Using the branchline directional coupler in the design of microwave bandpass filters
ABSTRACT This paper addresses the application of the branchline directional coupler to the design of microwave bandpass filters. The basic idea consists of using the branchline coupler as a transversal filtering section by loading the coupled ports of the coupler with suitable transmissionline segments ending in an open circuit and taking the isolated port as the output node. Thus, under the signal interference philosophy involved in classic transversal filter schemes, bandpass transfer functions with perceptible stopbands and sharp cutoff slopes are derived. Furthermore, the main characteristics of the synthesized filtering response, such as the bandwidth or the position of the outofband power transmission zeros, can be easily controlled by means of the design parameters of the transversal section. Hence, a large variety of bandpass filtering profiles different from those offered by classical filter schemes can be realized. Finally, the experimental usefulness of the transversal filtering section based on the branchline coupler is proven with the design and construction in microstrip technology of two microwave bandpass filter prototypes at 5 GHz.

Conference Paper: Application of generalized Bagleypolygon fourport power dividers to designing microwave dualband bandpass planar filters
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ABSTRACT: A new type of microwave dualband bandpass planar filter based on signalinterference techniques is reported. The described filter approach consists of transversal filtering sections made up of generalized Bagleypolygon fourport power dividers. This transversal section, by exploiting feedforward signalinteraction concepts, enables dualband bandpass filtering transfer functions with several transmission zeros to be obtained. A set of closed formulas and guidelines for the analytical synthesis of the dualpassband transversal filtering section are derived. Moreover, its practical usefulness is proven with the development and testing of a 2.75/3.25GHz dualband microstrip prototype.Microwave Symposium Digest (MTT), 2010 IEEE MTTS International; 06/2010  [Show abstract] [Hide abstract]
ABSTRACT: A new class of microwave dualband bandpass planar filter is reported. It uses an original generalized branchline hybrid topology devised here, operating as dualpassband transversal filtering section. This type of filter network, under feedforward signalinteraction concepts, enables dualband bandpass filtering actions with transmission zeros to be generated. Some design formulas and guidelines helping to synthesize the proposed dualpassband transversal filtering section are given. Furthermore, to prove its practical viability, theoretical, simulated and experimental results of two manufactured sharprejection dualband bandpass microstrip filter prototypes are shown.IEEE Transactions on Microwave Theory and Techniques 01/2011; · 2.23 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A novel wideband bandpass filter with a very compact size is presented in this article. Using a sidecoupled stepped impedance resonator, wideband characteristics with adjustable centre frequency and 3dB fractional bandwidth can be obtained easily. Finally, a filter sample is designed and fabricated to provide an experimental verification on the proposed topology. Good insertion/return losses, flat group delay, wide bandwidth as well as ultra compact size are achieved as demonstrated in both simulation and experiment, which makes this filter a very promising candidate for applications in future wideband communication system.International Journal of Electronics. 08/2011; 98(8):11151121.
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IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 10, OCTOBER 2005 3221
Using the BranchLine Directional Coupler in the
Design of Microwave Bandpass Filters
Roberto GómezGarcía, Student Member, IEEE, José I. Alonso, Member, IEEE, and
Daniel AmorMartín, Member, IEEE
Abstract—This paper addresses the application of the branch
line directional coupler to the design of microwave bandpass fil
ters. The basic idea consists of using the branchline coupler as
a transversal filtering section by loading the coupled ports of the
couplerwithsuitabletransmissionlinesegmentsendinginanopen
circuitandtakingtheisolatedportastheoutputnode.Thus,under
the signal interference philosophy involved in classic transversal
filter schemes, bandpass transfer functions with perceptible stop
bands and sharp cutoff slopes are derived. Furthermore, the main
characteristics of the synthesized filtering response, such as the
bandwidth or the position of the outofband power transmission
zeros,canbeeasilycontrolledbymeansofthedesignparametersof
the transversal section. Hence, a large variety of bandpass filtering
profiles different from those offered by classical filter schemes can
be realized. Finally, the experimental usefulness of the transversal
filteringsectionbasedonthebranchlinecouplerisprovenwiththe
designandconstructioninmicrostriptechnologyoftwomicrowave
bandpass filter prototypes at 5 GHz.
Index Terms—Branchline directional coupler, microstrip,
microwave bandpass filter, transmission line, transmission zero,
transversal filtering section.
I. INTRODUCTION
V
modern wireless and highspeed data communication appli
cations [1]. In the design of passive filters, the major issue
is the realization of low insertionloss and highselectivity
filtering responses to accomplish appropriate band selections
by efficiently rejecting spurious signals and outofband noise
[2]. Regarding active filters, some other important factors, such
as linearity, noise performance, and power transmission gain,
must also be considered [3].
Over the last few years, one of the most followed choices in
research into novel microwave filter topologies is based on the
use of circuits providing more than one inputtooutput signal
propagation path. Microwave transversal filters are a good ex
ponent of this trend, appearing from the extrapolation to the
analogdomainofthetheoreticalconceptsinvolvedinclassicde
ERY sophisticated filter solutions are required in the
development of highperformance RF subsystems for
Manuscript received March 3, 2005; revised May 11, 2005. This work was
supported in part by the National Board of Scientific and Technology Research
under Project TIC200204569C0201 and Project TIC200202657, and in part
by the Spanish Ministry of Education and Culture under a doctoral scholarship.
R. GómezGarcía and J. I. Alonso are with the Grupo de Microondas
y Radar, Departamento de Señales, Sistemas y Radiocomunicaciones,
UniversidadPolitécnicadeMadrid,
roberto.gomez.garcia@ieee.org; ignacio@gmr.ssr.upm.es).
D. AmorMartín is with INDRA Sistemas S.A., 28850 Torrejón de Ardoz,
Madrid, Spain (email: damor@indra.es).
Digital Object Identifier 10.1109/TMTT.2005.855140
28040Madrid,Spain(email:
signtechniquesofdigitalfilters[4].Intransversalstructures,the
inputsignaltobefilteredissplitintoamultiplicityofsubcompo
nentspropagatingthroughthedifferentfeedforwardsignalpaths
that make up the overall filter. Thus, the filtering action comes
about through the combination of these signal subcomponents
oncetheyhavebeen processed.By forcinga passbandconstruc
tive interference and outofband signal energy cancellations to
produce power transmission zeros, highselective filtering re
sponses with sharp cutoff slopes can be derived. Furthermore,
since only feedforward techniques and not feedback principles
are used in microwave transversal filters, instability problems
caused by the presence of active devices to carry out active fil
tering functions are avoided [5].
Traditionally, the main drawback to overcoming in mi
crowave transversal filter design has been the large number of
transversal branches needed to synthesize highorder transfer
functions, usually leading to circuits with excessive physical
dimensions. In the most basic transversal structures, which are
made up of constant amplitudeweight and timedelay blocks
and where interactions between signals is the only available
medium to define the bandpass filtering response, this imped
iment has become unaffordable [6]. Lately, the inclusion of
frequencydependent processing blocks in the filter branches
or even the use of more advanced feedfordward architectures
emerging as generalizations of the transversal arrangement
have allowed the size constraint in highselectivity situations to
be partially circumvented, but at the expense of increasing the
design complexity [7], [8]. The introduction of monolithicmi
crowave integratedcircuit (MMIC) technology has also been
important to demonstrate transversal filtering concepts in small
circuits for lowpass, highpass, bandpass, and stopband appli
cations with the tunability as an added feature [9], [10].
A new alternative to designing microwave bandpass filters
using signalinterference sections based on the branchline di
rectional coupler is presented in this paper [11]. The idea is to
use the branchline coupler as a transversal filtering section by
connecting the coupled ports to opened load stubs. Thus, taking
the isolated port of the coupler as the output node, two inputto
output signal paths are generated so that bandpass transfer func
tionscanbeobtainedthroughthetransversalcombinationofthe
signal components derived from the input signal and traveling
through these propagation paths.
The main advantage of the proposed transversal filtering
topology with respect to the existing ones is its capability to
achievehighselectivityperformancesbyproducingappropriate
amplitude and phase relationships between the transversal sig
nals to be combined. This is experimentally proven in this study
00189480/$20.00 © 2005 IEEE
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3222IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 10, OCTOBER 2005
Fig. 1.
directional coupler.
Detail of the transversal filtering section based on the branchline
with the construction and characterization of two microstrip
filter prototypes at 5 GHz. Other relevant features to be re
marked upon are the simplicity of the design process and the
flexibility to adjust the main characteristics of the transversal
section transfer function by acting on its design parameters.
II. TRANSVERSAL FILTERING SECTION BASED ON THE
BRANCHLINE DIRECTIONAL COUPLER
The detail of the transversal filtering section based on the
branchline directional coupler is given in Fig. 1. As shown, the
transversal section is made up of a typical branchline coupler
whosecoupledportsareloadedwithtransmissionlinesegments
ending in an open circuit. The output node of the transversal fil
tering section is just the isolated port of the branchline coupler.
The characteristic impedances of the transmission lines making
up the coupler are denoted as
and characteristic impedances of the load stubs are referred to
as
,, and,, respectively.
The operating principle involved in the proposed filtering
section consists of obtaining the overall frequencyselective
transfer function from the feedforward combination of the
signal components derived from the input signal, and propa
gating through the different paths of the branchline coupler
in the transversal configuration. The basic idea is to generate
power transmission zeros out of the intended filter passband
through destructive signal interactions, preserving a construc
tive interference at the center frequency. Thus, sharprejection
bandpass filtering responses with perceptible stopbands can be
derived.
The theoretical analysis of the branchline coupler in the
transversal configuration is described later. The aim is to es
tablish some design rules to obtain sharprejection bandpass
filtering responses by means of a suitable selection of the values
for the design parameters of the transversal section.
and. The electrical lengths
A. Design of the Load TransmissionLine Segments
A branchline directional coupler designed for a 3dB cou
pling factor is considered as an initial approach, i.e.,
and, where
The characteristic impedances
mission line segments are assumed to be equal to
generator and output load connected to the ports 1 and 4 of the
coupler are matched to the impedance
is the reference impedance.
and of the load trans
. The input
.
Under the previous conditions, the following matrix system
ofequationsrelatingtheincidentandreflected powerwavesref
erenced to the impedance
in the four ports of the coupler
andcan be established:
(1)
where
3dB branchline coupler [12], and
nary unity. Note that only four different scattering parameters
for characterizing the branchline coupler are needed as a result
of being a reciprocal lossless network with two planes of sym
metry. Furthermore, the following relationships derived from
the termination conditions of the directional coupler operating
as a transversal filtering section have been used:
are the scattering parameters of the
is the imagi
(2)
From solving (1), the following analytical expressions for
the reflection and transmission coefficients
transversal filtering section are obtained:
andof the
(3)
The bandpass filtering response of the transversal section
is strongly influenced by the action of the transmissionline
segments loading the directional coupler. The main task of
these load stubs is to generate appropriate amplitude and phase
relationships between the signal components to be combined.
Therefore, the suitable design of the electrical lengths
becomes a key issue to achieving highselective filtering
responses.
The following considerations are established to synthesize a
symmetrical bandpass transfer function with a maximum power
transmission at a specified center frequency
1) The power transmission maximum condition is fulfilled
by forcing a constructive interference at
passband of the transversal section transfer function is
not destroyed by the signal interaction. As the result of
the passiveness and lossless property satisfied by the
and
.
. Thus, the
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GÓMEZGARCÍA et al.: USING BRANCHLINE DIRECTIONAL COUPLER IN DESIGN OF MICROWAVE BANDPASS FILTERS 3223
transversal filtering section (i.e.,
), the aforementioned condition is achieved by
imposing
,
(4)
assuming that the 3dB branchline coupler is designed
for a perfect power division at
, i.e.,
(5)
where
coupler.
Equation (4) results in
is the scattering matrix of the 3dB branchline
(6)
where
is considered without a loss of generality.
2) The symmetry condition of the transversal filtering sec
tion response in relation to the center frequency
obtained from imposing the appropriate relationship be
tween the electrical lengths of the load transmissionline
segments. As is well known, the following properties are
satisfied by the scattering parameters of the 3dB branch
line coupler designed at
is
:
(7)
where
and the phase of a complex number.
From the definitions given in (3), it is deduced that the
properties (7) will also be assured for the reflection and
transmission coefficients of the transversal filtering sec
tion only if the following relation is imposed on the elec
trical lengths of the load stubs:
anddenote, respectively, the absolute value
(8)
Taking into account the linear frequency dependence of
the electrical length of a transmissionline segment as
, the previous condition leads to
(9)
From (6) and (9), the following relations are obtained:
(10)
Thus, by using the above expressions to design the load stubs,
symmetrical bandpass frequencyselective responses are de
rived from the transversal filtering section.
(a)
(b)
Fig. 2.
filtering section on the electrical lengths of the load stubs (?
? ? ?
? ?
? ? ). (a) ? ? ?. (b) ? ? ?.
Dependence of the power transmission response of the transversal
? ? ???,
The effect of the electrical lengths
transmission response of the transversal filtering section is
analyzed in Fig. 2. As shown, narrower bandwidths and
sharper passbandtostopband transitions are obtained when
higher values for the index
by increasing the frequency variation velocity of the phase
difference generated between the feedforward signals to be
combined since a faster phasedifference rotation implies a
signal interaction going from a passband constructive interfer
ence to a stopband suppression in a narrower frequency span.
Nevertheless, signalamplitude relationships are also important
in the proposed transversal scheme to obtain filtering responses
exhibiting a good outofband rejection performance. Thus, the
best transfer function regarding closetopassband selectivity
and stopband rejection is achieved for
(,
outofband power transmission zeros through a mutual can
cellation between signal components with optimum amplitude
and phase characteristics.
andon the power
are selected. This is caused
and
) as the result of generating
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3224IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 53, NO. 10, OCTOBER 2005
B. Control of Bandwidth and Power Transmission Zeros
The passband behavior and the outofband performance of
the transversal filtering section are strongly dependent on the
characteristic impedance design parameters. Thereby, these pa
rameterscanbeusedforadjustingthemaincharacteristicsofthe
resulting filtering response such as the bandwidth, the location
of the outofband power transmission zeros, or the attenuation
levels in the stopbands.
Here, the following considerations are taken into account.
1) The transversal filtering section behaves as a
teristicimpedanceandquarterwavelengthlongtransmis
sionline segment at the center frequency
(,
relation
is forcedto accomplish thepowertrans
mission maximum requisite at
2) The symmetry condition of the transversal filtering sec
tion response in relation to
(10), not depending on the characteristicimpedance de
sign parameters.
Thus, the design parameters to be used for controlling the
transversal filtering section performance are
The dependence of the transversal section transfer function on
these parameters has been researched (case
The main results are described below.
charac
for,
). Therefore, the
.
is assured by satisfying
,, and.
and).
• The characteristic impedances
stubs are useful to adjust the bandwidth of the transversal
section filtering response. This is proven in Fig. 3(a) for
the design parameter
. As shown, a narrower band
widthisobtainedwhenahigher
thermore,althoughboththeattenuationlevelsandspectral
width of the stopbands are slightly reduced with the de
crease in the bandwidth, the general shape of the outof
band filtering response is preserved.
• The characteristic impedance
the branchline coupler is appropriate to control the
outofband performance of the transversal filtering sec
tion. This is demonstrated in Fig. 3(b). As observed, both
the spectral width and power rejection levels of the stop
bands are varied considerably by actingon the
the result of modifying the position of power transmission
nulls. Moreover, stopband control is achieved without
distorting the filter passband.
Theoutofbandpowertransmissionzerosareessentialtoob
tain filtering responses exhibiting a sharp filterflank steepness
and high attenuation levels in the stopbands. Therefore, the se
lection of the values for the transversal section design param
eters must always be directed to the generation of the power
transmission nulls.
The generation of outofband power transmission zeros de
pending on the characteristic impedance design parameters has
been researched. Specifically, the curves corresponding to the
3dB relative bandwidth
dBand the relative spectral sep
aration between the adjacent transmission zeros
and of the load
valueisselected.Fur
corresponding to
value as
of the
(a)
(b)
Fig.3.
the 3dB relative bandwidth. ?
between adjacent transmission zeros. (a) Influence of the characteristic
impedance ?
on the power transmission response of the transversal filtering
section (? ? ?, ? ? ?, ?
? ? ???, ?
control. (b) Influence of the characteristic impedance ?
transmission response of the transversal filtering section (? ? ?, ? ? ?,
?
? ?
? ? ? ? ). Outofband performance control.
Controlofthetransversalfilteringsectionperformance.?dBdenotes
refers to the relative spectral separation
? ?
? ? ). Bandwidth
on the power
transversal section transfer function as a function of the param
eters
,(,,
in Fig. 4. These curves have been obtained numerically, being
drawn only for the
value range in which the power trans
mission nulls are produced. As shown, the
be used is reduced when a lower
from Fig. 4 that there is an infinite number of solutions for the
parameters
andsatisfying a fixed bandwidth specifica
tion for the transversal filtering section. As a design guideline,
bandpass transfer functions with higher outofband attenuation
levels are obtained when lower
This is done at the expense of increasing the difference between
dBand
, i.e., decreasing the sharpness of the pass
bandtostopband filter transition.
) are detailed
value range to
value is chosen. Note also
andvalues are selected.
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GÓMEZGARCÍA et al.: USING BRANCHLINE DIRECTIONAL COUPLER IN DESIGN OF MICROWAVE BANDPASS FILTERS3225
Fig. 4.
and the relative spectral separation between the adjacent transmission zeros
?
(dashed line)of the transversal filtering section on the design parameters
? , ?
(? ? ?, ? ? ?, ? ? ?
Dependence of the 3dB relative bandwidth ?dB(continuous line)
? ? ).
III. EXPERIMENTAL RESULTS
The usefulness of the transversal filtering section based on
the branchline directional coupler in microwave bandpass filter
design is experimentally validated here.
Specifically, the design, construction in microstrip tech
nology, and characterization of both a passive and an active
microwave filter prototype at 5 GHz using the proposed
transversal filtering section is described below.
A. Microwave Passive Filter With SharpRejection Stopbands
The design of a microwave passive bandpass filter with
sharprejection stopbands has been approached. This kind of
filter is especially suitable for the input duplexer of transceiver
subsystemsdirectedatfullduplexcommunicationapplications,
where hard isolation levels between the adjacent channels cor
responding tothetransmitter and receiver modules are required.
The overall filter has been derived empirically, but using the
design rules provided in Section II, starting from the cascade
connection of two identical transversal filtering sections based
on the branchline coupler. Thus, without coupling gaps be
tween the transversal sections and the input/output lines, the
filter insertion losses are minimized by avoiding any radiation
and interstage mismatching losses. Consequently, the filter
noise performanceis alsoimproved.The initialspecificationsto
be met are a 5GHz center frequency, a 3dB relative bandwidth
equal to 10%, and a power transmission rejection level higher
than 40 dB in the bands allocated at 3.3–4.2 and 5.8–6.7 GHz.
Note that if the two transversal filtering sections are directly
cascaded, then the power transmission parameter of the overall
filter is
(11)
where
tering parameters of the transversal section. Obviously, the re
and are the reflection and transmission scat
Fig. 5.
filter.
Simulated power transmission response of the ideal designed passive
sult
range since the condition
fore, the overall filter should be optimized as a unit using the
design parameters of the transversal filtering section, the trans
missionline segment cascading the sections, and the filter input
line as degrees of freedom.
The commercial simulator HPEEsof Libra has been used in
the design and optimization process of the filter. For both the
transversal filtering sections, the obtained values for the char
acteristic impedances of thetransmissionline segmentsmaking
upthecouplerare
and
is thereference impedance. Theloadstubs are designed as 50
(,) line segments with electrical lengths
andat 5 GHz. The filter input line and the line
cascading the transversal sections are implemented as quarter
centerwavelengthlonglinesegmentswith25and 100
acteristic impedance, respectively.
The simulated power transmission response of the ideal de
signed overall filter is shown in Fig. 5. The attenuation mask
to be satisfied is also provided. As shown, a highly selective
filtering response with sharprejection stopbands is achieved
avoiding the use of crosscouplings.
The designed ideal filter has been constructed in microstrip
technology. The parameters of the selected Cuclad microstrip
substrate are a relative dielectric constant
thickness
m, and metal thickness
dimensionsofthelinesmakingupthefilterhavebeencomputed
making use of the line calculator LineCalc. A photograph of the
developed microstrip filter prototype is given in Fig. 6. When
the circuit size is a critical issue, further reductions in the filter
surface area can be achieved by using fractaltype geometrical
arrangements for the couplers and the load stubs [13] or other
miniaturization techniques [14]–[16].
The measured and simulated power reflection and transmis
sion responses of the constructed filter prototype are shown in
Fig. 7(a). The results corresponding to the single transversal fil
tering section are given in Fig. 7(b). These measurements have
is not satisfied in all the specified frequency
is not always met. There
,where
char
, dielectric
m. The