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The use of liquid crystals for tunable filter in planar technology is proposed. The design is based on a dual-behavior resonator (DBR) topology. These resonators are based on the association of different parallel open-ended stubs and allow the designer to independently control the in-band and out-of-band responses of the filter. To benefit from liquid crystal anisotropy and thus obtain agility, a bias voltage is applied. The simulated results are compared with measured data, and good agreement is obtained.

The demand on tunable or reconfigurable components at micro- and millimeter waves increased during the last years. Today, these difficult problems are the subject of intensive studies in microwave planar filters [

This paper propose device using planar technology-based on LC materials. This choice allows the LC anisotropy property to the excited radio frequency field [

LCs are attractive substrates for microwave devices. They possess a significant tuneable dielectric constant in the microwave band, which can be exploited in compact and reconfigurable devices such as phase shifters, antennas, and filters. When designing such devices two main problems are normally encountered. Depending on the temperature, liquid crystal phase exists in a mesophase between a crystalline solid and an isotropic liquid [

Schematic of a typical LC molecule (K15) and its temperature dependency.

In an aligned nematic liquid crystal, linearly polarized light incident as an extraordinary wave will see a permittivity

In this paper, LCs are used in the nematic phase, where the molecules float around as in the liquid phase but are still ordered in their orientation[

The director vector

Studies over the last two decades have also conclusively demonstrated their unusually large electro- and all-optical (i.e., nonlinear-optical) response associated with the field induced director axis reorientation. These unique optical properties [

A DBR results from the association of two different parallel bandstop structures. Each of them brings its own transmission zero with respect to its fundamental resonant condition. At the same time, their association is transparent within a given operating frequency once the bandstop structures have been properly connected under constructive recombination criteria. These result in a bandpass response created between the abovementioned lower and upper rejected bands. According to the number of available parameters and to the initial behavior of each bandstop structure, DBR allows an independent control of the following.

Thus, a DBR structure allows one to independently control [

one pole in the operating bandwidth;

one transmission zero in the lower attenuated band;

one transmission zero in the upper attenuated band.

Among the numerous DBR topologies available, one of the simplest ones is based on the parallel association of two open-circuited stubs of different lengths. Its easy implementation drove us to choose this topology for the design of our circuits. The inherent specifications of the filter are entered in equations called synthesis [

The generic structure (see Figure

DBR, the basic resonant structure.

This equation shows that the stub association has no incidence on the frequencies of the transmission zeros that always appear when

After modifications and optimization of the circuit, an electromagnetic calculation of the global structure is carried out with the HFSS design based on a finite element method. This method permits one to consider all of the structure elements: alumina, dielectric ink, liquid crystal, and PTFE.

Figures

Structure filter DBR with an LC cavity by HFSS.

Transversal cut of the filter DBR structure.

After its insertion in the cavity, LC is considered by the microstrip line as the main substrate; thus, its properties are modified by application of an external static electric field. As the technological process is set, we have to choose a topology filter that presents a narrow bandpass to clearly show the shift of the function. Nevertheless, we have to implement simple structure like stubs with inductive feeding to avoid the use of an anisotropic substrate model and to facilitate the bias voltage. In this way, DBR filter appears as a very good compromise. Indeed, the central frequency and the bandwidth can be tuned by only modifying lengths [

Figures

Simulated return losses for two different permittivities by (HFSS).

Simulated insertion losses for two different permittivities by (HFSS).

The simulated return losses are centred around 5 GHz. The return loss achieved −20 dB from 4 to 5 GHz. The resonance frequency variation (ΔFr) is 240 MHz corresponds to a frequency agility of 4.8%. The insertion losses achieved −60 dB from 4 to 5 GHz.

Figures

Simulated and measured return losses without applied DC voltage.

Simulated and measured return losses with applied DC voltage.

It can be seen from Figure

Simulations and measurement insertion losses with and without applied DC voltage show good agreement, as depicted in Figures

Simulated and measured insertion losses without applied DC voltage.

Simulated and measured insertion losses with applied DC voltage.

Structure of LC-based reconfigurable tunable filter with DBR topology accordable in a band and a frequency is designed and simulated. The observation of the results confirms the potential frequency agility of the device that uses LCs. This agility was obtained by varying the LC dielectric permittivity, established by dielectric characterisation, from two different permittivities. The accuracy of simulation was verified by comparison with experimental data.