具有共模抑制功能的紧凑型差分超宽带带通滤波器
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456 IEEE MICROWAVE AND WIRELESS COMPONENTS LETTERS, VOL. 22, NO. 9, SEPTEMBER 2012
Compact Differential Ultra-Wideband Bandpass
Filter With Common-Mode Suppression
Xiao-Hu Wu and Qing-Xin Chu, Senior Member, IEEE
Abstract—A compact differential ultra-wideband (UWB) band-
pass filter (BPF) with planar structure is presented. The circuit
of the proposed filter is analyzed, showing that the differen-
tial-mode circuit, which consists of two short-ended stubs and
an open-ended parallel-coupled lines, performs as a UWB BPF.
While the common-mode circuit performs as a UWB bandstop
filter, and the common-mode responses are suppressed across
the whole UWB differential passband. A microstrip differential
UWB BPF is designed, simulated and measured. Simulated and
measured results show that the proposed filter has compact size
, UWB differential passband (3 dB fractional
bandwidth 119%) and UWB common-mode suppression (
10 db
attenuation across 2.6–12 GHz).
Index Terms—Common-mode, compact, differential-mode,
ultra-wideband bandpass filter (UWB BPF), ultra-wideband
bandstop filter (UWB BSF).
I. INTRODUCTION
D
IFFERENTIAL filters are essential in the d e sign of
balanced circuits, which have the advantages o f high
immunity to the environm ental noise, interference and crosstalk
between different elements. For the differential bandpass filters
(BPFs), only the differential-mode sig nals could pass throu gh,
while the common -m od e responses s hou ld be supp ressed at
least across the differential passband.
Due to the increase demand for w ideb and applications, es-
pecially the ultra-wideband (UWB 3.1
10.6 GH z) systems,
numerous interest has b een aroused from both academ ic and
industrial areas towards differential U WB techno logy [1]– [6]
In [1], a branch-line structure with additio nall y attached open-
circuited stubs was constructed and applied to simultaneously
achieve a wide differential-mode passband and com mon-mode
stopband with a fractional bandwi dth of only 65%. With six-
stage branch-lines, it was extended to design differential UWB
BPF with fractional ban dwi dth 117% in [2], however, the cir-
cuit size was absolutely enlarged. In [3], the double-sided par-
allel-strip line with branch-line s tr ucture was proposed to desig n
differential UWB BPF. A com pact differential UWB BPF based
on th e transversal signal-interference concept was designed in
[4], w hile the b alanced input/output ports were in the oppo-
site position, which was unsuitable in real applications. In [5],
a d ifferential BPF using a three-layer broadside-coupled struc-
ture was presented with compact size, UWB differential pass-
Manuscript received June 0 5, 2012; accepted August 03, 2012. Da te of pub-
lication August 21, 2012; date of current version August 30, 2012. This work
was supported by the National Natural Science Foundation of China (61171029
and 61101016).
The authors are w ith the School of Electronic and Information Engi-
neering, South China University of Technology, Guangzhou, China (e-mail:
xiaohu.wu@mail.scut.edu.cn; qxchu@scut.edu.cn).
Color versions of one or more of the figures in this letter are available online
at http://ieeexplore.ieee.org.
Di
gital Object Identifier 10.1109/LMWC.2012.2213075
Fig. 1. (a) Circuit scheme of the proposed differential UWB BPF. (b ) Differ-
ential-mode circuit. (c) Common-mode circuit.
band and all-stop pe rforman ce under com m on-mode operation.
BasedonT-shapedstructure,widebanddifferentialBPFin[6]
was designed with 3 dB fractional b andwidth of 70%. Conse-
quently, it is sill a challenge to design BPF w ith com pact size,
UWB differential passband and UWB common-mode suppres-
sion performances.
A differential U W B with compact size and planar structure is
presented. The presented circuit operates as an U WB BPF under
differential-mode, whereas it performs as a UWB bandstop filler
(BSF) und er common-m ode. The measured resu lts show that
the 3 dB differential passband covers frequenc y range of 2.6
10.3 GHz while the 10 dB stopband of commo n-mo de re-
sponses covers from 2.6 to 12 GHz. The measured results are in
good agreement with the simulated pr edict ions.
II. D
ESIGN AND CHARACTERISTICS OF THE
PROPOSED DIFFERENTIAL UWB BPF
Fig. 1(a)
is the circuit scheme of the proposed differential
UWB BPF.
The four-port circuit is ideally symmetric with re-
spect t
o the central plane
. Here, all the transmission lines
inclu
ding the parallel-coupled lines are
with respect to the
centr
al frequency
of the differential passband, i.e.,
. For differential-mode operation, the central plane be-
com
es a perfect electric wall, and the differential-mode circuit
is
as shown in Fig. 1(b). It is a two-stage branch line structure,
1531-1309/$31.00 © 2012 IEEE
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