Design, Analysis And Implementation Of Orthogonal Frequency Coding In Saw Devices Used For Spread Spectrum Tags And Sensors

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

Weighted SAW reflector gratings for orthogonal frequency coded SAW tags and sensors

Weighted surface acoustic wave reflector gratings for coding identification tags and sensors to enable unique sensor operation and identification for a multi-sensor environment. In an embodiment, the weighted reflectors are variable while in another embodiment the reflector gratings are apodized. The weighting technique allows the designer to decrease reflectively and allows for more chips to be implemented in a device and, consequently, more coding diversity. As a result, more tags and sensors can be implemented using a given bandwidth when compared with uniform reflectors. Use of weighted reflector gratings with OFC makes various phase shifting schemes possible, such as in-phase and quadrature implementations of coded waveforms resulting in reduced device size and increased coding

Delayed Offset Multi-track OFC Sensors and Tags

Apparatus, systems, devices and methods for providing an orthognal frequency coding technique for surfaced acoustic wave sensors incorporating the use of multiple parallel acoustic tracks to provide increased coding by phase shifting and delaying a code sequence. The surface acoustic wave sensor includes parallel tracks with multiple reflectors with differing delay offsets to form a complex code sequence. The reflectors may be uniform, but alternatively could include fingers withdrawen, have reflector position modulation, differing frequencies or be spatially weighted

Orthogonal Frequency Coding for Surface Acoustic Wave Communication, Tag and Sensor Applications

A secure apparatus, system, device and method for coding surface acoustic wave identification tags and sensors to enable unique sensor operation and identification for a multi-sensor environment. In an embodiment, a pseudo noise sequence is applied to the orthogonally coded signal for increased security. An orthogonal frequency coding technique is applied to the SAW tag using periodic reflector gratings for responding to an orthogonal interrogation signal to transmit the sensor identification and sensed data. A transceiver interrogates the sensor with a stepped up chirp corresponding to the orthogonal frequency coded chip frequency response, receives a response from the SAW device, applies an oppositely stepped chirp to the response and then uses matched filtering to produce a compressed pulse. The orthogonal frequency coding technique has an inherent advantage of processing gain, code division multiple access, spread spectrum and security

Weighted SAW Reflector Gratings for Orthogonal Frequency Coded SAW ID Tags and Sensors DIV

Weighted surface acoustic wave reflector gratings for coding identification tags and sensors enable unique sensor operation and identification for a multi-sensor environment. In an embodiment, the weighted reflectors are variable while in another embodiment the reflector gratings are apodized. The weighting technique allows the designer to decrease reflectively and allows for more chips to be implemented in a device and, consequently, more coding diversity. As a result, more tags and sensors can be implemented using a given bandwidth when compared to uniform reflectors. Use of weighted reflector gratings with OFC makes various phase shifting schemes possible, such as in-phase and quadrature implementations of coded waveforms resulting in reduced device size and increased coding

Weighted SAW Reflector Gratings for Orthogonal Frequency Coded SAW ID Tags and Sensors

Weighted surface acoustic wave reflector gratings for coding identification tags and sensors to enable unique sensor operation and identification for a multi-sensor environment. In an embodiment, the weighted reflectors are variable while in another embodiment the reflector gratings are apodized. The weighting technique allows the designer to decrease reflectivity and allows for more chips to be implemented in a device and, consequently, more coding diversity. As a result, more tags and sensors can be implemented using a given bandwidth when compared with uniform reflectors. Use of weighted reflector gratings with OFC makes various phase shifting schemes possible, such as in-phase and quadrature implementations of coded waveforms resulting in reduced device size and increased coding

Implementation Of Orthogonal Frequency Coded Saw Devices Using Apodized Reflectors

Recently, orthogonal frequency coding (OFC) has been presented as a novel method for coding SAW tags and sensors [1]. Orthogonal frequency coding is a spread spectrum technique and has been shown to provide enhanced processing gain and reduced time ambiguity resulting in greater range and increased sensitivity when compared with single carrier frequency devices. The sensor works both as a tag and a sensor with the ability to send back tagged sensor information in a multi-sensor environment. The tag information is provided by a series of reflectors which map into a known chip sequence. The time-chip- sequence is coded by differing OFC and PN sequences. Therefore, the implementation of an OFC sensor requires reflectors having differing local carrier frequencies. In the case of narrow fractional bandwidths or high reflectivity (such as on LiNbO3), it is desirable to adjust the reflectivity per electrode in the various chips. For varying system requirements, the use of weighted reflectors is an option; both apodization and variable weighted reflectors are investigated. Experimental results on cosine weighted apodized reflectors will be compared to COM predictions on YZ LiNbO3. This paper presents several OFC SAW device embodiments that are employed using wideband input transducers and multiple weighted reflector gratings. The devices operate in the differential mode using gratings on either side of the transducer. The advantages of using the weighted reflector gratings in OFC SAW devices are discussed. OFC temperature sensor device experimental results are presented using weighted reflectors to obtain the proper time coded response and compared to predictions. © 2005 IEEE

Lgx Pure Shear Horizontal Saw For Liquid Sensor Applications

This paper reports predicted and measured properties of the pure shear horizontal (SH) mode for the LGX family of crystals, which includes langasite (LGS), langanite (LGN), and langatate (LGT). These crystals are of the trigonal class 32 group, as quartz, and they exhibit the SH symmetry type uncoupling for the Euler angles (0°, θ, 90°). This surface acoustic mode, also know as surface transverse wave (STW), is especially attractive for liquid sensing due to the moderate damping observed in liquid or viscous environments. The numerical and experimental propagation data presented for the SH mode on LGX (0°, θ, 90°) includes phase velocity (vp), electromechanical coupling coefficient (K2), temperature coefficient of delay (TCD), fractional change in frequency with respect to temperature (Δf/fo), penetration depth, metal strip reflectivity, and excitation of spurious plate modes as a function of θ. High electromechanical coupling and zero temperature coefficient of delay (TCD) along LGX Euler angles (0°, θ, 90°), 9 between 10° and 25°, with penetration depths comparable to surface acoustic wave (SAW) devices are disclosed. In particular, along LGT (0°, 13.5°, 90°), the experimental results reported with resonators and delay line structures verify the high electro-mechanical coupling (0.8%) for a SH SAW mode, about 10 times stronger than the 36° Y rotated quartz SH orientation, and the existence of zero TCD around 140°C. The phase velocity of 2660 m/s is within 0.2% of the calculated value, which is about 55% below the phase velocity of 36°Y quartz, thus leading to smaller STW devices. The penetration depth of 6.5 wavelengths is eight times more shallow than 36°Y quartz, thus providing significant SH mode energy trapping close to the surface. With such positive predicted and measured coupling and propagation characteristics, these orientations are appropriate for the fabrication of high coupling, zero TCD, smaller, and highly sensitive STW devices for filtering, frequency control, and liquid sensor 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. © 2006 IEEE

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), Ca 3TaGa3Si2O14 (CTGS), and Ca 3NbGa3Si2O14 (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 (\u3e 1000°C). 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. © 2007 IEEE