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Design of Compact and Sharp-Rejection Ultra
Wideband Bandpass Filters Using Interdigital
Stepped-Impedance Resonators
IEICE TRANS. ELECTRON., VOL.E90–C, NO.8 AUGUST 2007
Cheng-Yuan HUNG,, Min-Hang WENG,, Yan-Kuin SU,
Ru-Yuan YANG, and Hung-Wei WU.
Adviser : Hon Kuan
Min-Hang Weng
Reporter : Yi-Hsin Su
Date : 2010/3/24
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Outline
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Introduction
UWB-BPF: Schematic and Principle
Experimental Results and Discussion
Conclusion
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Introduction
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A small size and high selectivity microwave bandpass filter (BPF)
is widely used to enhance the performance of radio frequency (RF)
front end.
In the past, conventional microstrip interdigital bandpass filters
(IDT-BPFs) using quarter-wavelength resonators were compact
while they require short-circuit connections with via holes.
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J. S. Hong and M. J. Lancaster, Microstrip Filters for RF/MicrowaveApplications. New York: Wiley, 2001, ch. 3.
Introduction
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New type of miniaturized IDT-BPFs by using pseudo-IDT structure
(PIDT) without via holes ground
These microstrip PIDT-BPFs fabricated on PCB have a relatively
small bandwidth, since the coupling level of interdigital electrode
using conventional design methods were limited.
This paper develop a very compact UWB-BPF with FBW larger
than 95% based on PIDT-SIRs. It is able to place two transmission
zeros near the passband edge so that higher selectivity with fewer
resonators could be obtained.
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UWB-BPF: Schematic and Principle
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Practical layout of the designed UWB-BPF
designed on a 0.787 mm-thick substrate with
a dielectric constant of 2.2.
The proposed UWB-BPF using
PIDT-SIR on commercial PCB
RT/Duroid 5880. This filter basically
consists of two identical SIR and
two tapped I/O at the two sides.
The SIR is symmetrical and has two
different characteristic impedance
lines, low-impedance (Z1) line in
center and two identical highimpedance (Z2) lines in two sides.
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UWB-BPF: Schematic and Principle
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The typical SIR with R = Z2/Z1 < 1, has shorter electrical length
and shifts the first spurious response mode fs1 to higher frequency.
The proposed UWB-BPF, the first two resonant modes ( f0 and fs1)
are used and taken into account together and they are applied to
create a wide passband. Therefore, the SIR with R = Z2/Z1 > 1.
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UWB-BPF: Schematic and Principle
1  x   t

1 
R  tan 1 tan 2
or
R   cot 1 tan 2


(1)
(2)
2
x t
2 
2
(3)
The low-impedance wavelength θ1 is not equal to high impedance
wavelength θ2.
The stepped percentage x is defined as the portion of θ1 and θ2 to the
total wavelength of the SIR (θt = 2(θ1 + θ2)).
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UWB-BPF: Schematic and Principle
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Resonant electric length of first spurious
response mode versus stepped percentage x
with impedance ratio R = 1, 1.2, 2, 3 as a
parameter.
It is found that the first
spurious response mode fs1
goes toward the fundamental
response mode f0 as stepped
percentage x is around 0.76.
The first spurious response
mode fs1 in SIR with more
lager R value is closer to the
fundamental response mode f0.
The first two resonant modes
can be combined together to
create a wide passband.
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UWB-BPF: Schematic and Principle
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Three cases are designed and simulated, including
case 1, case 2 and case 3 of SIR with the parameters.
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UWB-BPF: Schematic and Principle
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The simulated frequency responses of case 1,
case 2 and case 3.
In case 2 with R = 2 and x = 0.76, a
satisfied band at center frequency f0
= 7.1 GHz and bandwidth of 3.74–
10.43 GHz (3-dB FBW= 94%) is
obtained.
The frequency skirts of the passband
edge are very sharp, since two
transmission zeros are produced
from the multi-path effect of the
PIDT-SIRs
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Experimental Results and Discussion
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The fabricated filter consists of
two SIRs having the lowimpedance (Z1=69Ω) line section
with a strip width of 1.44mm and
the high-impedance (Z2=138Ω)
line sections with a strip width of
0.3 mm.
At the stepped percentage x =
0.76, the length L is 11.2 mm, t is
7mm
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Experimental Results and Discussion
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Predicted and measured frequency response of
the fabricated UWB-BPF. The designed
parameters are those in case 2.
The predicted and measured results
of designed UWB-BPF at center
frequency f0 = 7.1 GHz.
The measured results of the
fabricated BPF have low insertion
loss of −0.5±0.4 dB and bandwidth
of 3.68–10.46GHz (3-dB FBW=
95%).
The finite transmission zeros occur
in the lower side of passband edge at
3.26 GHz with −35 dB attenuation,
and in the higher side of passband
edge at 11.3 GHz with −38 dB
attenuation.
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Conclusion
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New type of miniaturized IDT-BPFs by using pseudo-IDT structure
(PIDT) without via holes ground.
UWB-BPF using pseudo-interdigital stepped impedance resonators
(PIDT-SIRs), with improved skirt characteristics and very compact
size.
The first two resonant modes of the SIR with stepped impedance
ratio larger than 1 are taken into account together and they are
applied to make up a wide dominant passband.
The multi-path effect of the PIDT-SIRs is a very useful feature for
enhancing the attenuation characteristics in the stopband of the
filter.
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