Design Methodologies of LTCC Bandpass Filters, Diplexer, and ...

IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 54, NO. 2, FEBRUARY 2006 717
Design Methodologies of LTCC Bandpass Filters,
Diplexer, and Triplexer With Transmission Zeros
Ching-Wen Tang, Member, IEEE, and Sheng-Fu You
Abstract—A new method for designing the multilayered filter,
diplexer, and triplexer is proposed in this paper. The main structure
of the multilayered filter is a parallel-coupled line connected
with a capacitor. By properly controlling the coupling coefficient
between the parallel-coupled line and the capacitor, the transmission
zero will appear at the lower or higher passband’s skirt. Moreover,
such characteristics can be employed to design the diplexer as
well for dual- or quad-band portable telephones. In order to miniaturize
the size of the circuit and to improve its performance, multilayered
structure and the low-temperature co-fired ceramic technology
are employed to design and fabricate the filter. Theoretical
analysis and design procedures are also provided. Measurement results
of fabricated examples are shown to match well with the electromagnetic
simulation, which validate the proposed structure.
Index Terms—Bandpass filter, diplexer, low-temperature cofired
ceramic (LTCC).
Fig. 1.
Architecture of the proposed second-order combline bandpass filter.
I. INTRODUCTION
MULTIPLE OR wide bands are the current trends in the
modern telecommunication system. In order to realize
multiband behavior, RF transceivers with larger bandwidth and
higher flexibility are utilized. To simultaneously achieve multifunction,
high performance, and smaller chip size, the technologies
of integrating passive circuits are attractive for microwave
and millimeter-wave applications. To this end, low-temperature
co-fired ceramic (LTCC) [1]–[9] is one of the most efficient
methods for miniaturizing and packaging technologies
[10]–[13], as LTCC can bundle both passive and active components
into a single module to meet the system-in-a-package
(SiP) requirement.
The bandpass filter is one of the most important components
in the RF front-end. It can select passband frequencies and reduce
interferences arising from adjacent frequency channels.
Bandpass filters are used to design the diplexer and triplexer
[14]–[18].
In this paper, we propose a novel design of a bandpass filter,
as shown in Fig. 1, which has the advantage of controlling the location
of a transmission zero via adjusting the coupling scheme
of the parallel-coupled line and cross-coupled capacitor. A multilayered
diplexer and triplexer developed with the proposed
Manuscript received July 20, 2005; revised October 5, 2005 and October 24,
2005. This work was supported in part by the National Science Council, Taiwan,
R.O.C., under Grant NSC 94-2213-E-194-028.
C.-W. Tang is with the Department of Electrical Engineering and Department
of Communications Engineering, Center for Telecommunication Research,
National Chung Cheng University, Chiayi 621, Taiwan, R.O.C. (e-mail:
cwtang@ccu.edu.tw).
S.-F. You is with the Department of Electrical Engineering, National Chung
Cheng University, Chiayi 621, Taiwan, R.O.C.
Digital Object Identifier 10.1109/TMTT.2005.862638
Fig. 2. Equivalent circuit of the second-order combline bandpass filter.
(a) Transformed cross-coupled capacitor C . (b) Transformed parallel-coupled
line.
bandpass filter are presented. This paper is organized as follows.
In Section II, we first present a theorem that is required for filter
synthesis. Structures of filters with transmission zeros are then
provided in Section III. Finally, some designs and fabrications
of both the diplexer and triplexer are given in the Section IV.
Our conclusion is given in Section V.
II. DESIGN THEORY
To analyze the proposed filter, we will adopt the technique of
the immittance inverter [19], [20]. The second-order combline
bandpass filter is composed of a cross-coupled capacitor ,a
parallel-coupled line, and two grounded capacitors . Fig. 2
shows the equivalent circuits of the transformed cross-coupled
capacitor and the transformed parallel-coupled line. The
equivalent circuit and even- and odd-mode line impedances can
be expressed as
(1)
0018-9480/$20.00 © 2006 IEEE

718 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 54, NO. 2, FEBRUARY 2006
Fig. 3.
filter.
Equivalent circuit of the proposed second-order combline bandpass
Fig. 5. Transformations for the matching circuit in the source and load ports.
(a) Positive J-inverter. (b) Negative J-inverter.
is resonance at the central frequency, which will result
in . The capacitor can be derived as
Fig. 4.
Equivalent circuit of the generalized bandpass filter.
where and are, respectively, the odd- and even-mode
admittances of coupled transmission line, and is the corresponding
electric length.
Adding two circuits shown in Fig. 2, and shunting with two
grounded capacitors at two ports separately yields the complete
equivalent circuit of the bandpass filter, as shown in Fig. 3.
Thus, we can apply the immittance inverter to analyze and design
bandpass filters. The admittance inverter and capacitor
are derived as
Following the equivalent circuit in Fig. 3, the equivalent circuit
of the generalized bandpass filter can also be expressed as
Fig. 4, which includes the admittance inverter. The admittance
inverters, susceptance, and its slope parameter are, respectively,
given by
(2)
(3)
(4)
(5)
(6)
(7)
(8)
(9)
(10)
where the ’s are the element values of the prototype lowpass
filter, is the fractional bandwidth, and and are
the impedances of source and load transmission lines, respectively
[19].
(11)
where and are electrical length and angular frequency at
the central frequency, respectively.
The bandpass filter with a transmission zero located at the
angular frequency will make . at the center
frequency and can be derived as
when
when
(12)
(13)
The matching circuits located at source and load ports,
as shown in Fig. 1, can be implemented using the following
methods. Note that the quarter-wavelength transmission line is
the simplest form of inverters and, on the other hand, the inductance
or capacitance -network to substitute the -inverter
is the other method. Since the negative inductance or capacitance
cannot be employed to the source or load impedance, a
transformation is needed, as shown in Fig. 5. These equations
are revealed as
III. FILTER DESIGN WITH TRANSMISSION ZERO
(14)
(15)
(16)
(17)
Here, two examples are presented to explain the location of
the frequency of the transmission zero, which will appear at the
lower or higher skirt of the passband.

TANG AND YOU: DESIGN METHODOLOGIES OF LTCC BANDPASS FILTERS, DIPLEXER, AND TRIPLEXER WITH TRANSMISSION ZEROS 719
Fig. 6. 2.4-GHz bandpass filter with the transmission zero at the frequency of
1.9 GHz. (a) Filter architecture. (b) Simulated results.
A. Circuit Model
The first example is a 2.4-GHz bandpass filter. Its center frequency
is set at 2.4 GHz and the frequency of the transmission
zero is set at 1.9 GHz. The ripple, bandwidth, impedance, and
electric length of the coupled transmission line are chosen as
0.01 dB, 6%, 30 , and 12 , respectively. By making use of
(1)–(17) derived in Section II, one can show that the parameters
of the cross-coupled capacitor , even- and odd-mode impedances
and of the parallel-coupled line, and grounded capacitors
are equal to 3.93 pF, 39.24 , 24.28 , and 6.47 pF,
respectively. Therefore, is 0.0206. A quarter-wavelength
transmission line is utilized to form the matching circuit, and,
hence, the impedance of this transmission line is 48.47 , which
is close to 50 , which is the vale of the system impedance.
Therefore, the matching circuit can be neglected and the secondorder
combline bandpass filter can be connected directly to the
input and output ports, as shown in Fig. 6(a). These derived parameters
and are substituted into the circuit
simulator such as ADS or equivalent software to carry out the
circuit simulation. Simulation results of the 2.4-GHz bandpass
filter are presented in Fig. 6(b).
Fig. 7. 2-GHz bandpass filter with the transmission zero at the frequency of
2.5 GHz. (a) Filter architecture. (b) Simulated results.
A 2-GHz bandpass filter is taken as the second example. Its
center frequency is set at 2 GHz and the frequency of the transmission
zero is set at 2.5 GHz. The ripple, bandwidth, impedance,
and electric length of the coupled transmission line are set as 0.01
dB, 7%, 30 , and 30 , respectively. The structure in Fig. 5(b)
can be applied to the transformation for the matching circuit in
the source and load ports. Fig. 7(a) shows the equivalent circuit
of the bandpass filter with the transmission zero at the higher skirt
of the passband. Here, the parameters of the cross-coupled capacitor
, even- and odd-mode impedances and of the parallel-coupled
line, the grounded capacitors , and source and
load capacitors are calculated as 1.26 pF, 54.96 , 20.63 ,
2.54 pF, and 0.8 pF, respectively. Simulation results of this 2-GHz
bandpass filter are presented in Fig. 7(b).
B. EM Simulation and Measurement
Prior to designing the circuit, the exact parameter values of
ceramic sheets such as the dielectric constant and layer thickness
should be known first. These values are very significant to
extract physical parameters and can be critical in constructing
the equivalent circuit [21].

720 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 54, NO. 2, FEBRUARY 2006
Fig. 8. Fabricated 2.4-GHz bandpass filter with transmission zero at the
frequency of 1.9 GHz. (a) 3-D structure. (b) Measured and EM simulated
results.
Two filters mentioned above are fabricated with the substrate
of Dupont 951. Their dielectric constant and loss tangent are 7.8
and 0.0045, respectively. The 2.4-GHz LTCC filter is designed
on one upper layer with the sheet of 1.57 mil, six middle layers
with the sheet of 3.6 mil, and three lower layers with the sheet
of 1.57 mil. Its overall size is 110 mil 92 mil 28 mil. After
circuit simulation, these values are converted into the LTCC
structure. The simulation is carried out with the assistant of the
full-wave electromagnetic (EM) simulator Sonnet from Sonnet
Software Inc., North Syracuse, NY. In the 3-D structure, the parallel-coupled
line is placed on the lower layer to reduce the coupling
effect with other capacitors. Fig. 8(a) shows the three-dimensional
(3-D) structure of 2.4-GHz LTCC bandpass filter.
The on-wafer tester has been chosen to improve the accuracy of
measurement. The network analyzer Agilent N5230A PNA_L
is used to measure, and the short-open-load-through (SOLT) is
adopted to calibrate.
Fig. 9. Fabricated 2-GHz bandpass filter with transmission zero at the
frequency of 2.5 GHz. (a) 3-D structure. (b) Measured and EM simulated
results.
As shown in Fig. 8(b), the frequencies of measured and EM
simulated transmission zeros are 1.84 and 1.9 GHz, respectively.
At the frequency of 2.4 GHz, the measured and EM simulated
insertion losses are less than 1.44 and 1.3 dB, respectively; the
return losses are greater than 15 and 17 dB, respectively.
A 2-GHz bandpass filter is given as another design example.
This 2-GHz LTCC filter is designed on five upper layers with the
sheet of 3.6 mil, four middle layers with the sheet of 1.57 mil,
and two lower layers with the sheet of 3.6 mil. Its overall size
is 107 mil 102 mil 32 mil. The 3-D structure of the 2-GHz
LTCC bandpass filter is shown in Fig. 9(a). In the 3-D structure,
the parallel-coupled line is the same placed on the lower layer
to reduce the coupling effect with other capacitors. As shown
in Fig. 9(b), the frequencies of the measured and EM simulated
transmission zeros are 2.45 and 2.5 GHz, respectively. At the
frequency of 2 GHz, the measured and EM simulated insertion
losses are less than 1.45 and 1.26 dB, respectively; the return
losses are greater than 19 and 39 dB, respectively.
The LTCC technology is a kind of thick-film process. In order
to realize the physical 3-D circuit, the parasitic effect among ca-

TANG AND YOU: DESIGN METHODOLOGIES OF LTCC BANDPASS FILTERS, DIPLEXER, AND TRIPLEXER WITH TRANSMISSION ZEROS 721
Fig. 11. Diplexer designed with multilayered structure. (a) Photograph.
(b) Measured responses.
Fig. 10. Diplexer designed with multilayered structure. (a) Equivalent circuit.
(b) EM simulated responses.
pacitors may cause their model differing from the ideal values
provided by the circuit simulator. Figs. 8(b) and 9(b) show the
measured and EM simulated responses, which involve the parasitic
effect and substrate loss. These effects may cause the measured
and EM simulated results such as Figs. 8(b) and 9(b) to
not exactly be the same as the ones in Figs. 6(b) and 7(b).
In this paper, we design our examples with the substrate of
Dupont 951. Even though the thick-film process may provide
the linewidth variation within 5% in the plane surface, it can
still provide good repeatability. 1 Compared with their EM simulation,
the measured frequencies of transmission zeros in the
fabricated circuits merely shifted 3.2% and 2% downward, as
shown in Fig. 8(b) and 9(b), respectively. Moreover, with a fixed
sintering profile, stable dielectric constant and layer thickness
1 DuPont Green Tape Material Systems. [Online]. Available: http://www.
mcm.dupont.com/MCM/en_US/Products/greentape/green_tape.html
can be obtained in the LTCC process and the resultant fabricated
circuits are not sensitive to the temperature variation within the
range of 40 C– C.
IV. SYNTHESIS OF DIPLEXER AND TRIPLEXER
The proposed second-order bandpass filter can be employed
to develop the diplexer and triplexer.
A. Diplexer
The diplexer developed from 2- and 2.4-GHz bandpass filters
is taken as the design example. Two filters are connected
with the matching line to form a diplexer as shown in Fig. 10(a).
There are two transmission zeros, which can increase the isolation
between two frequency bands, generated by 2- and 2.4-GHz
bandpass filters. Their frequencies are 2.58 and 1.86 GHz, respectively.
Two passbands of 2- and 2.4-GHz bandpass filters
are within 1.8–2 and 2.4–2.5 GHz, respectively. The EM simulated
results of the diplexer are expressed in Fig. 10(b).
The diplexer is fabricated with the substrate of Dupont 951.
Its dielectric constant and loss tangent are 7.8 and 0.0045, respectively.
The LTCC diplexer is designed based on four upper

722 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 54, NO. 2, FEBRUARY 2006
Fig. 13. Triplexer designed with multilayered structure. (a) Photograph.
(b) Measured responses.
Fig. 12. Triplexer designed with multilayered structure. (a) Equivalent circuit.
(b) EM simulated responses.
layers with the sheet of 1.57 mil, six middle layers with the sheet
of 3.6 mil, and four lower layers with the sheet of 1.57 mil.
Its overall size is 250 mil 100 mil 34 mil. The photograph
of LTCC diplexer is shown in Fig. 11(a). The on-wafer tester
has been utilized in order to improve the accuracy of measurement.
As shown in Fig. 11(b), the frequencies of the measured
transmission zeros are 1.85 and 2.55 GHz. The measured insertion
losses within two passbands of 1.8–2 and 2.4–2.5 GHz
are less than 1.66 and 1.7 dB, respectively; the return losses are
greater than 11.2 and 14.4 dB, respectively. The measured isolation
(not shown in this paper) between two passbands is greater
than 30 dB. Figs. 10(b) and 11(b) show that the measured results
also match well with the EM simulation.
B. Triplexer
The diplexer developed in Section IV-A and the proposed
second-order bandpass filter connected with the matching line
could form the triplexer shown in Fig. 12(a). Three passbands of
1-, 2-, and 2.4-GHz bandpass filters are within 0.9–1, 1.8–2, and
2.4–2.5 GHz, respectively. There is a transmission zero generated
by the 1-GHz bandpass filter at the frequency of 1.9 GHz.
There are two transmission zeros generated by the diplexer composed
of 2- and 2.4-GHz bandpass filters at the frequencies of
2.56 and 1.88 GHz, respectively. The EM simulated results of
triplexer are expressed in Fig. 12(b).
The triplexer is fabricated with the substrate of Dupont 951
as well. Its dielectric constant and loss tangent are 7.8 and
0.0045, respectively. The LTCC triplexer is designed based on
four upper layers with the sheet of 1.57 mil, six middle layers
with the sheet of 3.6 mil, and four lower layers with the sheet
of 1.57 mil. Its overall size is 239 mil 214 mil 34 mil. The
photograph of the LTCC triplexer is presented in Fig. 13(a).
The on-wafer tester has been employed in order to improve
the accuracy of measurement. As shown in Fig. 13(b), the
frequencies of measured transmission zero are 1.88, 2.44, and
1.89 GHz for the 1-, 2-, and 2.4-GHz bandpass filters, respec-

TANG AND YOU: DESIGN METHODOLOGIES OF LTCC BANDPASS FILTERS, DIPLEXER, AND TRIPLEXER WITH TRANSMISSION ZEROS 723
tively. The measured insertion losses within three passbands
of 0.9–1, 1.8–2, and 2.4–2.5 GHz are less than 1.9, 1.55, and
2.4 dB, respectively; the return losses are the greater than 11,
13.6, and 10.8 dB, respectively. The measured isolation (not
shown in this paper) among three passbands is greater than
25 dB. As shown in Figs. 12(b) and 13(b), the measured results
also match well with the EM simulation.
V. CONCLUSION
The newly designed LTCC bandpass filters have been proposed
in this paper. This filter can be used to develop the
diplexer and triplexer as well. The fabricated bandpass filters,
diplexer, and triplexer with the advantages of high integration
and small size are very suitable for implementation in
the multichip module. With the parallel-coupled line and the
cross-coupled capacitor, transmission zeros, which can increase
isolation of different passbands, can be easily generated at lower
or higher passband skirts. Agreement between measurement
and theoretical prediction has evidenced the feasibility of our
study.
ACKNOWLEDGMENT
The authors would like to thanks the reviewers of this paper’s
manuscript for their helpful comments.
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Ching-Wen Tang (S’02–M’03) received the B.S.
degree in electronic engineering from Chung Yuan
Christian University, Chungli, Taiwan, R.O.C., in
1991, and the M.S. and Ph.D. degree in communication
engineering from National Chiao Tung
University, Hsinchu, Taiwan, R.O.C., in 1996 and
2002, respectively.
In 1997, he joined the RF Communication
Systems Technology Department, Computer and
Communication Laboratories, Industrial Technology
Research Institute (ITRI), Hsinchu, Taiwan, R.O.C.,
as an RF Engineer, where he developed LTCC multilayer circuit (MLC) RF
components. In 2001, he joined Phycomp Taiwan Ltd., Kaohsiung, Taiwan,
R.O.C., as a Project Manager, where he continues to develop LTCC components
and modules. Since February 2003, he has been with the Department of Electrical
Engineering and Department of Communications Engineering, Center
for Telecommunication Research, National Chung Cheng University, Chiayi,
Taiwan, R.O.C., where he is currently an Assistant Professor. His research
interests include microwave and millimeter-wave planar-type and multilayered
circuit design, and the analysis and design of thin-film components.
Sheng-Fu You was born in Changhua, Taiwan,
R.O.C., on December 12, 1980. He received the
B.S. degree in electronic engineering from Feng
Chia University, Taichung, Taiwan, R.O.C., in 2003,
and the M.S. degree in electrical engineering from
National Chung Cheng University, Chiayi, Taiwan,
R.O.C., in 2005.
His research interests include the design of
microwave planar and multilayered filers and associated
RF components.