D. Puccio

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 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.

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), Ca₃TaGa₃Si₂O₁₄ (CTGS), and Ca₃NbGa₃Si₂O₁₄ (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 (> 1000degC). 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.

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.

Multiple access SAW sensors using orthogonal frequency coding

Orthogonal frequency coding (OFC) provides a novel method of coding SAW sensors for use in multi-sensor environments. The OFC approach is general and should be applicable to physical, chemical, and biological measurands. OFC sensors offer increased range due to enhanced processing gain and reduced power spectral density. In addition, a reduction in compressed pulse time ambiguity results in increased sensitivity when compared with single frequency signals of similar duration. Successful implementation of an OFC SAW sensor system requires optimization of parameters related to sensitivity and range for the measurand and environment of interest. Several device embodiments are shown, and a discussion of design parameters such as SAW coupling, strip reflectivity, attenuation, and temperature coefficient, and optimal materials for various applications are given. Experimental sensor data are shown and compared with coupling-of-modes (COM) model predictions

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/spl deg/, /spl theta/, 90/spl deg/). This surface acoustic mode, also known as surface transverse wave (STW), is especially attractive for liquid sensing due to the moderate damping observed in liquid or viscous environments. Numerical and experimental propagation data presented for the SH mode on LGX (0/spl deg/, /spl theta/, 90/spl deg/) includes phase velocity (v/sub p/), electromechanical coupling coefficient (K/sup 2/), temperature coefficient of delay (TCD), fractional change in frequency with respect to temperature (/spl Delta/f/fo), penetration depth, metal strip reflectivity, and excitation of spurious plate modes as a function of /spl theta/. High electromechanical coupling and zero temperature coefficient of delay (TCD) along LGX Euler angles (0/spl deg/, /spl theta/, 90/spl deg/), /spl theta/ between 10/spl deg/ and 25/spl deg/, with penetration depths comparable to surface acoustic wave (SAW) devices are disclosed. In particular, along LGT (0/spl deg/, 13.5/spl deg/, 90/spl deg/), the experimental results reported with resonators and delay line structures verify the high electromechanical coupling (0.8%) for a SH SAW mode, about 10 times stronger than the 36/spl deg/ Y rotated quartz SH orientation, and the existence of zero TCD around 140/spl deg/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/spl deg/ Y quartz, thus leading to smaller STW devices. The penetration depth of 6.5 wavelengths is eight times more shallow than 36/spl deg/ 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.

Investigations of STGS, SNGS, CTGS, & CNGS materials for use in saw applications

Langasite structure compounds have been the focus of much consideration recently, given their higher electromechanical coupling and similar temperature behavior to that of quartz. Recent investigations have begun on four new materials, STGS, SNGS, CTGS, and CNGS. These langasite structure crystals show great promise for use in SAW applications. In some respects, STGS, SNGS, CTGS, and CNGS exhibit better SAW material properties than the LGX materials (langasite, langanite, and langatate) previously studied. For example, SNGS and STGS have higher electromechanical coupling than LGX, in excess of 0.6% and 0.5%, respectively. In addition, all four materials have lower densities resulting in higher phase velocities. Along with previously studied trigonal 32 class materials, there is a good possibility that temperature compensated cuts exist. Recent results are reported for SAW material parameters and device performance at various propagation directions on Y-cut STGS, SNGS, CTGS, and CNGS wafers. These results include coupling coefficient, phase velocity, transducer capacitance, and temperature coefficient of delay (TCD). Experimental measurements of metal strip reflectivity are shown for various metal thicknesses. Measured SAW resonator Q data are also presented. The measurement procedures and the parameter extraction methods for these experiments are discussed. A comparison of the data with previously reported parameters on LGX is presented and a discussion of the results is provided.

Investigations of langanite and langatate materials for use in SAW device applications

This paper will present recent results and investigations of langanite (LGN) and langatate (LGT) materials for use in SAW device applications. These substrates offer new possibilities for device applications given their slower phase velocities, higher coupling and high temperature operation without degradation of the piezoelectric properties. This paper will present recent results on the extraction of SAW material parameters and device performance for various cut angles. The SAW parameter extraction approach used for these experiments is summarized. Measured resonator Q of the order of 4000 has been obtained with the currently designed devices on Y-cut, showing promising behavior for these materials and orientations. Delay line data is used to extract the phase velocity, coupling coefficient and transducer capacitance. The work discusses the correlation between measured phase velocity, electromechanical coupling coefficient and IDT capacitance, and the predicted parameter performance. Measured SAW resonator temperature coefficient of delay (TCD) data will be presented and compared to expected values for the selected cut angles and propagation directions. The results reported are obtained from LGN and LGT materials from Crystal Photonics, Inc., (CPI) which provides boules that are typically clear, transparent and free of visible defects.