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. INTRODUCTION

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. DESIGN

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 εmicrostrip transmission line was created on the top of the sub-r = 10.2. A 50-Ω 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-

mm-high sidewalls of the channels. A 1-mm-thick piece of polystyrene forms the top layer of the fluidic channels.

Galinstan, a gallium-based alloy, was used as the liquid metal. A 0.2-mm-diameter needle was used to pump the liquid metal into the inlet of the fluidic channels, filling selected DGS elements (Fig. 3). Outlet holes were drilled into the top layer at the end of each DGS element, as an outlet to prevent the buildup of pressure within the fluidic channel during the liquid metal pumping.

Fig. 2. A fluidic channel for liquid metal tuning, built over a defected ground structure element.

(a)

(b)

Fig. 3. Photos of fluidic channels over the DGS side of the tunable filter. Selected DGS areas elements can be filled with Galinstan, which is silver-colored in these photos. (a) The tunable filter with no Galinstan. (b) The tunable filter with the middle five DGS elements filled with Galinstan.

C. Tuning Mechanism

The reconfigurable low-pass filter provides tuning to up to eight cutoff frequencies. In this paper, four of these frequen-cies were measured.

Fig. 4. Simulated S21 of the tunable filter.

State 1 is the original state of proposed filter where none of the DGS elements are filled with Galinstan. A full-wave EM simulation was performed using Sonnet, showing the cu-toff frequency of the filter in State 1 as 1.8 GHz (Fig. 4).

To achieve tuning of the cutoff frequency, the DGS ele-ments were filled in with liquid metal. The center DGS ele-ment was covered with liquid metal to create State 2. The cen-ter element has the largest size of all the elements, correspond-ing to the first finite pole near the cutoff frequency, and mak-ing this element the largest contributor to the cutoff frequency of the filter. Thus, after the center element is filled in with liquid metal, the 3-dB cutoff frequency shifts to a higher fre-quency, as expected. Simulations of State 2 confirm this trend (Fig. 4). State 3 corresponds to the center element and the two immediately adjacent elements being covered with liquid met-al. State 4 additionally fills the elements next to the filled ele-ments in State 3, as shown in Fig. 3(b).

III. MEASUREMENT

The SAgilent 8720ES network analyzer (Fig. 5). As the filter is 21 of the reconfigurable filter was measured using an tuned from State 1 to State 4, the 3-dB cutoff frequency in-creased from 1.85 GHz to 2.99 GHz, corresponding to a 62% tuning range. The simulated and measured 3-dB cutoff fre-quencies are compared in Table I. The elimination of the first finite pole near the cutoff frequency after each state change clearly shows the periodic dumbbell-shaped DGS lattice is reconfigured by the Galinstan in the fluidic channel. The 20-dB isolation bandwidths for States 1–4 are 6.17 GHz, 5.50

GHz, 5.23 GHz, 5.14 GHz, respectively (Fig. 6).

Fig. 5. Measured S21 of the tunable filter.

Fig. 6. Measured wideband response of the tunable filter.

TABLE I: SIMULATED VS. MEASURED CUTOFF FREQUENCY

State3-dB cutoff frequency (GHz)Simulated MeasuredState 11.891.85State 22.232.28State 32.612.65State 42.962.99

IV. CONCLUSION

Fluidic channels have been fabricated on the ground plane

of a tunable non-uniform DGS low-pass filter. Using Galins-tan as the liquid metal to fill the channels on different DGS lattices, the DGS can be reconfigured, resulted in shifting of the cutoff frequency. A tuning range of 62% was achieved while maintaining a stopband of more than 5 GHz This work demonstrates the applicability of liquid metal in tunable fil-ters.

ACKNOWLEDGMENT

This material is based upon work supported by the National Science Foundation under Grant No. ECCS-1101936.

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