<,~ Substrae
A New Transformation of Bandpass Filter to Bandstop
Filter Using Multilayer-Technique and U-Defected Ground
Structure DGS
Ahmed BoLJTEJDAR, Anatoliy BATMANOV', Jan MACHAci, Edmund BuRTE', Abass OMAR], IEEE, Fellow
'Chair of Microwave and Communication Engineering University of Magdeburg, Germany
2Faculty of Electrical Engineering, Czech Technical University, Prague, Czech Republic
Ahmed.Boutejdargovgu.de
Abstract. A novel compact microstrip bandpass/Bandstop
filters realized by combining new Multilayer Method with
electrically coupled microstrip U-shaped DGS resonators, is
proposed The proposed filters have low insertion loss and
are compact in size due to the slow-wave effect. They also
have sharp transition regions due to the presence of twotransmission zeros on both sides of their passband. The
measured center frequency, bandwidth and insertion loss are
3.4 GHz, 4O0'o, and 0.5 dB respectively. The simulated and
measured results show good agreement and validate the
proposed approach.
filters, especially narrow-band bandpass filters which play an
important role modem communication and electronic systems
[1]-[2]. In order to obtain compact three-pole microstrip
bandpass filters with two transmission zeros, low rejection
band performance, low insertion loss in passband and high
selectivity, several structures were proposed such as the filters
using end-coupled slow-wave resonators, slow-wave openloop resonator filters and slow-wave open stub-tapped
resonator filters. Another techniques involve employing cross
coupling and Quasi-elliptic function filters which are able to
place the transmission zeros near the cutoff frequencies so
that higher selectivity with less resonators can be obtained.
Keywords
U-Resonator
50Q.-Microstrip line
DGS, Microstrip slots, Suspended layer, BPF, BSF.
Substrate
IUha G
1. Introduction
Metallic Ground
plane
Recently, defected ground structures (DGS) and
electromagnetic band gap (EBG) structures have received
increased attention because of their use in mobile
communication systems which require compact high
performance filters and couplers. One of the very successful
approaches to achieve siginifcant size reduction is to use DGS
components which also has the capability of suppressing
harmonics. DGS elements can be used in various kinds of
components such as lowpass filters and bandpass filters as
well as RF phase shifters. DGS, which evolved from EBG are
realized by etching certain patter in the metallic ground
plane of a microstrip line which perturbs the curent
distribution causing an increases in the effective inductance
and capacitance of the line. Thus, a DGS elements is
equivalent to an LC circuit. Planar bandpass filters [1] have
been extensively studied and exploited as key circuit blocks
wit oprtn fucin of inbn trnmiso an out-of-1
ban reecio. To mee th reureet in moe
.irls
communication, much effort has been made in the past years.
to develop a variety of compact bandpass filter with sharp and
dee reeto ousd the pasbn by genrain
tranmisionzersorattnuaionpols. Rcen adanc in
high-empeaturesupecondcting (HTS circits nd
,
. has
. , .,
microwave monolithic integratedl circuits ..(MMIC)
additionally stimulated the development Of various planar
978-1-4244-2138-1/08/$25.OO ©)2008 IEEE
Fig. 1. Three-dimensional view of the U-Head-DGS cell.
In order to improve the selectivity and efficiency of spectrum
..
utilisatn,
erswit two or mrestorasmission zerosthav
bed aed . Usn wo 4resnto with a
stub[3
trappe inveerr lolwa proposedstopachiein ahsingle
tansmssonzero at origh toto zero mpedance
i ch t
attached open-ended stub iS
at
attributed
i
X/2l. ine
Another ttwo ttransmission zeros
resonator).
s
. A
in [4], were realized using open stub lines and DGS-circles. In
[3], a slow-wave resonator filter using two coupled hairpin
microstrip folded Lines was presented. The bandpass filter,
designed at fundamental resonant frequency of the resonator,
show low insertion loss in the passband.
In this paper, a new type of compact microstrip bandpass
'
-G
fitr,wt'lwwv fetraie sn
resonators and Multilayer Method has been developed.
Transmission zeros can be implemented on both sides of the
passband. The proposed filter iS fabricated and measured. The
results were in good agreement of the
~~measurement
siuaonrul.
thseparated
2. Characteristics of the DGS
3. Design of the proposed Compact BPF
The proposed U-DGS in Fig. 1-2 is etched in the ground
plane and consists of two capacitive arms, which are
connected to a rectangular slot, with the same dimension as
shown in Fig. 2. The etched slot (DGS) is equivalent to a
capacitance. The metal area between the two arms on the top
layer corresponds to an inductance. The conventional circuit
parameters can be extracted from an electromagnetic
simulation by matching to a one pole Butterworth band stop
filter response, as in [5]. The U-Slot in the ground plane
excited by the 50Q line acts as a parallel resonant circuit [5].
It can be modeled by a parallel LC circuit as shown in Fig. 2.
The values of L and C can be computed using:
In order to improve the performance of the previous bandpass
filter shown in Fig. 3, a multilayer structure is used. The new
structure is similar to the original filter but the central DGS
resonator is moved to the bottom layer as shown in Fig. 3.
This was found to improve the performance and reduce the
overall size of the filter. The new bandpass filter has the same
bandwith (400 o) and the same center frequency (fO:3.4GHz)
but with improved passband characteristics. In order to obtain
the coupling matrix of the new topology, the specifications of
the filter are defined and then the desired parameters are
extracted by using an optimization-based scheme [6-7]. The
coupling coefficient and quality factor curves are then used
25
2 pF & C=
( )J
f Cp
C
nH
(1)
The values of the cut-off frequency f, and resonance
frequency fp can be found from the transmission
characteristics of the U-slot. The simulation results of the Uslot show one-pole low-pass filter characteristics. It is clear
that employing the slot in the metallic ground plane increases
the effective permittivity, leading to an increase of the
effective inductance of the microstrip line. From the Figs. 2.
we can see the dependence of the resonance frequency (cutoff
frequency) on the dimensions of the U-shape on the top and
bottom layers. The etched arm has a significant effect on the
resonance frequency. Actually, it is well known that an
attenuation pole can be generated by a combination of
inductive and capacitive elements. This explains the
frequency characteristic of the proposed U-DGS-element. Fig.
2 shows that if b is kept constant while varying d, it is easy to
control the positions of the (cutoff) and attenuation poles.
This means that the length of gap (arm) controls the effective
series inductance of a microstrip. The microstrip
12
to realize the obtained coupling coefficients. In our case the
third order filter is required to designed to have a bandwidth
BW = 1300 MHz, return loss RL = 20 dB, and center
frequencyfo = 3.4 GHz. The obtained coupling matrix from
the optimization scheme is
0
M
L0.721
0.721
(2)
0
and the external quality factors are qin= qout= 1.24. To realize
the normalized coupling matrix and quality factors, we use
the required fractional bandwidth FBW = BW/fo. The actual
(denormalized) coupling matrix becomes and Ql = Q2 = 3.1
where m = FBW x M , and Q = q/ FBW. The m-coupling
coefficients will be inserted in the The unknown distance s is
2mm. The proposed DGS-Bandpass filter was
0mF
0.288
o.288 o
(3)
50Q-Microstrip line
U-Resonator
10 < \3-jX ~~~~~c
10
trate
0
b >
~~ 4
''
-
-
<
2o
U- DGS
0
0
11
22
33
44
Lenght (d) of the arm in mm
55
66
Fig.2. ComparisonofresonancefrequencyofDGS(-) and Mircostrip
resonator ).
line on the top of the substrate in Fig. 2 has a width of
w=1 .9mm to obtain a 50Q2 characteristic impedance of the
microstrip line. The substrate dielectric constant is 3.38 and
its height is h=0.8 13mm. The dimensions shown in Fig. 3 are
g =lmm, b =3mm and d =1 1mm. The DGS cell is simulated
using Microwave Office. Simulation results are depicted in
Fig. 2, which shows the characteristics of a one-pole LPF.
X
i~~~
F
Metallic
Ground
~~~~
~~~~~plane
Fig. 3. Three-dimensional view of the new U-Bandpass filter.
simulated on a Rogers R04003 substrate experimental curve
[6] in order to get the optimal distance between the DGS
relative dielectric constant £r of 3.38 and a
~~~~~~~~~~resonators.with
thickness h of 0.813mm. Simulation is performed using
Microwave Office and CST Microwave Studio.M. All the
dimensions ofthe U-slot are g=lmm, b=3mm and d=ll mm.
The simulation results ofthe bandpass filter are shown in Fig.
5. Although the filter consists of three resonators, only two
poles exist in the passband. This is because one of the
resonators is in the bottom layer.
4 Design and Fabrication of the BPF
The simulation results showed that the designed filter has a
good sharpness factor, symmetrical response and smaller
losses in the pass band as shown in Fig. 5. In order to verify
the simulation results, the filter was fabricated and measured
using an HP8722D network analyzer. The measurement
results are shown in Fig. 5. together with the simulation
results. A very good agreement between the simulated and
measured results is observed. In the passband, the measured
insertion and return loss were less than 0.7dB and 20dB,
respectively. The results shows significantly improved
performance over the filters previously presented in [5-6].
The BPF was simulated and fabricated on a substrate with a
relative dielectric constant £r of 3.38 and a thickness h of
0.813mm. Simulation is performed using CST Microwave
Studio and Mocrowave Office. Fig. 4 shows photographs of
the fabricated BPF filter. The total area is 20 x 15Mmm2.
50Q-Microstrip
line
Ue
a
U-Resonator
_
Metallic
Ground
plane
Fig. 6. Three-dimensional view of the cascaded U-bandstop filter.
performance because the losses is not negligible in pass band
and the stop band is not large enough. In order to improve
these characteristics and in the same time to reduce the size of
the filter, we have used another reduction method, which will
be shown in later chapter.
-10
~~~~~~-15
-2
-06
-35
pg1
3
! 1X1 * l | 5i1 E _ -40
i
Si11
s21
Fig. 4. Photograph of the fabricated U-DGS BPF.
P
-10
0
6
8
10
-L
m-30
uo -3011
-40
-50
4
Frequency[GHz]
Fig. 7. Simulated S-parameters ofthe proposed cascaded BSF.
---
-20
2
6. Design of the Multi-Layer BSF
In order to improve the performance of the Previously
structure and further reduce its size, we moved one of the
- Measurement
~EM-Simulation
-601 2 3 4 5 6 7 8 9
Frequency[GHz]
Fig. 5. Measured and Simulated S-parameters of the proposed BPF.
-
5. Design of the Cascaded BSF
microstrip resonators to the upper layer, thus the new compact
PSF will be consist of two connected microstrip U-resonators
on the Top layer and one DGS-U-slot on the ground plane.
This proposed geometrical idea is based on the use of several
layers on top of each other. The new structure is similar to
three cascaded structure but the central resonator is moved to
theBottom layer. The simulation results of the resultingcom-
The Fig. 6. shows that the transformation of band-pass filter
band stop filter is feasible by connecting the three Microstrip
U-elements together. The 3 U-resonators are similar and it
presents a 3 pole cascaded band stop filter. The Fig. 6. shows
the cascaded structure, which is designed on a R04003S.srt
substrate with a relative dielectric constant £r of 3.38 and a
thickness h of 0.8 13mm. in this case, the ground plane is full- /
copper with a Thick of 35p~m and it is separated from top /_
layer through the substrate. Fig. 7. shows the simulated
parameters ofthe cascaded band stop filter. The filters'answer
shows that the structure has an insertion loss of 2 dB from DC
to 4.3GHz and the return loss is - 6dB over the both passbands The esig
of te casadedstrucure desn' givegood
50Q-Microstrip line
/Rot
U-Resonator
111\
;
~_
U- DGS
__
_
7
\
Metallic
GIround
plane
Fig. 8. Three-dimensional view of the proposed U-DGS-bandstop filter.
the etched arm is equivalent to a series inductance. So, the
DGS unit is equivalent to a resonant circuit, which is shown
in Fig. 2. the etched arm length. The slot head dimensions
were kept constant and the length of the arms was varied. The
simulation results are shown in Fig. 10. and Fig. 11. the
attenuation pole location moves up to a higher frequency,
while the length decreases.
-pact structure are much better, compared to the conventional
BSF structure. As shown in Fig. 9. and Fig. 8. We designed
and simulated this filter using AWR. This new modification
of the structure does not have a significant influence on the
performance of the filter as compared to the previous ones.
We can use the top and Bottom layers without sacrificing the
good response of the filter. The cutoff and resonance
frequencies did not change from their previous positions, but
the advantage is that, the new BSF is 35% more compact than
the conventional [3], as Fig. 8 shows.
10
0~~~~~~~~~~~~~~~~~~~~~~~1
/
C~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~"
-10X
d=1om;
Ci) -30
&m
-06
-40 811~~~~~~~~~~~~~~~~-5
-50
j~ ~ ~
t
-60
2
4
6
2
8
Frequency[GHz]
~=m
Frequency[GHz]
l0
Fig. 11. Simulated 21-parameters for different length, d.
Fig. 9. Simulated results of the new compact BSF.
8. Conclusion
7. Tuning of The Filter Characteristics
A new U-bandpass filter with multiple transmission zeros for
sharp transition band has been presented in this paper. In
order to realise a compact, symmetrical structure and to
simplify the implementation, DGS-method and coupling
method were used. The measured insertion loss and return
loss are less than 0.7dB and 20dB in the passband of 1.3GHz
at center frequency 3.2GHz, respectively. The proposed Filter
was designed, simulated, fabricated and measured. Good
agreement between simulated and measured results has been
achieved.
In order to investigate the frequency characteristics of the
etched slot, we simulated the DGS unit section using
Microwave Office. The variation of the dimensions of the
etched gap shifts the cutoff frequency and the attenuation pole
location in the frequency domain. As is well known, a
resonance frequency can be generated by a combination of
inductive and capacitive elements [3]. The etched gap area,
which is placed under the microstrip line, corresponds to
capacitance and the metal area, which is between the both
arms is equivalent to a series inductance. So, the DGS unit is
equivalent to a resonant circuit, which is shown in Fig. 2. The
parameters of this DGS equivalent circuit have been found
using curve-fitting. The found results were: C =0.33pF and L
2.33nH. Next we investigated the effect of 0.33pF of
References
[1]
Awida, Mohamed; Boutejdar, Ahmed; Safwat, Amr; El- Hennawy,
Hadia; Omar, Abbas Multi-bandpass filters using multi-armed open
loop resonators with direct feed In: 2007 IEEE MTT-S International
Microwave Symposium , Honolulu, Hawaii, June 03 - 08, 2007. <zrX7
[Piscataway, NY]: IEEE Operations Center, S. 913-916
[2] A. Abdel -Rahman, A. K. Verma, A. Boutejdar and A. S. Omar,
,,Compact stub type microstrip bandpass filter using defected ground
-plane," IEEE Microwave and Wireless Components Letters, vol. 14,
---pp. 136-138, 2004. (MTT- Journal).
[3] Boutejdar, Achmed; Elsherbini, A.; Omar, Abbas Sayed
--"Improvement of bassband and sharpness factor of parallel coupled
microstrip bandpass filters using discontinuities correction"
Mediterranea Microwave Symposium 2007. Budapest, pp. 121-124
X
_ l[4]X S. Amari, "Synthesis of Cross-Coupled Resonator Filters Using an
o
-Y
-10
-1 5 - -{ -\
-20 -t
-25
-3o0
-X
;1
-
i
/_l
Gradiant-Based Optimization Technique", IEEE Trans.
-35 _ t ~~~~~~~~~~~Analytical
Theory Tech., vol. 48, No. 9, pp. 1559-1564, Sep. 2000.
a / \1 11~~~~~~~~~~~~Microwave
|C. Kim, J. S. Park, A. Dal, and J. Kim, "A novel 1-D periodic
-40 X _~~~~~~~~~~~[5]
-45 ; :~~~~~~~~~~~~~~defected ground structure for planar circuits," IEEE Microwave
^{
-50( 3) 1 2 4
s
Frequency[GHz]
6
Fig. 10. Simulated 511-parameters for different length, d.
7
t
Wave Lett., vol. 10, pp. 131-133, Apr. 2000.
~~~~~~~~~~~~~~~~Guided
J. S. Hong and M. J. Lancaster, Microstrip Filters for RF/Microwave
~~~~~~[6]
[7]
Applications. New York: Wiley, 2001.
D. AHN, J. S. PARK, C. S. KIM, Y. QIAN, AND T. ITOH, "A design of
the low-pass filter using the novel microstrip defected ground
structure," IEEE Trans. Microwave Theory Tech., vol. 49, no. 1, pp.
86-93, Jan. 2001.