International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 1, January 2012)
Review of Impedance Matching Networks for Bandwidth
Enhancement
Rashmi Khare1, Prof. Rajesh Nema2
1
M.Tech Scholar, Department of Electronics And Communication, NIIST, Bhopal, M.P. India
2
Department of Electronics And Communication, NIIST, Bhopal, M.P. India
1
rashmi.khare17@gmail.com
rajeshnema2010@rediffmail.com
2
Abstract— A number of techniques can be used to eliminate
reflections when the characteristic impedance of the line and
the load impedance are mismatched. Impedance matching
techniques can be designed to be effective for a specific
frequency of operation (narrow band techniques) or for a
given frequency spectrum (broadband techniques). The
purpose of this paper is to make a comparative study on
bandwidth enhancement techniques of impedance matching
networks that help to overcome the bandwidth constraint of
transmission line. In this paper work narrowband impedance
matching networks like lumped element, single stub, double
stub, quarter-wave transformer and broadband impedance
matching techniques like binomial, and chebyshev are studied.
Depending on the application, matching may be required
over a band of frequencies such that the bandwidth of the
matching network is an important design parameter. If the
load impedance varies over a given range, a matching
network which can be adjusted or tuned as necessary. In
general, matching networks are constructed with reactive
components only so that no loss is added to the overall
network.
T-line or waveguide to termination matching network
Keywords— Bandwidth, Narrowband matching, Characteristic impedance, Broadband matching, Reflection coefficient
T-line or waveguide to T-line or waveguide matching
network
I. INTRODUCTION
In many cases, loads and termination for transmission
lines in practical will not have impedance equal to the
characteristic impedance of the transmission line. This
result in high reflections of wave transverse in the
transmission line and correspondingly a high VSWR due to
standing wave formations, one method to overcome this is
to introduce an arrangement of transmission line section or
lumped elements between the mismatched transmission
line and its termination/loads to eliminate standing wave
reflection. This is called as impedance matching. Matching
the source and load to the transmission line or waveguide
in a general microwave network is necessary to deliver
maximum power from the source to the load. In many
cases, it is not possible to choose all impedances such that
overall matched conditions result. These situations require
that matching networks be used to eliminate reflections.
In this paper, we will try to focus ourselves in
narrowband frequency impedance matching and broadband
frequency matching and compare their bandwidth.
Basically, we will try to match a load and transmission line
at a particular frequency and usually the fractional
bandwidth of the impedance matching is of the order of
few percentages only. In narrowband frequency impedance
matching we will discuss about quarter-wave
transformation, l-section impedance matching, single stub
matching and double stub matching and in broadband
frequency impedance matching or multi-section matching
transformer we will discuss about binomial multi-section
matching transformer and chebyshev multi-section
matching transformer.
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International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 1, January 2012)
Impedance matching networks at a single frequency can
This work is the continuation of the first national work
be designed without much difficulty to provide a reflection
on fabrication of RF MEMS devices. The device in this
coefficient of zero at the desired frequency. However, in
work is fabricated using the surface micromachining
many applications it is desirable to match impedances over
technology in the microelectronic facilities of Middle East
Technical University.
a range of frequencies. One method of improving control
over the bandwidth of the matching network is by adding
A paper by Alfred R. Lopez was published in 2004
one more element to the simple L-section lumped-element
which states many antennas can be characterized by their
(lumped-elements are feasible for frequencies up to about 1
radiation Q (The ratio of reactance to radiation resistance.
GHz.) matching circuit thereby creating a T-section.
A classic problem is determining the maximum possible
However, with the use of an iterative optimization routine,
bandwidth.
L-sections can simply be added to provide the required
impedance matching over a given bandwidth. It should be
noted that an iterative procedure is not necessary for fixed
known loads. However, it is very useful for loads that vary
Constrained by the maximum permissible reflection
with frequency and parasitic effects.
magnitude R, and the no. of tuned circuit n in the
One way of designing broadband matching networks is
impedance matching circuit.This paper presents the fano’s
relationship among Bn, Q, R , for any number of tuned
to use multiple sections of transmission line rather than just
circuits and these fano’s equations are solved by using
one section as in the case of the quarter wave transformer.
MATHCAD software. This paper presents a multiple
In order to simplify the analysis of these multiple section
tuning impedance-matching network it is clear that increase
matching networks, the theory of small reflections is
in bandwidth occurs when impedance matching circuit
utilized.
increase by one tuning level.
II. LITERATURE SURVEY
A paper by Bo-Yang Chang was published in 2005 that
describes a low-voltage; low-power ultra-wideband (UWB)
low-noise amplifier (LNA) for IEEE 802.15.3a. A
simplified Chebyshev filter is used to achieve the input
broadband matching. This input network has lower
complexity and good reflected coefficient from 3.1GHz to
10.6GHz. An output-matching buffer is designed specially
to match for maintaining high gain at upper frequency.
Therefore, it can both achieve flat gain over the whole
bandwidth and generate more output current. The LNA is
simulated based on TSMC 0.18μm mixed signal/RF
process. With only 1V bias voltage, the LNA can achieve
power flat gain of 10dB with input matching of -9.76dB;
the minimum noise figure 3.7dB; and input third-orderintercept point (IIP3) of -1dB. The power dissipation is
only 7.2mW.
A paper by Unlu Mehmet was published in 2003 that
describes how to design, model, and fabricate an RF
MEMS adjustable impedance matching network. The
device employs the basic triple stub matching technique for
impedance matching. It has three adjustable length stubs
which are implemented using capacitive loaded coplanar
waveguides. The capacitive loading of the stubs are
realized using the MEMS switches which are evenly
distributed over the stubs. There are 40 MEMS bridges on
each stub which are separated with λ/40 spacing making a
total of 120 MEMS switches in the structure. The
variability of the stub length is accomplished by closing the
MEMS switch nearest to the required stub length, and
making a virtual short circuit to ground. The device is
theoretically capable of doing matching to every point on
the Smith chart.
The device is built on coplanar waveguide transmission
lines. It has a center operating frequency of 10GHz, but
because of its adjustability property it is expected to work
in 1-40GHz range. It has dimensions of 8950 × 5720μm2.
A paper by S. F. Liu and X. W. Shi was published in
2007 that states Impedance-Matching technique is in
common use for antennas to broaden their bandwidth. Its
application in high temperature superconducting microstrip antennas is studied theoretically in this paper.
93
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 1, January 2012)
It is found that employing an impedance-matching network
In this way the ripple in the balun response due to the
directly to HTS micro-strip antennas to broaden their
variation of the UWB dipole impedance is reduced. A
bandwidth is of little significance.
balun for a UHF and UWB dipole antenna working from
220 to 820 MHz (bandwidth of 4:1) has been achieved with
A paper by G. Castaldi was published in 2008 that
losses lower than 1 dB. These types of baluns are
describes an exact synthesis method which allows the
particularly useful in the low microwave frequency band.
design of dual-band transformers with an arbitrary even
number of uniform sections giving equi-ripple impedance
III. PROBLEM ANALYSIS
matching in two separate bands centered at two arbitrary
For frequencies up to approximately 1 GHz, matching
frequencies. This method is a generalization of the exact
networks containing lumped elements (L-networks) may be
Collin-Riblet synthesis of Chebyshev single-band
used. The circuit elements (capacitors and inductors) must
transformers. As compared to a single-band Collin-Riblet
be small enough relative to wavelength so that the normal
transformer encompassing both required pass-bands, the
circuit equations for voltage and current are valid. This is
proposed design yields significantly better performance in
used for narrowband frequency impedance matching. We
terms of pass-band tolerance and width. Reflection
can obtain any value of reactance or susceptance with the
coefficient equation for chebyshev
proper length of short-circuited or open-circuited
transmission line, we may use these transmission line stubs
as matching networks. A single stub network suffers from
the disadvantage of requiring a variable length of t line
A paper by Y. Wu, Y. Liu and S. Li was published in
between the load and the stub. This may not be a problem
2008 that describes a compact pi-structure transformer
for fixed transformation network, but would pose some
operating at arbitrary dual band is proposed in this paper.
difficulty if an adjustable tuning network is desired.
To achieve the ideal impedance matching, the exact design
formulas with no restrictions are obtained. In addition, it is
Short-circuited or open-circuited transmission line, we
found that there are infinite solutions for this novel
may use these transmission line stubs as matching
transformer considering the fact that three independent
networks. A single stub network suffers from the
variables exist in two equations. Furthermore, to verify the
disadvantage of requiring a variable length of t line
design formulas, the reflection characteristics in different
between the load and the stub. This may not be a problem
cases are shown by numerical simulations. The horizontal
for fixed transformation network, but would pose some
length of this transformer is half of the Monzon’s dual band
difficulty if an adjustable tuning network is desired.
transformer. The proposed dual band transformer can be
A quarter-wave transformer (QWT) is a simple and
used in many compact dual band components such as
useful circuit for matching real load impedance to a
antennas, couplers and power dividers.
transmission line. An additional feature is that it can be
A paper by Vicente Gonzalez-Posadas was published in
extended to multi-section design for broader bandwidth.
2008 that describes a semi-lumped balun transformer for
Although quarter-wave transformer can in theory used to
UHF and ultra wideband (UWB) dipole antennas. The
match complex Impedance, it is more common to use it to
proposed structure is based on two asymmetric filters that
match real impedance. At the operating frequency fo, the
also transform the variable antenna impedance into the
electrical length of the matching section is o/4. But at
desired source impedance. These asymmetric filters make
other frequencies the length is different, so a perfect match
use of a binomial impedance transformer in each filter
is no longer obtained. The quarter wave transformer has a
section. The asymmetric filters (one low pass filter, LPF,
limited bandwidth, like other transformation methods and
and other high pass filter, HPF) allow the balun bandwidth
the transmission line must be placed between the load and
to be increased while the binomial transformer matches the
the feed line.
variable balanced dipole impedance.
94
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 1, January 2012)
For applications requiring more bandwidth than a single
In order to determine the variation of Zin with respect to
quarter wave section can provide, multi-section
frequency, we need to know the variation of the load
transformers can be used.
impedance with respect to frequency. According to the
design equations, we will find out the input impedances for
BINOMIAL TRANSFORMER
the two networks looking into the matching network input
ports are
•Impedance of consecutive 1/4 wave lines are proportional
to binomial coefficients.
•Gives maximally flat pass-band characteristic.
A Binomial Multi-section matching network will have a
perfect match at the frequency where the section lengths
are a quarter wavelengths!
CHEBYSHEV TRANSFORMER
•Wider bandwidth than Binomial Transformer for the same
number of ¼ wave sections.
• Ripple over pass-band.
For the case of the shunt stub networks, the input
admittance looking into the matching network is
A Chebyshev multi-section matching transformer can
provide even larger bandwidths than a binomial multisection matching transformer for a given number of
transmission line sections. The increased bandwidth of the
Chebyshev transformer comes at the cost of increased
ripple over the pass-band of the matching network.
However, we may still designate some maximum allowable
reflection coefficient for the design of the Chebyshev
transformer. The Chebyshev transformer exploits the
characteristics of the Chebyshev polynomials
We find Zin, l by inserting (l1,d1) and find Zin, 2 by inserting
(l2, d2).
Compare the frequency responses of the lumped element
matching networks and the stub tuners.
IV. METHODOLOGY
Ideal lumped element and single stub matching
networks provide perfect matching (Ã=0) at only one
frequency. However, the component configuration in a
lumped element matching network and the stub position in
a stub matching network will affect the frequency response
of the network away from the design frequency. We may
plot the frequency response of the reflection coefficient to
illustrate the different responses. Given either type of
matching network, the reflection coefficient looking into
the matching network may be written as
In case of binomial multisection matching transformer
the general form of the reflection coefficient approximation
for the N section matching transformer can easily be
written in terms of a binomial series according
to
Where A is the general amplitude coefficient and Cn is the
binomial coefficient given by
95
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 1, January 2012)
The percentage bandwidth of the transformer may be
References
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This study provided an insight in determining the
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matching transformer provide more enhanced bandwidth
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