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.

Meander line millimetre-wave liquid crystal based phase shifter

Dr. Senad Bulja — Mon, 09 Jan 2023 09:14:34 +0000

The abundance of a widely available spectrum at a frequency band of around 60 GHz (mm-wave region) displays potential to support high data rate, short range wireless communications. This has led to an increased demand for cost effective solutions for the RF front end, such as antennas, phase shifters and filters. Preferably, these mm-wave devices need to be reconfigurable and compact.

Liquid crystals (LCs) are promising materials for a range of microwave devices, owing to its dielectric anisotropy and relatively low millimetre-wave losses. Its main advantage stems from the fact that its dielectric properties can be controlled by an external electric or magnetic field. If an electric field is used in the control of the LC molecules orientation (and thus dielectric permittivity), the bias circuit has virtually no power consumption. As such, LCs have already found use in the design of microstrip line phase shifters, reflection-type phase shifters, voltage tuned microstrip antennas and reflect arrays.

In this paper an LC-based inverted microstrip meander line phase shifter is presented. The control of phase shift is achieved through use of an external electric field. To facilitate on-wafer measurements and the application of the electric field to the LC, a specially developed transition from the inverted microstrip line to the CPW is used. The performance of the proposed meander line phase shifter is tested using two LC mixtures: first, the standard LC mixture, commonly referred to as E7, while the second mixture is known as MDA-00- 3506. A differential phase shift of 2028 is achieved with the phase shifter using E7 and 2708 using MDA-00-3506 at 50 GHz for a 50 mm-long microstrip line meandered into a length of 7.67 mm.

Fig. 1 Inverted microstrip meander line LC based phased phase shifter

Dr. Senad Bulja, PhD, FIET, SMIEEE

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

Meander line millimetre-wave liquid crystal based phase shifter