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Circuit Theme

by Olivier Pigaglio - published on , updated on

One of the group main topics deals with high frequency circuits design especially concerning the tunability and the use of metamaterials as shown in the following.

Tunable filters

Three main concerns are essential when dealing with circuit design and especially the design of reconfigurable filters:

  • Topology: find the topology with the best electrical performance in terms of losses and selectivity while keeping a high sensitivity for frequency tuning without any significant loss in the electrical response.
  • Synthesis: determining the synthesis of nominal function taking the frequency tuning element into account.
  • Technology: implement appropriate technologies allowing both to achieve good filter performance and an efficient tuning element.

In this context, the GRE team has developed a number of innovative topologies, promoting pseudo-elliptic solutions, which allow a more flexible frequency tuning, either in center frequency, frequency rejection or bandwidth. Among these topologies, ring resonators filters have shown are very promising potential. By integrating a dual tuning (working frequency and bandwidth), the electrical response was tuned beyond 40%, while maintaining the quality of the electric response (losses, matching). In addition, cascading ring resonators allows to develop multi-band solutions whose center frequencies can be controlled. Those topologies and the associated synthesis development are carried out through a collaboration with the University of Mara (Malaysia).

Filtre bi-mode pseudo-elliptique pour application spatiale

Dual mode pseudo-elliptic filter for space application

The implementation of tunable functions in the sub-millimeter band is a major challenge. In addition to the function control ensured by the synthesis, it is crucial to precisely control the technological process to meet the high requirements at sub-millimeter frequencies. These aspects, studied in collaboration with IEMN but also with LAAS, let to suggest an innovative resonator topology which allows a frequency tuning from the W-band (94 GHz) to the V band (60GHz). The proposed solution uses a MEMS whose geometry has been specifically developed for this frequency range. The technological implementation required the development and the implementation of a specific process, for the nominal passive function on the one hand and on the other hand for the MEMS. The figures below show the realization and the associated result for three configurations.

Filtre reconfigurable à MEMS en bandes V et W

MEMS reconfigurable filter in V and W bands

Still looking for tunable filters, the group studied technological breakthroughs, consisting in modifying a conventional approach in order to develop new microwave tunable solutions. Within this breakthroughs, tunable circuits based on a microfluidic technology were developed by the group in the ANR RF-IDROFIL project carried out by the Lab-STICC in Brest and in collaboration with the LAAS. The developped circuits were resonators, planar filters and SIW filters, whose working frequencies were changed when injecting dielectric liquids and / or conductor (GALINSTAN) in microfluidic channels. The use of these techniques is also interesting for bio-sensor applications because the frequency response provides an efficient way to characterize the liquid injected in the microfluidic channels. The figures below show a schematic view of the device, as well as a picture ans associated measurements.


Use of metamaterials

Between 2007 and 2010, the group studied (at the request of ONERA with the thesis of Nicolas Capet) opportunities to strengthen the decoupling between adjacent patchs antennas within a large network. The GRE was then able to show very substantial effects by exploiting mushrooms resonators dual ultra-compact level. The coupling after a half-wavelength has been lowered from -15dB down to -35dB as shown below for two antenna patchs with a E-plane coupling. This result sets this particularly favorably strategy and was followed with surface wave works realized by the group.

Patch Antennas decoupling with High Impedance Surface

The use of metamaterials printed on the walls of a circular or rectangular waveguide can change the electrical properties like the propagation constant or the field shape. Therefore analytical studies were carried out in collaboration with CNES to characterize these properties (2 co-supervised Masters in 2012 and 2013 and continued with a thesis in 2013). Based on the metamaterials modelling, an adequated choice of the surface has improved the properties obtained in these waveguides. In comparison with conventional waveguides, these new guides allow obtaining a better polarization for the circular guide, reducing the section of the rectangular and circular guides for the same cutoff frequency, decreasing the cut-off frequency for the same section guide and then expand the single-mode band. The results, in terms of metasurface geometry and cut-off lowering, is displayed below.

Metamaterials for waveguide walls