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IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 20, NO. 3, MARCH 2010 145

Compact UWB Bandpass Filter WithUltra Narrow Notched Band

Xun Luo , Student Member, IEEE , Jian-Guo Ma , Senior Member, IEEE , Kaixue Ma , Senior Member, IEEE , andKiat Seng Yeo

Abstract—A compact ultra-wideband (UWB) bandpass filterwith an ultra narrow notched band is proposed using a hybridmicrostrip and coplanar waveguide (CPW) structure. The CPWdetached-mode resonator (DMR) composed of a quarter-wave-length ( 4 ) nonuniform CPW resonator with a short-stub anda 4 single-mode CPW resonator (SMCR) can allocate threesplit frequencies at the lower end, middle, higher end of the UWBpassband. The conventional broadside-coupled microstrip/CPWstructure is introduced to improve the bandwidth enhancementaround the split frequencies, which leads to good UWB operation.To avoid the interferences such as WLAN signals, the 4 me-ander slot-line structure embedded in the DMR is employed toobtain the notched band inside the UWB passband. The designis then verified by experiment. Good passband and stopbandperformances are achieved. Specifically, the fabricated filter hasa 10 dB notched fractional bandwidth (FBW) of 2.06% at thenotched center frequency of 5.80 GHz.

Index Terms—Bandpass filter (BPF), coplanar waveguide(CPW), detached-mode resonator (DMR), microstrip, notchedband, ultra-wideband (UWB).

I. INTRODUCTION

ULTRA-WIDEBAND (UWB) technology has risen dra-matically, since the Federal Communication Commission

(FCC) authorized UWB (range of 3.1–10.6 GHz) for the unli-

censed use of short-distance communication in early 2002 [1].

As one of the key components in the UWB systems, the UWB

bandpass filters (BPFs) are developed with variant structures

[2]–[6]. However, existing undesired narrow band radio sig-

nals, such as wireless local-area network (WLAN), may inter-

fere with the UWB range defined by the FCC. In order to avoid

the interferences from the WLAN signals, UWB BPFs with

notched band using different structures are proposed [7]–[10].

However, the narrowest 10 dB notched fractional bandwidth

Manuscript received October 13, 2009; revised November 09, 2009. Firstpublished January 26, 2010; current version published March 10, 2010. Thiswork was supported by the National Science Foundation of China (NSFC:60688101) and National 111 program.

X. Luo is with the School of Electronic Engineering, University of Elec-tronic Science and Technology of China, Chengdu 610054, China (e-mail:[email protected]).

J.-G. Ma was with the School of Electronic Engineering, University of Electronic Science and Technology of China, Chengdu 610054, China. He isnow with the School of Electronic Information Engineering, Tianjin University,Tianjin 300072, China.

K. Ma is with the ST Electronics, Singapore.K. S. Yeo is with the Center for Integrated Circuits and systems, Nanyang

Technological University, Singapore 639798.

Digital Object Identifier 10.1109/LMWC.2010.2040212

(FBW) reported in the literature is around 4.6%. Much narrower

FBW is required [10].

In this letter, a compact UWB BPF with an ultra narrow

notched band is proposed using a hybrid microstrip and

coplanar waveguide (CPW) structure. Following our early

research in [5], the implemented filter is composed of the CPW

detached-mode resonator (DMR) and conventional broad-

side-coupled microstrip/CPW structure to achieve the UWB

frequency response. Furthermore, with the meander slot-line

structure embedded in the DMR, a desirable notched band

to cancel the WLAN signals in standard UWB passband isachieved. The proposed filter is constructed on the RT/5880

with and . Both simulated and

measured results demonstrate good passband and stopband

performances of the implemented filter.

II. UWB BPF WITH NOTCHED BAND:

SCHEMATIC AND PRINCIPLE

The configuration and transmission line model of the pro-

posed UWB BPF with notched band using the hybrid struc-

ture of microstrip and CPW are shown in Fig. 1. As depicted in

Fig. 1(a), the microstrip structure on the top is used as the input/ output (I/O) feed lines through the broadside-coupled transition

to the CPW resonant structure on the bottom. The microstrip

layer and CPW layer share the same substrate and signal ground.

As shown in Fig. 1(b), the DMR on the CPW layer is composed

of three parts: a nonuniform CPW resonator with characteristic

impedance and electric length , a shunt-stub of ,

and a single-mode CPW resonator (SMCR) of . Inside

the DMR, two symmetrically embedded meander slot-lines with

length and width (i.e., ) are implemented.

A. Detached-Mode Resonator

The DMR is employed to occur three split frequencies at thelower end , middle , and higher end to meet the

concern of the UWB operation. First, the nonuniform CPW res-

onator with quarter-wavelength is introduced to resonate

at the center frequency , where is the microstrip guided

wavelength at . Secondly, for a resonance at , a SMCR

is inserted and separates the nonuniform CPW resonator into

two identical parts. Meanwhile, a higher unwanted harmonic

introduced by this SMCR that could dominate the stopband

bandwidth of the proposed filter. Thirdly, a short-stub connected

to the nonuniform CPW resonator is employed to allocate the

and compose the whole DMR. This lower resonance can be de-

termined when shown in Fig. 1(b) is about 90 at .

1531-1309/$26.00 © 2010 IEEE



146 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 20, NO. 3, MARCH 2010

Fig. 1. (a) Configuration of the proposed filter. (b) Transmission line model.

B. Broadside-Coupled Microstrip/CPW Structure

The conventional broadside-coupled microstrip/CPW struc-

ture is introduced to provide strong enough capacitive coupling

for bandwidth enhancement [4],[5]. It is found that by extending

the microstrip electric length to90 (i.e., ) atthe center

frequency of , the insertion loss around the split frequencies isreduced and becomes smooth to meet the UWB passband limit.

C. Embedded Meander Slot-Line Structure

To cancel the interferences such as the WLAN signals, the

meander slot-line structure embedded in the DMR is employed.

This novel structure could easily achieve a desirable notched

band inside the UWB operation. It is found that the center fre-

quency of the notched band can be finely adjusted when the

meander slot-line length is about at . Besides, the

bandwidth of the notched band can be controlled by tuning the

width of the meander slot-line structure.

D. Filter Design

To demonstrate the operation of the proposed structure

above, the full-wave EM simulator [11] is used. Three cases

are compared and studied as follow. Case (a) is the structure of

the DMR. The nonuniform CPW resonator with characteristic

impedance of and electric length of 91.8 and the

short-stub with of and of 32.2 at 6.85 GHz are

chosen, respectively. The SMCR has characteristic impedance

of and electric length of 90.4 at 9.34 GHz. Case

(b) is the DMR with conventional hybrid broadside-coupled

microstrip/CPW structure, where the microstrip characteristic

impedance and electric length are chosen to be

and 90.2 at 6.85 GHz, respectively. Case (c) is the structure of case (b) with the meander slot-line structure embedded in the

Fig. 2. Frequency responses of the configuration of the proposed filter.

Fig. 3. Effects of embedded meander slot-line structure on notched band.(a) Curve to relate the center frequency of notched band to slot-line length L .(b) Curve to relate the bandwidth of notched band to slot-line width W .

DMR. These meander slot-lines shown in Fig. 1 have width of

0.2 mm and electric length of 91.1 at 5.80 GHz. The results

of three cases are compared in Fig. 2, respectively. Curve (a)

in Fig. 2 shows that three split resonant frequencies could be

allocated by the DMR at the lower end, middle, and higher end

of the UWB band, i.e., 3.79, 6.81, and 9.34 GHz. Besides, due

to the use of the SMCR which makes the spurious passband

about the triple of the center frequency at 6.85 GHz, higherunwanted harmonic resonance appears around 17.2 GHz.

Curve (b) depicts that good UWB response can be achieved by

the broadside-coupled microstrip/CPW structure. In addition,

with the advantages of the microstrip feed-line structure, two

transmission zeros close to the passband are formed, which lead

to good selectivity. Curve (c) illustrates that the UWB operation

is still obtained and a notched band with center frequency

of 5.80 GHz is achieved.

Fig. 3(a) and (b) show the simulated frequency response of

the proposed filter with tuning and of the embedded

meander slot-line structure, respectively. It is clearly observed

that with increasing length , the notched band is shifted to

lower frequency and with decreasing width , the bandwidthof the notched band slightly decreases. After choosing of



LUO et al.: COMPACT UWB BANDPASS FILTER 147

Fig. 4. Layout of the fabricated filter. (L = 9 : 6 , L = 1 : 3 , L = 0 : 8 ,L = 6 : 2 , L = 6 : 0 5 , L = 1 : 3 , L = 4 : 1 5 , L = 6 : 1 , L = 0 : 8 ,L = 1 0 : 3 , W = 5 : 3 , W = 0 : 6 , W = 0 : 8 , W = 0 : 2 , W = 1 : 6 ,W = 1 : 2 , W = 1 : 0 5 , and W = 0 : 2 .).

Fig. 5. Measured and simulated results of the fabricated UWB BPF with anultra narrow notched band.

10.3 mm and of 0.2 mm, a 10 dB notched FBW of 2.06%

at the notched center frequency of 5.80 GHz can be achieved.

III. FABRICATION AND EXPERIMENTAL RESULTS

The investigation above shows the principle of the proposed

filter. To further improve the passband/stopband performances

and demonstrate this type of filter experimentally, the fabri-

cated filter is slightly modified and displayed in Fig. 4. -pa-

rameters and group delay response measurements are performed

using Agilent 5230A network analyzer over the frequency range

from 10 MHz to 18 GHz. Fig. 5 demonstrates the simulated

and experimental results of the proposed filter, where excel-

lent agreement is obtained. The proposed filter exhibits a good

UWB bandpass performance from 2.96 to 10.72 GHz with a

fractional bandwidth (FBW) of 113.5% at a midband frequency

of 6.84 GHz. The measured results show the emergence of a

notched band at 5.80 GHz with 24.7 dB insertion loss and 10 dB

notched FBW of 2.06%. The measured minimum insertion loss

is found to be 0.5 and 0.9 dB in the lower and upper passbands,

respectively. The measured return loss is better than 14.3 dB

over the lower band and better than 11.6 dB over the upper band.

Two transmission zeros are found at 0.86 and 11.8 GHz, and

the upper stopband is observed from 11.4 to 16.4 GHz. Mean-

while, the implemented filter exhibits a flat group delay response

below 0.48 ns and group delay variation less than 0.28 ns overthe lower and upper passbands. In addition, the proposed filter

has the merit of a compact size ( by , where

is the guided wavelength of microstrip structure at the center

frequency of 6.84 GHz) when compared to the published UWB

BPF with a notched band.

IV. CONCLUSION

In this letter, a compact UWB BPF with an ultra narrow

notched band is proposed based on a hybrid microstrip and

CPW structure. The DMR on the CPW layer and conventional

broadside microstrip/CPW coupled structure are constructed to

meet the UWB limit. To achieve the notched band in the UWBpassband, the meander slot-line structure embedded in the

DMR is employed. As such, the UWB filter with notched band

is designed and implemented. Specifically, the proposed filter

exhibits an ultra narrow notched band at 5.80 GHz with 24.7 dB

insertion loss and 10 dB notched FBW of 2.06%. With good

frequency performance and a compact size, the proposed filter

is attractive to the UWB systems for the purpose of blocking

unwanted radio signals.

REFERENCES

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[8] S. W. Wong and L. Zhu, “Implementation of compact UWB bandpassfilter with a notch-band,” IEEE Microw. Wireless Compon. Lett., vol.18, no. 1, pp. 10–12, Jan. 2008.

[9] G. M. Yang, R. Jin, C. Vittoria, V. G. Harris, and N. X. Sun, “Smallultra-wideband (UWB) bandpass filter with notched band,” IEEE Mi-crow. Wireless Compon. Lett., vol. 18, no. 3, pp. 176–178, Mar. 2008.

[10] H. Shaman and J. S. Hong, “Ultra-wideband (UWB) bandpass filterwith embedded band notch structures,” IEEE Microw. WirelessCompon. Lett., vol. 17, no. 3, pp. 193–195, Mar. 2007.

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