4
New Tunable Coplanar Microwave Phase Shifter With Nematic Crystal Liquid Fehim Sahbani 1,2 , N.Tentillier 1 , A.Gharsallah 2 ,A.Gharbi 2 ,C.Legrand 1 LEMCEL 1 ,LPMM 2 E-mail: [email protected] Abstract: The liquid crystals known for their applications as viewers have interesting features for a microwave circuit design’s so-called ”agile frequency”. These circuits offer the advantage to change their frequency response according to an external command. Here we present a new phase shifter agile frequency liquid crystal substrate. Indeed the nature of viscous liquid crystal requires development ad-hoc structures.The measurements are characterised in [26-40]GHz band. This phase shifter with agile frequency will be integrated into an antenna network and will be used to control the radiation pattern of the synthesised array antenna. Keywords:liquid crystals:LC , phase shifter, antenna, microwave, frequency agility 1. Introduction The devices are agile circuits whose frequency response can be controlled through an external command. A major application relates to the miniaturisation of embedded systems, replacing a set of circuits by another.In this sense we can use a substrate product called ”agile” composed of materials whose electromagnetic properties can be changed through an electric or outside magnetic field. Among these materials we cite the ferrites [1] and ferroelectric ceramics [2] but also the liquid crystal [3-6]. Indeed they are anisotropic and therefore have different permittivity depending on their orientation in relation to the electromagnetic field. in this article we propose to use LC in the design of phase shifter . As a first step we will consider the phase nematic liquid crystal.We present an new phase shifter liquid crystal substrate a characterise it up to 40GHz. Finally we chose a possible application of this phase shifter in an antenna structure in order to change the orientation of the main lobe of the radiation pattern. 2. Nematic liquid crystal Here we present liquid crystal LC. Under the applications we use liquid crystal nematic phase in an ambient temperature. The nematic liquid crystals are characterised by their center of gravity,the molecules showing no order of position. However molecules procure an orientation order incase of long distance . their long-distance and long axis is parallel to an average direction defined by the vector director n (Figure 1). x y z Figure 1. Representation of molecules CL in phase nematic In this phase the liquid crystals are anisotropic materials with complex permittivity presented in the form of a tightening (Relationship 1). It then defines the dielectric anisotropy by the relationship2. ε ε ε ε * // * * * = 0 0 0 0 0 0 (1) ∆ε = ε // - ε (2) Measures in two different directions of the material are needed to determine each of its parameters. It is possible to orient the liquid crystal by applying an outside magnetic field of typical 0.4 Tesla. therefore the molecules align their long axis in its direction. The permittivity analysed by the microwave field E HF measure is amended following this trend. Here we will choose two configurations in measurement of permittivity ε r// and ε r(Figure 2). 3. Characterisation of the microwave dielectric nematic liquid crystal To characterise the liquid crystal, we chose a new method without calibration of the vector network analyser developed in the laboratory [7]. This method which is named ∆γ is based on two measures. 978-1-4244-3477-0/08/$25.00 ©2008 IEEE 78

[IEEE 2008 3rd International Design and Test Workshop (IDT) - Monastir, Tunisia (2008.12.20-2008.12.22)] 2008 3rd International Design and Test Workshop - New tunable coplanar microwave

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Page 1: [IEEE 2008 3rd International Design and Test Workshop (IDT) - Monastir, Tunisia (2008.12.20-2008.12.22)] 2008 3rd International Design and Test Workshop - New tunable coplanar microwave

New Tunable Coplanar Microwave Phase Shifter With Nematic Crystal Liquid

Fehim Sahbani 1,2

, N.Tentillier1, A.Gharsallah

2,A.Gharbi

2 ,C.Legrand

1

LEMCEL1 ,LPMM

2

E-mail: [email protected]

Abstract: The liquid crystals known for their

applications as viewers have interesting features

for a microwave circuit design’s so-called ”agile

frequency”. These circuits offer the advantage to

change their frequency response according to an

external command. Here we present a new phase shifter

agile frequency liquid crystal substrate. Indeed the nature

of viscous liquid crystal requires development

ad-hoc structures.The measurements are

characterised in [26-40]GHz band. This phase shifter

with agile frequency will be integrated into an

antenna network and will be used to control the

radiation pattern of the synthesised array

antenna.

Keywords:liquid crystals:LC , phase shifter,

antenna, microwave, frequency agility

1. Introduction

The devices are agile circuits whose frequency

response can be controlled through an external

command. A major application relates to the

miniaturisation of embedded systems, replacing

a set of circuits by another.In this sense we can

use a substrate product called ”agile” composed

of materials whose electromagnetic properties

can be changed through an electric or outside

magnetic field. Among these materials we cite the

ferrites [1] and ferroelectric ceramics [2] but also

the liquid crystal [3-6]. Indeed they are anisotropic

and therefore have different permittivity

depending on their orientation in relation to the

electromagnetic field. in this article we propose

to use LC in the design of phase shifter . As a

first step we will consider the phase nematic

liquid crystal.We present an new phase shifter

liquid crystal substrate a characterise it up to

40GHz. Finally we chose a possible application

of this phase shifter in an antenna structure in

order to change the orientation of the main lobe

of the radiation pattern.

2. Nematic liquid crystal

Here we present liquid crystal LC. Under the

applications we use liquid crystal nematic phase in

an ambient temperature. The nematic liquid crystals

are characterised by their center of gravity,the

molecules showing no order of position. However

molecules procure an orientation order incase of long

distance . their long-distance and long axis is

parallel to an average direction defined by the vector

director n (Figure 1).

x

y

z

Figure 1. Representation of molecules CL in phase

nematic

In this phase the liquid crystals are anisotropic

materials with complex permittivity presented in

the form of a tightening (Relationship 1). It then

defines the dielectric anisotropy by the

relationship2.

εε

εε

*/ /*

*

*

= ⊥

0 0

0 0

0 0 (1)

∆ε = ε’

// - ε’⊥ (2)

Measures in two different directions of the material are needed to determine each of its parameters. It

is possible to orient the liquid crystal by applying

an outside magnetic field of typical 0.4 Tesla.

therefore the molecules align their long axis in its

direction. The permittivity analysed by the

microwave field EHF measure is amended following this trend. Here we will choose two configurations in

measurement of permittivity ε∗r// and ε∗

r⊥ (Figure 2).

3. Characterisation of the microwave

dielectric nematic liquid crystal

To characterise the liquid crystal, we chose a new

method without calibration of the vector network

analyser developed in the laboratory [7]. This

method which is named ∆γ is based on two measures.

978-1-4244-3477-0/08/$25.00 ©2008 IEEE 78

Page 2: [IEEE 2008 3rd International Design and Test Workshop (IDT) - Monastir, Tunisia (2008.12.20-2008.12.22)] 2008 3rd International Design and Test Workshop - New tunable coplanar microwave

BB

E HFConfiguration parallèle Configuration perpendiculaire

BBBB

E HFE HFConfiguration parallèle Configuration perpendiculaire

Figure 2. Configurations for measuring permittivity

ε∗r// and ε∗

r⊥. The LC is guided by an outside magnetic

field. .

The permittivity presented by the liquid crystal K15

is constant over the entire frequency range, with an

average direction perpendicular 2.52 and 2.95 to the

parallel direction.

4. Phase Shifter agile frequency

liquid crystal substrate

4.1.Order of liquid crystal within a microwave

sustrate

In the case of a line micro strip classic, the gap

reached between input and output is fixed at a given

frequency. This phase shift depends on effective permittivity and the line length of the following:

c

refffL ε

φ...360

= (3)

when we use a substrate of a liquid crystal structure

defined above, it will be possible to vary the effective

permittivity on the substrate using in addition to the

microwave signal, a low frequency voltage

command (Figure 3).

Figure 3. Influence of an electric field command on

the orientation of liquid crystal molecules

The electrodes by treating surface (planar ori-

entation). The permittivity seen by the microwave

signal is noted εreff(0). This permittivity is related mainly to the permittivity crystal liquid frequency

microwave signal εr⊥ . As a result of the electric field command, the molecules of liquid crystal will gradually move

perpendicular to the electrodes ( n // E ) to

saturation permittivity. The east view εreff(E) . The

saturation permittivity εreff(E) is related mainly to the

liquid crystal permittivity εr// This variation of permittivity will induce a change in the wavelength

guided, therefore a change in phase following the

following relationship:

c

reffErefffL −−

=∆)0()(

...360 εεφ (4)

4.2. Phase Shifter coplanar access

The main structure presented in Figure 3 is not

feasible because of the nature of viscous liquid

crystal. It is therefore necessary to develop a phase

shifter suited for this material. The phase shifter

adapted to this constraint is characterised by an access of coplanair lines engraved on a substrate

such as”RT / DUROD 5880” 381µm of depth and

relative permittivity near 2.16 .To achieve the party

agile, a ground (GND) copper form a cavity of 60

µm of arrivals in which the liquid crystal is inserted

by capillary (Fig. 4)

Figure 4. Structure of coplanar phase shifter.

HFSS simulations have confirmed in a mode of

spread of type micro-strip inside this cavity. As a

result, maintaining a characteristic impedance of 50 is obtained throughout the structure by changing the

width of the central ribbon taking into account the

characteristics of the dielectric liquid crystal made pre-

viously. That is why, in the coplanar access, it has a

width of 980µm and outside and inside of the cavity

its width is 180 µm. In this new structure, the

electromagnetic field is essentially confined to the

anisotropic material and is directly influenced by

changing the permittivity. So,from the point view of ”agility”, this structure should be effective. The

order is obtained by applying a voltage, adjustable

from 0 to 10 Volts between the tape and central layout.

A surfactant-based PVA (PolyVinylAlcool) was

deposited on the tape and in the cavity, in order to

give direction within the liquid crystal in the absence of electric field command. The maximum change

measured phase is 100° to 35 GHz for a voltage of 10

v in an electric field of 0.16v/µm.

The variation of this phase shifter is higher than

0.71°/GHz/cm (Fig. 5).

60 µm

3 cm

Plan de Masse

Cristal Liquide

79

Page 3: [IEEE 2008 3rd International Design and Test Workshop (IDT) - Monastir, Tunisia (2008.12.20-2008.12.22)] 2008 3rd International Design and Test Workshop - New tunable coplanar microwave

0 5 10 15 20 25 30 35 40 45

-100

-80

-60

-40

-20

0

PH

AS

E S

21(°

)

Fréquency

Figure 5. Change phase maximum phase shifter

depending on the frequency orders for 10 Volts.

Moreover, the shifter in access change in phase is a

linear function of frequency. This phase shifter in

the range of insertion’s analysis has losses of

between -1 dB and -7 dB for a ROS average less than 2

(Figures 6 ,7).

0 5 10 15 20 25 30 35 40 45

-9

-8

-7

-6

-5

-4

-3

-2

-1

Mo

du

le S

21

(dB

)

Fréquency

Figure 6. Losses in reflection of the phase shifter

coplanair depending on the frequency.

0 5 10 15 20 25 30 35 40 45

-50

-40

-30

-20

-10

0

Mo

du

le S

11

(dB

)

Fréquency

Figure 7. Losses in transmission phase shifter coplanair

depending on the frequency.

4.3. Scanning Antenna The development of wireless telecommunications

(Wi-Fi technology, Bluetooth,) results in an increase

of electromagnetic pollution. One way to reduce it

is to issue only in the direction of the element with

which it communicates. Thus, a limited area is

affected by communication and power can be

reduced. In this sense we can use an antenna to scan.

Using the phase shifter associated with patch antennas, it is possible to achieve antenna scanning. In this case,

the network of antennas is devoid of mechanical

elements that leads to the lobe emission-reception.

The latter is amended by playing on the phase of the

microwave signal from each antenna. A prototype Ka-band is currently underway in the laboratory

and will soon be characterized.

5. Conclusion

This study focuses on the characterisation of a phase shifter ”agile frequency.” A dielectric prior

characterisation of liquid crystal in the frequency

band used is essential for an optimal size. Moreover,

the nature of viscous liquid crystal makes it necessary

to define an appropriate topology. Taking into account

these constraints, a new structure phase shifter”Frequency agile” liquid crystal substrate is

presented. Starting with the dielectric liquid crystal,

it was designed, produced and characterised until

40GHz. This phase shifter offers interesting

performances (0.71°/GHz/cm to 35 GHz) for a low

voltage command (about 10Volts to get 80% of agility). The integration of this phase shifter is planned and

will lead to the realisation of a scanning antenna

whose lobe radiation will be electrically controlled.

References

[1] F. DE FLAVIIS, N.G. ALEXOPOULOS, O.

STAFSUDD, Planar microwave integrated

phase-shifter design with high purity ferroelectric material, IEEE Trans. on MTT, Vol.45, n6;

pp963-969, 1997.

[2] A. PETOSA, R.K. MONGIA, M. CUHACI, J.S. WIGHT, Magnetically

tunable ferrite resonator antenna, Electronics

Letters, Vol.30, n13, pp1021-1022, 1994.

[3] F. GUeRIN, J.M. CHAPPE, P. JOFFRE, D.

DOLFI, Modeling, Synthesis and characterisation

of a millimetre-Wave Multilayer Micro strip Liquid Crystal Phase Shifter, Jpn. J. Appl. Phys., Vol.37,

Part 1, pp926-928, 1997.

[4] B. SPLINGART, N. TENTILLIER, F. HURET, C. LEGRAND, Liquid Crystals

Applications to R.F. and Microwave Tunable

Components, Molecular Crystals and Liquid

Crystals. Vol 368, pp 183-190, 2001.

[5] N. TENTILLIER, B. SPLINGART, F. HURET, P. KENNIS, C. LEGRAND, Nouvelles

Structures de Dephaseurs Agiles en Frequence

Substrat Cristal Liquide, 12mes Journees

Nationales Microondes, Poitiers, mai 2001.

[6]N.MARTI,N.TENTILLIER,P.

80

Page 4: [IEEE 2008 3rd International Design and Test Workshop (IDT) - Monastir, Tunisia (2008.12.20-2008.12.22)] 2008 3rd International Design and Test Workshop - New tunable coplanar microwave

LAURENT,B.SPLINGART, F. HURET,PH.

GELIN, C.LEGRAND, Electrically microwave

tunable components using liquid crystals,

EuMC 2002, M7.

[7] N. TENTILLIER, Contribution la caracterisation dielectrique micro-onde de cristaux liquides :

Application aux circuits agiles en frequence,

These de doctorat de lUniversite de Lille I,

Decembre 2003.

81