Patrick Turner

Multimode bandpass SAW filter using reconfigurable resonance technology

The trend to include more frequency bands in mobile phones has so far been addressed by adding more fixed frequency Surface Acoustic Wave (SAW) filters and switching the RF path to accommodate each band. As more bands are needed, a reconfigurable SAW filter is of great interest if performance is not compromised. Here we present the results of a new approach to designing multiband filters. The SAW resonators remain common to all the modes of the device, and non-resonant elements, (inductors and capacitors) are adjusted to reconfigure the passbands. A two-mode proof of concept is presented that uses the same fixed SAW resonators and demonstrates a six percent (6%) change in passband frequencies.

Hierarchical cascading in 2D FEM simulation of finite SAW devices with periodic block structure

Application of finite element methods (FEM) to the simulation of SAW devices has been constrained by the large number of degrees-of-freedom required, resulting in large memory usage and long computation times. Here, we propose a hierarchical cascading approach which takes advantage of the periodic structure typical of SAW devices. In this approach, each unique electrode period is modeled and simulated only once. Smaller blocks are cascaded to describe larger blocks until the whole device is modeled. This is equivalent to full simulation of the device with FEM, however - for structures with high degree of periodicity - with drastically reduced memory consumption and simulation time.

Interaction between the Rayleigh-type SAW and the SH-wave in a periodic grating on a 128°-LN substrate

Ladder-type filters employing SAW resonators demand high Q-factors at both the resonance and anti-resonance frequencies. Also, spurious-free, low-loss response is especially important for novel filters having multiple passband modes in which one or more of the passbands is far from the resonant frequency of a SAW device. 128° LiNbO3 is traditionally considered as a substrate in which SH-SAW modes are very weakly coupled to transducer electric fields and thus it may provide a good avenue for improving SAW resonator loss performance. We have experimentally studied a large number of synchronous SAW resonators on 128° LiNbO3 substrate with different aluminum relative thickness (h/ λ) and resonant frequencies near 900MHz. All theoretical modeling of the resonators was performed using the FEM/BEM simulation tool FEMSAW. In almost all measured devices and FEMSAW simulations we observed an interaction of the SAW with another acoustic mode. However, the very weak coupling of the IDT to the SH-SAW mode does not explain the data. The observed interaction is likely to be due to mechanical scattering of the strongly excited Rayleigh-type SAW into a shear SAW.

Lamb plate modes and surface acoustic wave resonator microwave filters

For sufficiently thin piezoelectric (PZ) wafers, Lamb wave plate modes can interfere with the response of the SAW resonator by introducing spurious modes in the vicinity of the resonant and anti-resonant frequencies of the IDT. We present data taken on individual SAW resonators constructed on thin wafers of LiTaO3 bonded to silicon. Finite element methods are employed to simulate the effects of bulk acoustic excitations. From these simulations, a model is developed to account for these plate modes and their effect on a SAW resonator. Finally, a Band 3 (uplink frequencies 1710-1785 MHz, downlink frequencies 1805-1880 MHz) duplexer was constructed, and good agreement is found between the measured filter response and the simulation incorporating plate modes.

Acoustic radiation from ends of IDT in synchronous LSAW resonators

In order to minimize losses due to bulk-acoustic radiation, ‘leaky’ SAW resonators on rotated Y-cut LiTaO3 typically employ synchronous design, with all the electrodes in the IDT and in the reflectors sharing the same periodicity and electrode shape. The change in the electric connection pattern between the IDT and the reflectors creates an electric discontinuity, which can serve as an additional source of bulk wave radiation. In this work, numerical FEM simulations are employed to characterize this additional loss mechanism.

COMSOL modeling of SAW resonators

Here we demonstrate some generic 2D and 3D routines for SAW analysis employing the commercial COMSOL Multiphysics platform for finite element analysis (FEA). More specifically, we consider the analysis and optimization of high performance LSAW resonators for RF filters. The LSAW nature - being well studied and sufficiently complicated - is chosen as a suitable example for analysis and the results are compared to state-of-the-art knowledge. We found very good agreement between the results of the analytical scheme proposed here and the state-of-art findings. Finally, we demonstrate a TC-SAW piston-mode device simulated using SiO2/128°Y-X LiNbO3.

FEM modeling of an entire 5-IDT CRF/DMS filter

A CRF/DMS filter is simulated using “Layers” software [1-2]. We analyze a few different CRF/DMS structures, including the 10-port CRF device with passband frequency close to 1842.5 MHz including 5 IDTs, 8 “gap IDTs” and 2 reflectors. This TC SAW structure has a thick SiO2 over layer covering 274 Cu electrodes on a 128° LiNbO3 substrate. In addition to the electrical network parameters for the filter, a visualization of acoustic field and power flow is presented. The simulation reveals the resonances in the structure, radiation of energy at the interfaces between IDTs, and helps to see the origin of notches in passband including parasitic acoustic modes. We simulate 801 frequency points and present acoustic fields, power flows, and generated bulk waves inside the entire device for select frequencies of interest. On a PC with 32 processors and 128 GB RAM, a complicated device having 15 “building blocks” (274 electrodes) was simulated, with about 6 second simulation time per frequency point (∼1.3 hours total time). Losses due to bulk wave generation are numerically estimated.