect to range, detection, and noise. The minimum detectable signal (MDS) at the ADC is used as the measure for prediction of the maximum range. The loop gain of the system, MDS, noise and processing gain bound the predicted achievable range for a correlator receiver. It will be shown that in a correlator receiver nano- to micro-joules of energy can obtain ranges greater than 50 meters, dependent on the key system parameters. A model is developed for prediction of the maximum range as a function of center frequency, output signal and power, MDS, synchronous interrogation, and loop gain. From the predictions, it appears feasible to have a sensor range of over 100 meters with modest interrogation energy in a pulsed correlator system.

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.

Surface Acoustic Wave Pulsed-Correlator Transceiver for Aerospace Applications

This paper will present current efforts on wireless passive surface acoustic wave (SAW) sensor transceiver development for aerospace applications. Our groups SAW sensor work has been sponsored by NASA for the past ten years in efforts to develop wireless sensing for ground base and space exploration in extreme environments. The devices are radiation hard and work from cryogenic to several hundred degrees centigrade with standard device processing. The focus of this paper will be on a synchronous, pulsed correlator transceiver for interrogation of SAW delay-line sensors. The 915-MHz system has a bandwidth of 15 MHz and uses a pulsed noise-generated signal of 1-\(\mu \) s duration. The design principles, system analysis, and receiver measurements are given. The prediction of signal-to-noise ratio versus range is shown for given system parameters. The system development leading to the first testing of SAW liquid level sensors at NASA KSC will be discussed.

Analysis of inter-sensor interference for wireless SAW sensors

This paper presents a theoretical development of the effects of inter-sensor interference applicable to surface acoustic wave (SAW) multi-sensor systems. An approach is presented that statistically estimates desired wireless sensor signal degradation due to interference from other sensors in the system. A simple theoretical model is developed to estimate the inter-sensor interference. Simulations are performed that model the overlap of SAW signals, with consideration given to single frequency, single chip sensors as well as OFC SAWsensors. The simulations predict the degradation of the desired signal to noise due to other sensor responses moving into the desired time window. Results presented in this paper show that the SNR degrades rapidly as undesired sensors move into the desired sensor time window. The model is applicable to a broad range of possible sensor embodiments.

Spread Spectrum Orthogonal Frequency Coded SAW Tags and Sensors Using Harmonic Operation

This paper presents a preliminary investigation of orthogonal frequency coding (OFC) surface acoustic wave tags and sensors at a 500- and 900-MHz operational frequency range using conventional contact lithography techniques. The higher frequency devices are achieved using second harmonic operation; eliminating the need for advanced lithography techniques. The objective of this effort is to increase the technology readiness level of wideband OFC tags at higher frequencies. Measured device results are presented and compared with coupling of modes model predictions to demonstrate performance. The fundamental and harmonic device response is compared for devices designed for operation in the 900-MHz frequency band. The experimental correlation against an ideal matched filter is also presented.