可调低通滤波器

A Tunable Low-Pass Filter Using a Liquid-Metal Reconfigurable

Periodic Defected Ground Structure

Shuyan Guo, Bao Jun Lei, Wenqi Hu, Wayne A. Shiroma, and Aaron T. Ohta Department of Electrical Engineering, University of Hawaii at Manoa, Honolulu, HI 96822 USA

Abstract — A new type of tunable low-pass filter is demon-

strated that uses liquid metal to reconfigure a defected ground

structure (DGS). By filling in different DGS lattices with Galins-

tan liquid metal, the tunable low-pass filter provides tuning of up

to eight cutoff frequencies. Measurements of four of the cutoff

frequencies show a 62% tuning range while maintaining a stop-

band of more than 5 GHz.

Index Terms — Tunable low-pass filter, defected ground struc-ture (DGS), liquid metals.

I. I NTRODUCTION

Tunable RF filters are widespread in communication sys-tems, wireless devices, and transceiver systems. Currently, most tunable filters are achieved using RF-MEMS switches [1], [2]. However, the number of possible tunable states is dependent on the number of MEMS switches; to increase the number of states, more switches are needed. The use of liquid metals in RF devices can potentially provide a higher tuning resolution than MEMS switches.

Different research groups are exploring the feasibility of us-ing liquid metals in RF devices as functional components [3]-[8]. Some devices use mercury [3], [4], although it is prefera-ble to use a non-toxic metal, such as Galinstan, which is a eutectic alloy of gallium, indium and tin. Galinstan has been used in RF switches [6], antennas [7], and resonators [8]. This paper presents a liquid-metal reconfigurable low-pass filter with a defected ground structure (DGS), using Galinstan to reconfigure the distribution of DGS lattices to realize cutoff-frequency tuning.

DGSs have been widely used in RF circuits, offering advan-tages such as compact physical dimensions, spurious response suppression, and bandgap characteristics in some frequency bands [9], [10]. Nonuniform configurations of DGSs have also been investigated to obtain a wide or an ultra-wide stopband [11], [12]. Recently, DGSs have been applied to the design of tunable bandstop resonators that have a tuning range of ap-proximately 20%, centered about the resonance frequency [13], [14]. In this paper, cutoff-frequency tuning of a low-pass DGS filter is achieved using Galinstan liquid metal, while maintaining a wide stopband.

Fig. 1. Layout of the cascaded defected ground structure used in the reconfigurable filter.

II. D ESIGN

A. Non-Uniform Dumbbell DGS

The dumbbell-shaped DGS, first proposed by Kim et al. [9],

has a stopband effect due to the effective inductance of the

lattice. The cutoff frequency is primarily determined by the

area that is etched from the RF device ground plane (Fig. 1). If

the etched area is decreased while the etched gap is kept con-

stant, the cutoff frequency increases. The DGS lattice period

also has a slight influence on the stopband center frequency,

although this is a smaller effect compared to changing the

dimensions of the DGS elements. The tunable low-pass filter

presented here employs an exponential distribution of DGS

lattices (Fig. 1). An exponentially resized DGS provides a

wide stopband and excellent ripple suppression [12]. Further-

more, the gradual variation of DGS lattice dimensions in this

configuration enables a tunable filter with fine resolution.

The dimensions of the square lattices are varied proportion-

ally to the amplitude distribution of the exponential function

e1/n. The center element of the thirteen-element array corres-ponds to n = 1, and has sides that are 10 mm long. Hence, the

sides of the square elements, moving from the center element

outwards, have dimensions as follows (in mm): 10, 6.07, 5.13,

4.72, 4.49, 4.35, 4.24. The period of the thirteen nonuniform

DGS elements is 10 mm, and the gap distance is 0.6 mm.

B. Materials and Fabrication

The DGS circuit was etched on the ground plane of a 1.27-

mm-thick RT/Duroid 6010 substrate with ε

r

= 10.2. A 50-Ω

microstrip transmission line was created on the top of the sub-

strate, with a width of 1.15 mm.

Rectangular fluidic channels were fabricated over each DGS

lattice element, allowing the etched areas to be filled in by

liquid metal (Fig. 2). Double-sided polyimide tape forms 0.3-