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Stopband-Extended Balanced Bandpass Filter
Using Coupled Stepped-Impedance Resonators
IEEE MICROWAVE AND WIRELESS COMPONENTS
LETTERS, VOL. 17, NO. 7, JULY 2007
Chung-Hwa Wu, Chi-Hsueh Wang, and Chun Hsiung Chen,
Adviser : Hon Kuan
Min-Hang Weng
Reporter : Yi-Hsin Su
Date : 2010/4/28
Outline




Introduction
Balanced Filter Structure
Design Of The Stopband-Extended Balanced Filter
Filter Responses
 Differential-Mode Response
 Common-Mode Response
 Conclusion
Balanced Circuits
 Balanced circuits play an important role in building a modern
communication system.
 Balanced circuits with differential operation show higher
immunity to environmental noise
 A well-designed balanced bandpass filter (BPF) should exhibit
the desired differential-mode frequency response and should also
be capable of reducing the common-mode signal at the same time.
Introduction
 Recently, a fourth-order balanced coupledline BPF using parallel-coupled-line
structures was proposed, which presents
both high selectivity and good commonmode rejection within the differential-mode
passband.
 However, This filter still has several
drawbacks, such as requiring via-hole
process.
 A novel fourth-order balanced BPF based on
λ/2 stepped-impedance resonators is
proposed to realize the desired differentialmode characteristics in addition to
overcoming the shortcomings
C.-H.Wu, C.-H.Wang, and C. H. Chen, “Novel balanced coupled-line bandpass filters with common-mode noise
suppression,” IEEE Trans. Microw. Theory Tech., vol. 55, no. 2, pp. 287–295, Feb. 2007.
Balanced Filter Structure
 The proposed fourth-order
balanced filter is composed of
four symmetric λ/2 SIRs, The
proposed balanced filter has
used the balanced feed structure
 This balance BPF is designed
and fabricated on FR4 substrate
having a thickness of 1mm, a
dielectric constant of 4.4, and a
loss tangent of 0.02.
Physical layout of the proposed fourth-order balanced
filter using symmetric=λ/2 SIRs.
Balanced Filter Structure
 Under differential-mode operation,
a virtual-short (PEC) would appear
along the symmetric-line, therefore
d
each resonator, resonating at f 0 ,
may be treated as a shorted λ/4 SIR.
 Under common-mode operation, a
virtual-open (PMC) would present
along the symmetric-line, thus each
resonator, resonating at
(
),
c
c
d
f0 f0 ＞ f 0
may be treated as a λ/2 resonator.
differential-mode
common-mode
Design Of The Stopband-Extended
Balanced Filter
differential-mode and common-mode external quality factors
against the tap position of I/O resonators.
dd
e
Q
g 0 g1

 9.55
FBW
( f 0 =1GHz and 3-db FBW=10%)
d
mixed coupling
M
dd
ab
FBW

 0.087
g1 g 2
Design Of The Stopband-Extended
Balanced Filter
M
dd
bb
FBW  J 2

 0.076
g2
d
f
0
(
=1GHz and 3-db FBW=10%)
M
dd
aa
FBW  J1

 -0.017
g1
Filter Responses
 The differential-mode response, the center frequency is at 1.02 GHz,
with a minimum insertion-loss of 3.51 dB and a bandwidth of 12%.
 The differential-mode spurious passband has been pushed up to 5.65 f 0d.
Filter Responses
 The common-mode signal is suppressed with a level of -34.46 dB
d
around f 0 ,and almost below 30 dB from 0.5 to 6 GHz.
 This spurious response has been suppressed due to the relatively weak
signal along the main path and the poor common-mode external quality
factor value associated with I/O resonators.
Conclusion
 In this letter, a novel fourth-order balanced BPF using symmetric λ/2
SIRs is proposed.
 To obtain the desired differential-mode frequency response and also
to avoid the undesired via-hole process.
 The open stub associated with tap position Lt has created an
additional. transmission zero