Switchability of a single-port SAW resonator using the electric Bragg bandgap

Surface acoustic wave filters play a crucial role in radio frequency communication systems. A joint work between Thales Research Technology and IEMN demonstrates that it is possible to tune these filters electrically by exploiting the electrical bandgap concept.

Surface acoustic wave filters play a crucial role in radio frequency communication systems. However, these devices do not currently allow voltage-controlled adjustment of their working frequency or bandwidth. Whatever the application, the ability to tune these filters in an efficient and deterministic way is a significant issue for the improvement of embedded RF signal processing modules.

To this end, the research activities presented in this APL paper aim at exploring the concept of electrical bandgap for surface acoustic wave filtering devices (see Patent). In particular, one of the classical components for RF communications is the single port resonator, a piezoelectric block with a central transducer (interdigitated combs) on its surface and mirrors on both sides (figure 1).  The assembly of several resonators forms bandpass filters, classical components present in cell phones.

Figure 1: Surface wave resonator, consisting of a central transducer (red) between two electrode-based mirrors, on a piezoelectric substrate. The electrodes of the mirrors are conventionally connected to ground and are here progressively open-circuited (from the electrodes closest to the central transducer in red to the outside)

In this APL paper, it was shown that changing the electrical connection of the mirror electrodes from “grounded” to “open circuit” from the electrodes closest to the central transducer to the electrodes furthest from it, allows the control of wave propagation by changing the frequency of the band gap of the mirrors, the band of frequencies in which the waves are reflected, depending on the electrical connection. Based on this observation, a resonator was designed with several operating points, at different frequencies (Figure 2)

Figure 2: Parameter S11 of the LiNbO3 resonator (transducer + two mirrors with 72 electrodes on each side) if the mirror electrodes are progressively set from the “grounded” condition to the “floating potential” condition. Resonances are marked by the bright color. They vary according to the number of electrodes in floating potential (NOC, the electrodes of the two mirrors are put in floating potential symmetrically).

In collaboration with project partner Thales Research Technology, device fabrication, characterization and testing have effectively shown a device agility of about 3% (Figure 3). This first result serves as a building block for the assembly of future agile components. Further studies will focus on the simplification of the electrical connection between the different states (grounding / open circuit) to consider the integration of these agile resonators in real devices.

Figure 3: S11 parameter of the LiNbO3 resonator (transducer + two mirrors with 72 electrodes on each side) if the mirror electrodes are partially put in floating potential condition (0 / 10 / 30 / 50 and 72). Left: images of the fabricated resonators, right: measured (blue) and simulated (green) responses.

The research was carried out in the framework of the ANR Astrid Maturation FORMOSA (MicrO-fabricated Radiofrequency phOnonic Filters) 2019-2022.

Patent : ” SURFACE ACOUSTICAL WAVE DEVICE “, 02/07/2020 (IEMN, Thales RT and Frec’N’Sys)

Papier :
Switchability of a single port SAW resonator using the electrical Bragg band gap
Appl. Phys. Lett. 120, 203504 (2022); https://doi.org/10.1063/5.0093357