Mani Yazdanpanahi

Millimeter-wave liquid-crystal-based tunable bandpass filter

For the first time, a tunable parallel-coupled microstrip lines filter based on nematic liquid crystals (LC) substrate for applications at mm-wave frequencies is presented. This 33 GHz filter has three-poles, about 10% bandwidth (3.3 GHz) and a tunability range of 2 GHz over bias voltages 0 to 10 V. Theoretical and experimental responses of the filter are in good agreement. The insertion loss of the filter is about 4.5 dB, but this is mainly due to the microstrip line to CPW transitions used for measurement of the filter by a probe-station.

Finite element analysis of a balanced microstrip line filled with nematic liquid crystal

Liquid crystals are an attractive medium for millimetre-wave band communications devices due to their large birefringence and reconfigurability. However, characterizing these materials is challenging at these frequencies. This work details simulation tools that have been developed to aid in this process, through the accurate analysis of the liquid crystal orientation and wave propagation.

60 GHz Reflection Type Phase Shifter based on liquid crystal

For the first time, a compact 60 GHz band phase shifter incorporating liquid crystal (LC) is presented. Based on the Reflection Type Phase Shifter (RTPS) structure, it comprises a 3-dB coupler and tunable reflective loads formed on a nematic LC substrate. The LC used to form the substrate is known as E7. Biasing of the LC is achieved using an external electric field, allowing for the control of the reactance of the reflective loads. To facilitate on-wafer measurements and the application of the electric field to the LC, a transition from the microstrip line to the CPW is used. Preliminary measurement results for this RTPS show a differential phase shift of over 130 degrees, a figure of merit of 11.4°/dB and return losses better than -10 dB at 61 GHz.

Tunable liquid-crystal millimeter-wave bandpass filter using periodical structure

For the first time, a tunable liquid crystal (LC) bandpass filter using a microstrip periodical structure for applications at millimeter-waves is presented. In the LC default state, it operates at 49 GHz with about 20% fractional bandwidth (9 GHz). The filter tunability range is 3.2 GHz over bias voltages 0 to 10 V and its insertion loss is about 4.7 dB. Dimensions of the filter can be scaled to increase its operating frequency to 77 GHz and hence, is a promising component for 60 GHz systems.

Planar transmission line method for measurement of dielectric constants of liquid crystals in 60 GHz band

Recent characterizations of liquid crystal (LC) materials invariably use magnetic field for biasing LC. In this paper a transmission line method is proposed for the characterization of the dielectric properties of LC in the 60 GHz band, using electric field bias. In this method a balanced strip line is used to accommodate the LC sample. To facilitate the application of the electric field to the LC and to ease the measurement of the scattering parameters, a unique transition from the balanced strip line to the finite ground coplanar waveguide (FGCPW) is developed. The LC investigated in this paper is the E7 mixture.

Effect of test device thickness on liquid crystal characterisation

A liquid crystal (LC) test device to operate in the frequency band of 57-66 GHz is designed and the effect of its thickness in determining the dielectric properties of the LC materials is investigated. For this purpose, two LC test devices one with a thick profile and another one with a thin profile are designed and investigated. These devices are then used to extract the dielectric properties of two dielectric test materials. When comparing the dielectric constant extraction from these test devices, the thin LC test device shows better accuracy for a wider frequency band.

Investigation of coupling based on liquid crystal in two end-coupled microstrip resonators

A new tunable planar coupled resonator structure based on nematic liquid crystals (LC) substrate for applications at mm-wave frequencies is presented. The proposed structure allows the use of electric field bias to align the LC molecules, offering direct control over the dielectric properties (anisotropy) of the LC substrate. The structure is designed for use of a LC mixture commonly referred to as E7. Symmetrical and asymmetrical couplings are investigated using this coupled resonator structure. In both cases, coupling tunability of about 20% is achieved in the 30 – 40 GHz range. The proposed structure can also be exploited to realize miniaturized tunable filters with wide tuning range at mm-wave frequencies.

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

In this paper the current state of research in our labs on the mm-wave characterization of LCs and the application of LC in development of two devices were presented. It has been shown that the characterization of LCs can be done using the resonant and broadband methods, but each has its own advantages and disadvantages. Also shown were a LC-based reflection type phase shifter, providing a large phase shift compared to the conventional transmission line phase shifters and a long LC-based resonator with multiple tunable resonant frequencies.