Dr. Senad Bulja, PhD, FIET, SMIEEE

Liquid crystal based phase shifters in 60 GHz band

Dr. Senad Bulja — Wed, 15 Feb 2023 07:36:25 +0000

Liquid Crystals (LCs) are promising dielectric media for a range of mm-wave devices. Their main advantage stems from the fact that their dielectric properties can be externally controlled by an electric or magnetic field. As such, LCs have already found some uses in the design of mm-wave devices such as tunable microstrip line phase shifters [1-2]. In [1-2], the phase shifters are simply made of a length of a microstrip line including a LC substrate. Therefore, the amount of the phase shift is dependent on the length of the microstrip line exposed to the LC. This becomes highly impractical when high values of phase shift are required as long length microstrip lines are needed. For example, in [2] the 50 mm length of the line achieves a differential phase shift of about 90 degrees at 24 GHz, when the LC mixture is K15.

Preliminary structures and performance results of two different 60 GHz band planar phase shifters using liquid crystal (LC) are presented in this paper. The first phase shifter is based on a meandered microstrip line structure with the conductor pattern exposed to a layer of a nematic LC, while the second phase shifter is Reflection-Type Phase Shifter (RTPS) whose tunable reflective loads are formed on a nematic LC substrate. The LC mixture used in these two phase shifters is E7 with a low temperature coefficient over a wide range of temperatures. In both phase shifters, the LC is biased through a variable low voltage external source allowing for the control of the dielectric constant of the LC. To facilitate the on-wafer measurement and the application of the electric field to the LC, a broadband transition from the microstrip line to the CPW is developed and used in both phase shifters. The best figure of merit of the meandered line phase shifter, 20.8° dB, occurs at 60.7 GHz coinciding with a differential phase shift of 243 degrees, while the RTPS achieves a maximum figure of merit, 12.3° dB, at 61.31 GHz corresponding to a differential phase shift of 170 degrees.

Fig. 1 Structure of LC-based meandered microstrip line phase shifter (Left) and RTPS operating at a centre frequency of 60 GHz (Right).

[1] N. Martin, P. Laurent, G. Prigent, P. Gelin and F. Huret, “ Improvement of an inverted microstrip-line microwave phase-shifter using liquid crystal”, 33rd European Microwave Conference (EuMC 2003), pp. 1417-1420, 2003, Munich, Germany.

[2] S. Mueller, P. Scheele, C. Weil, M. Wittek, C. Hock and R. Jakoby, “Tunable passive phase shifter for microwave applications using highly anisotropic liquid crystals”, IEEE, MTT-S Int. Microwave Symp.Dig., pp. 1153- 1156, 2004.

Advances in LC characterization with application to development of mm-wave devices

Dr. Senad Bulja — Mon, 06 Feb 2023 08:15:09 +0000

Nematic liquid crystals (LCs), largely used in display technology, are liquid anisotropic materials that exhibit a variation of relative dielectric permittivity when subjected to an external electric or magnetic field. The local LC molecular ordering is described by a vector called director. The relative dielectric permittivity of the LC can be described in terms of this tensor, normally by a uniaxial tensor with nonzero principal components and corresponding to the relative dielectric permittivities in the normal and parallel directions of the director with respect to the applied bias field. LCs have already found some uses in the design of mm-wave devices such as phase shifter, variable delay lines and voltage tuned antennas. A number of different factors influence the choice of an LC material for a particular mm-wave application. These include its mm-wave dielectric anisotropy, nematic-isotropic transition temperature and switching voltage threshold. The value of the dielectric anisotropy ( Δε ) is of paramount importance as it directly determines the ability to control an LC-based device.

The aim of this paper is twofold. First, it presents our work on two different methods for the characterization of dielectric properties of LCs at mm-waves (broadband and spot frequency) as described in [1] and [2] and, second, it covers a summary of the development of two novel LC-based mm-wave devices (reconfigurable phase shifter and resonator) in our labs, using the characterization data as described in [3] and [4].

Fig. 1 Broadband measurement devices (left) and spot frequency measurement device (right)

[1] S. Bulja, D. Mirshekar-Syahkal, M. Yazdanpanahi, R. James, F. A. Fernandez and S. E. Day, “Measurement of dielectric properties of nematic liquid crystals at milimeter wavelength”, IEEE Trans. Microwave Theory and Tech., vol. 58, issue 12, 3493-3501, 2010.

[2] M. Yazdanpanahi, S. Bulja, D. Mirshekar-Syahkal, R. James, S. E. Day and F. A. Fernandez, “Measurement of dielectric constants of nematic liquid crystals at mm-wave frequencies using a patch resonator”, IEEE Trans. Instrum. Meas., vol. 59, issue 12, pp. 3079-3085, 2010.

[3] S. Bulja, D. Mirshekar-Syahkal, M. Yazdanpanahi, R. James, F. A. Fernandez and S. E. Day, “60 GHz Reflection Type Phase Shifter Based On Liquid Crystal”, Proc. IEEE RWS 2010, New Orleans, USA, pp. 697- 699, 10-14 Jan. 2010.

[4] M. Yazdanpanahi, S. Bulja, D. Mirshekar-Syahkal, R. James, S. E. Day and F. A. Fernandez, “Liquid-crystal-based mm-wave tunable resonator”, Proc. EuMC 2010 Paris, France, 12331236, Oct. 2010.