Errors of phases and group delays in SAW RFID tags with phase modulation.

To achieve high-volume code capacity for SAW-based radio frequency identification(RFID) tags, it is very important to improve the delay time resolution. An efficient encoding method is to use the phase delay of the carrier wave in the pulses, but one has to solve the issues of the phase ambiguity at unknown temperatures and the location of reflectors to exact positions. In this paper, a method is proposed to obtain a high-phase delay resolution by measuring group delays and constructing a certain restriction on the exact positions of reflectors. To define the restriction parameter for a SAW RFID system with large code capacity, it is imperative to have a priori knowledge of the errors of the phases and group delays. The experimental and simulation errors for both the phases and the group delays, originated from the design procedure, the temperature effect, the fabrication process and the measurement, are presented. The error probability distribution curves in simulation and experiments are plotted. The maximum error of phase delay is about +/- 14 degrees, and the maximum error of group delay is about +/- 4 periods. The temperature range in investigation is from -5 to 45 degrees C.

Reflection and scattering characteristics of reflectors in SAW tags.

As a foundation of an optimal design for SAW tags,the reflection, scattering, and transmission of the reflector composed of a few electrodes are discussed. A source regeneration method based on Green's function and the finite element method/boundary element method is used to obtain the reflection coefficient and the scattering coefficient for the short-circuited and open circuit reflectors. Examples are presented to show how one can use structure variance to satisfy the requirements for SAW tag design according to the reflection and scattering characteristics.

Rayleigh wave reflection and scattering calculation by source regeneration method.

The analysis and precise calculation of reflection and scattering of Rayleigh wave by electrodes is important for surface acoustic wave (SAW) device models, especially for SAW identification (ID) tag design. We present a source regeneration method, which utilizes Green's function combined with finite-element method (FEM)/boundary-element method (BEM) to obtain accurate values of reflection, transmission, and scattering coefficients in a direct way. We take one electrode as the reflector on 128 degrees YX-LiNbO3 substrate to show the result as an example. The results are very accurate and are obtained with short computing time. The new analysis way shows its powerful ability for other advanced discussion of SAW devices.

Minimizing the bulk-wave scattering loss in dual-mode SAW devices.

The losses arising from the scattering of SAW into bulk waves in the nonsynchronous areas of SAW devices are studied numerically using the boundary element method combined with the finite element method. As a reference structure, we use a typical one-port hiccup resonator on 42 degrees Y-LiTaO3. Strong scattering into bulk wave occurs in the central gap due to an abrupt change in periodicity. To reduce the scattering, we replace the gap with electrodes having reduced pitches. We show that it is possible to significantly increase the Q-factor of the resonator while keeping the resonant frequency unchanged. Two types of structures are studied: the "distributed" gap and the "accordion" gap. To minimize the bulk-wave scattering in dual-mode SAW filters, we replace the metallized gaps in the traditional filter with distributed gaps. We find an optimal combination of pitch and metallization ratio in the gaps, reducing the insertion loss by 0.3 dB.

Optimization of single-phase, unidirectional transducers using three fingers per period.

Electrode width controlled (EWC) single-phase, unidirectional transducers (SPUDT) is widely used for low loss surface acoustic wave (SAW) filters. The insertion loss and fractional bandwidth of the filters are strongly related to the reflectivity of EWC cells. In order to achieve wide band and low loss simultaneously, it is necessary to obtain higher reflectivity. The relationship between geometrical configuration of EWC cells and reflection coefficient, (and transduction coefficient as well) is studied. Simulation results indicate that the reflectivity of the EWC SPUDT cell could exceed 5% on a 128 degrees Y-X lithium niobate (LiNbO3) substrate. Using such structure, low loss SPUDT test filters without weighting are fabricated. The measured 3 dB bandwidth is 3.9% and the insertion loss is 2.9 dB. The theoretical calculation is verified by the experiment.

Fast, precise, and full extraction of the COM parameters for multielectrode-type gratings by periodic Green's function method.

It is very important to extract all four coupling-of-modes (COM) parameters of the electrode cells for the simulation and optimal design of a low loss surface acoustic wave (SAW) filter. A new approach for fast and full extraction of the COM parameters for multielectrode-type grating is proposed. The field distribution of the wave under the periodic shorted grating is calculated by periodic Green's function method. The phase of the reflection is determined from the positions of the standing wave node. The transduction coefficient and its phase are determined by the charge distribution at low frequency. The COM parameters of the commonly used electrode width controlled (EWC) single phase unidirectional transducer (SPUDT) is computed. It shows that this is a simple and direct way to extract all the COM parameters for SPUDT and, accordingly, is a powerful tool for the optimization of the filter structure.