A multifunctional strain sensor based on Surface Acoustic Wave (SAW) Orthogonal Frequency Coding (OFC) technology on a Langasite substrate has been investigated. Second order transmission matrix models have been developed and verified. A new parameterizable library of SAW components was created to automate the layout process. Using these new tools, a SAW strain sensor with OFC reflectors was designed, fabricated and tested. The Langasite coefficients of velocity for strain (γS = 1.699) and Temperature (γT = 2.562) were experimentally determined. The strain and temperature characterization of this strain sensor, along with the coefficients of velocity, have been used to demonstrate both the ability to sense strain and the capability for temperature compensation. The temperature-compensated SAW OFC strain sensor has been used to detect anomalous strain conditions that are indicators of fastener failures during structural health monitoring of aircraft panels with and without noise on a NASA fastener failure test stand. The changes in strain that are associated with single fastener failures were measured up to a distance of 80 cm between the sensor and the removed fastener. The SAW OFC strain sensor was demonstrated to act as an impact sensor with and without noise on the fastener failure test stand. The average measured signal to noise ratio (SNR) of 50, is comparable to the 29.1 SNR of an acoustic emission sensor. The simultaneous use of a high pass filter for impact detection, while a low pass filter is used for strain or fastener failure, demonstrates the multifunctional capabilities of the SAW OFC sensor to act as both as a fastener failure detector and as an impact detector.

SAW based sensors can offer wireless, passive operation in numerous environments and various device embodiments are employed for retrieval of the sensed data information. Single sensor systems can typically use a single carrier frequency and a simple device embodiment, since tagging is not required. In a multi-sensor environment, it is necessary to both identify the sensor and retrieve the sensed information. This dissertation presents the concept of orthogonal frequency coding (OFC) for applications to SAW sensor technology. 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 COM model predictions to demonstrate performance. Devices are then used in computer simulations of the proposed transceiver design and the results of an OFC sensor system are discussed.

Ultra-wideband (UWB) communication new technology with ability to share the FCC allocated frequency spectrum, large channel capacity and data rate, simple transceiver architecture and high performance in noisy environments. Such communication advantages have paved the way for emerging wireless technologies such as wireless high definition video streaming, wireless sensor networks and more. This thesis examines orthogonal frequency coded surface acoustic wave (SAW) correlators for use in advanced UWB communication systems. 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 device consists of a dispersive OFC transducer and a wideband 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. Experimental correlator results of an OFC SAW correlation filter are presented. The dispersive filter is 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. 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.

Passive sensors that vary its impedance per measured parameter may be used with surface acoustic wave (SAW) reflective transducers (SRT) for wireless acquisition of the measurand. The device is composed of two transducers, where one, which may be attached to an antenna, is used to launch the wave within the device substrate, and the other is where the sensor load is attached to. The latter is able to reflect the incident wave. How much power is reflected is determined by the attached sensor load. Amplitude variations as well as peak frequency variations of the SRT reflectivity response are explored in this thesis. SAW passive temperature sensors with an orthogonal frequency coded (OFC) time response were previously investigated and prove to be ideal for use in harsh environments. Each sensor is distinguishable from the other due to the OFC code embedded within its time response. However, this coding technique poses a difficulty in designing antennas for the sensor due to its inherently wide bandwidth, and capacitive, non-uniform input impedance. This work covers antenna design and testing for the 250MHz wireless temperature acquisition prototype with a 28% fractional bandwidth, and for the 912MHz system which has 10% fractional bandwidth. Apart from the tag, antennas for the transmitter and receiver were designed for 50 Ohm matching with the required bandwidth maintained. Wireless temperature acquisition runs for the 250MHz prototype were successfully performed and show good agreement with measurements made by a thermocouple. Since a transceiver for the 912MHz system is not complete, the performance of the antennas was gauged by observing the signal transmitted wirelessly by the SAW tag and by comparing this with the sensor time response measured directly by a vector network analyzer.

Ultra-wideband (UWB) communication technology offers inherent advantages such as the ability to coexist with previously allocated Federal Communications Commission (FCC) frequencies, simple transceiver architecture, and high performance in noisy environments. Spread spectrum techniques offer additional improvements beyond the conventional pulse-based UWB communications. This dissertation implements a multiple-access UWB communication system using a surface acoustic wave (SAW) correlator receiver with orthogonal frequency coding and software defined radio (SDR) base station transmitter. Orthogonal frequency coding (OFC) and pseudorandom noise (PN) coding provide a means for spreading of the UWB data. The use of orthogonal frequency coding (OFC) increases the correlator processing gain (PG) beyond that of code division multiple access (CDMA); providing added code diversity, improved pulse ambiguity, and superior performance in noisy environments. Use of SAW correlators reduces the complexity and power requirements of the receiver architecture by eliminating many of the components needed and reducing the signal processing and timing requirements necessary for digital matched filtering of the complex spreading signal. The OFC receiver correlator code sequence is hard-coded in the device due to the physical SAW implementation. The use of modern SDR forms a dynamic base station architecture which is able to programmatically generate a digitally modulated transmit signal. An embedded Xilinx Zynq ™ system on chip (SoC) technology was used to implement the SDR system; taking advantage of recent advances in digital-to-analog converter (DAC) sampling rates. SDR waveform samples are generated in baseband in-phase and quadrature (I & Q) pairs and upconverted to a 491.52 MHz operational frequency. The development of the OFC SAW correlator ultimately used in the receiver is presented along with a variety of advanced SAW correlator device embodiments. Each SAW correlator device was fabricated on lithium niobate (LiNbO3) with fractional bandwidths in excess of 20%. The SAW correlator device presented for use in system was implemented with a center frequency of 491.52 MHz; matching SDR transmit frequency. Parasitic electromagnetic feedthrough becomes problematic in the packaged SAW correlator after packaging and fixturing due to the wide bandwidths and high operational frequency. The techniques for reduction of parasitic feedthrough are discussed with before and after results showing approximately 10:1 improvement. Correlation and demodulation results are presented using the SAW correlator receiver under operation in an UWB communication system. Bipolar phase shift keying (BPSK) techniques demonstrate OFC modulation and demodulation for a test binary bit sequence. Matched OFC code reception is compared to a mismatched, or cross-correlated, sequence after correlation and demodulation. Finally, the signal-to-noise power ratio (SNR) performance results for the SAW correlator under corruption of a wideband noise source are presented.