International Journal of Engineering Trends and Technology (IJETT) – Volume 34 Number 7- April 2016
A Survey on Narrowband filtering Design
using Microstrip filters
Akhilesh J Malse1, Dr Reena Kulshreshtha2,
1
ME Student, 2Professor, Department of electronics, AISSMSCOE,
Kennedy Road, Pune-1, Maharashtra, India.
Abstract The increase in wireless communication
applications in the Radio frequency range has led to
an increase in the demand for compact microstrip
circuitss. With high performance circuits becoming
a trend, more components need to be placed into per
square inch area on the circuit, leading to compact
structures such as filters finding high utility in these
circuits. This paper reviews the basic structures and
modes of microstrip filters. Four modes of filtering
have been introduced with an attempt to give a
novice a general understanding of microstrip
resonators. Also, a single mode high-Q narrowband
filter along with its generic coupled microstrip lines
is presented in the paper.
mode resonator is the basic structure for the design
of a low pass, high pass, band pass and band stop
filters. N-number of single mode resonators is
required for an N-th order filter. One way of
reducing the filter size is to use multimode
resonators. If each multimode resonator has Nmodes, the number of resonators in the filter design
is reduced by a factor of N. Thus, if dual mode
resonators (N=2) are used, the filter size is halved,
which can be interpolated with other designs as well.
Multimode resonators can also be categorized
according to the number of resonant modes. They
include the dual-mode, triple-mode and quadruplemode resonators.
Keywords — High-Q, resonators, microstrip, filter.
II. WORKING PRINCIPLE OF MICROSTRIP
I. INTRODUCTION
Previously all microwave equipments utilized
waveguides, coaxial or parallel strip line circuits.
Recently, monolithic microwave integrated circuits
(MMICs) have been introduced which facilitates the
use of micro-strip lines and coplanar strip lines. An
advantage of using micro-strips is that they provide
accessible surface on which solid state devices can
be mounted. Conventionally, two conductors are
used for transmitting microwave energy.
The efficiency of a conductor can be made
maximum if its characteristic impedance is matched
at each of its terminals. All impedance elements are
assumed to be lumped constants. However, this
varies for long transmission lines spreading over a
wide range of frequencies. The frequency of
operation is high; as a result the inductances and
capacitances of short lengths of conductors are taken
into consideration.
Many applications in telecommunication and
wireless technology require compact radio frequency
(RF) and microwave bandpass filters. Compact
microstrip resonators with superior performance are
continuously developed to meet various demands.
These microstrip resonators can be classified into
single mode and multimode. The single-mode
resonators can be classified into six sub classes.
They are, half wavelength resonators, quarter
wavelength
resonators,
stepped
impedance
resonators, quarter-wavelength stepped impedance
resonators, patch resonators and ring resonators.
Each varying in shape and performance The single
ISSN: 2231-5381
RESONATORS
To understand the working of microstrip filters,
they have to be considered as coupled transmission
lines of ideal topology. For obtaining the desired
frequency variable, the shorted stubs take on the
appearance of an inductor and an open circuited stub,
that of a capacitor. The structure so formed consists
of assemblages of shorted stubs and open stubs and
cascaded transmission lines all having the same
phase length. This gives a transmission line
equivalent of lumped element components for a
filter. The filter characteristics are then obtained by
equating the impedances of the transmission lines to
the impedances of lumped elements of a previously
designed filter. A resonator is characterized by a
resonant frequency o. This frequency gives a rate
of periodic exchange of electric and magnetic energy.
Every resonator has a quality factor Q, given by
Q = *(time-average energy stored
system)/energy loss per second in system
in
This is, the ratio of average energy stored in the
resonator to energy lost per second in the resonator
system multiplied by the resonating frequency. This
is also a measure of the circuit losses in the resonator
system.
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International Journal of Engineering Trends and Technology (IJETT) – Volume 34 Number 7- April 2016
(a)
Fig. 1: A “C-shaped” single mode trisection
bandpass filter [17].
(b)
Fig 2: dual mode resonators centrally loaded with (a)
stepped impedance open circuited stub [3], (b)
grounding via [7].
III. CLASSIFICATION OF RESONATORS
There are different types of resonators, and as
discussed above, depending upon the shape and
performance characteristic of the resonator, it can be
classified as:
i)
Single mode
ii)
Dual mode
iii)
Triple mode
iv)
Quadruple mode
Even though, there are more types of resonators and
even more modes of operation of a resonator, this
paper will limit the discussion up to quadruple mode
resonators.
1) Single mode resonators: Filters that provide only
a single pass band/stop band are known as single
mode resonators. Different types and stages of
microstrip sections can be provided to improve the
filter characteristics. For eg a trisection filter shown
in Figure 1 can provide better filter characteristics
than a single or a dual section filter. Each section
can be interpreted as one filter stage, which performs
cumulatively as the sections increase, thereby giving
better filter characteristics. A hairpin, c-shaped, Eshaped are the common filter shapes that can be
interpolated to give multiple stages.
2) Dual mode resonators: Dual mode resonators
basically have the following design constraints [3]:
1. There must be at least a 90◦ separation between
the phases of the EM energies at the input and output
ports.
2.
The
resonator
must
have
a
discontinuity/perturbation to generate a reflected
wave against an incident wave in the resonator, and
3. The resonator structure must be symmetric.
The introduction of stubs or cuts onto the
symmetrical planes of the resonators makes the ring
or patch to split the fundamental resonant frequency
into two. Further, by making the ring structure
meandered, the size of the filter can be reduced. The
stop-band response of dual-mode ring resonators are
.
ISSN: 2231-5381
improved by designing defected grounding structure
and by adding a T-shaped coupling feed structure
The basic structure of the dual-mode centrally
loaded openloop resonator is a half-wavelength open loop
resonator. The half wavelength open loop resonator
is then centrally loaded with an open-circuited stub,
a short-circuited stub or a grounded via as shown in
Figure 2.
3) Triple-mode resonators: Open loop resonators
when centrally loaded with an open-circuited stub
and a grounding via, becomes a triple-mode
resonator as proposed in [5]. The open-circuited stub
of a triple-mode resonator can be replaced with an
open-circuited stepped impedance stub as given in
[6]. Deng et al.[7][8] researched a series of work on
triple-mode resonator design using a centrally
loaded stub with open-circuited U-shaped resonators,
T-shaped stub and two short-circuited stubs as
shown in Figure 3.
4) Quadruple-mode resonators: Classification of
quadruple mode resonators is done into following
categories, depending to their structures.
(i) Coupled-ring resonators [12],[13].
(ii) Five-section stepped impedance resonators
(Figure 4(a)) [9].
(iii) Tri-section stepped impedance resonators with
embedded stubs (Figure 4(b)) [10].
(iv) Centrally loaded quadruple-mode resonators.
A straight line resonator is centrally loaded with an
open circuited stepped impedance stub and two
additional stepped impedance stubs at the symmetry
plane of the resonator (Figure 4(c)) to form a
quadruple-mode resonator as proposed in [11].
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International Journal of Engineering Trends and Technology (IJETT) – Volume 34 Number 7- April 2016
Fig 3: open loop triple mode resonator
Fig 5: S-parameter frequency response of the cshaped microstrip filter shown in figure 1.
(a)
(b)
(c)
Fig. 4: Quadruple mode resonators
IV. NARROWBAND BANDPASS FILTER
Narrowband RF filters that are able to pass a band of
frequencies and reject the remaining through the
mechanism of impedance transfer in the pass band
reflecting incident energy in the stop band have
attracted significant attention due to the following
remarkable advantages:
i) Theoretically infinite attenuation despite the
resonators finite Q, and,
ii) Inherent input/output matching that allows their
direct integration without the need for external
matching networks.
These unique performance merits are obtained by
cascading in parallel all-pass and bandpass
microstrip networks so that signal cancellation
occurs at the overall output node. A typical structure
of a narrowband filter along with the generic
coupled microstrip lines is as shown in Figure l. The
S-parameter frequency response of such a c-shaped
microstrip filter (Figure 1) is shown in Figure 5. It
can be observed that the filter stopband bandwidth
(BW) and the roll-off slope are a function of the
resonators Q. A narrower BW and a steeper slope
are attained as higher Q values are considered. The
application of such filters in an interference scenario
where strong jammers are closely spaced to the
receive band, narrowband filters with a manifold
value of Q need to be cascaded in series with a
preselect pass band in order to efficiently transmit
the passband and suppress interference without
affecting the receive band. As discussed in [1],
another method to realize a high Q narrow band
bandpass filter is the ABSF (absorptive band stop
ISSN: 2231-5381
filter). Such a filter can be realized using waveguide
resonators. However, their size is impractically large,
which hinders their applicability in mobile systems.
On the contrary, ABSFs with compact volume are
limited to fractional bandwidths >4% [14], [15], [16].
A trisection filter, as shown in fig. 1 is considered
here for understanding the high Q and filter response
characteristics of a narrowband filter.
The Q-factor is given by:
Q = /2
Therefore, for a C-shaped trisection filter, as shown
in figure 1, the Q factor is 14, as central frequency is
2.515 GHz and bandwidth is 0.09 GHz.
V. CONCLUSION
Modifications to the structure of microstrip
resonators provide different modes of operation,
thereby giving a wide frequency range for usability.
Moreover, the narrowband filters, characterised by a
high Q frequency response depicts the frequency
selectivity of a filter, thereby resulting in lower
bandwidths for filters with higher Q values. Most
multistage filters start with a relatively high-Q value
and exhibit a significant Q degradation over the
frequency range. This problem in an ABSF can be
easily overcome by employing a tuning device, to
maintain the filter characteristics over the entire
frequency range. A screw or a disk attached to a
moving mechanism and is penetrating inside the
resonator cavity to tune its resonance frequency is
one of the methods to overcome Q degradation. As
the resonator is tuned over a wide tuning range, the
screw/disk needs to have more penetration inside the
resonator, which reduces Q significantly. A similar
effect takes place when using MEMS/varactors to
tune resonators, such high loading capacitance can
significantly reduce the resonator Q value even if the
varactor is assumed loss less [1].
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