Narrow-Band, Band-Stop Filter Designs with Different Numbers of
L-Resonators
BUSRA OZTURK DOGANAY, OZLEM COSKUN
Department of Electrical-Electronics Engineering, Faculty of Engineering and Natural Sciences,
Suleyman Demirel University,
TURKEY
Abstract: - Filters have an important place in RF and microwave engineering. Filtering certain frequencies is
one of the most basic needs in receiver and transmitter systems. Today, microstrip filters are used in many areas
of microwave engineering such as radar systems, cellular communications, test/measurement systems, wireless
modems, radio and television receivers, and remote control systems. In these filter structures; high
performance, low loss, small size, and low-cost requirements are sought. In this study, narrow-band, band-stop
filter designs with different numbers of L-resonators at a frequency of 2.45 Hz were realized. The main goal of
this study is to design filters with higher performance, lower return loss, and more compact and useful sizes.
Thanks to these filter designs, undesirable signals are filtered and targeted signals are transmitted appropriately.
Key-Words: - 2.45 GHz, L-Resonator, Band-Stop Filter, Microwave Systems, Advanced Design System
(ADS), RF Engineering.
1 Introduction
Filters are a signal processing circuit that changes the
amplitude and/or phase characteristics of a signal on
the transmission line depending on the frequency.
They are frequently included in electronic circuit
designs to eliminate the unwanted frequency
component in a transmitted signal and to ensure the
transmission of desired frequency values, [1], [2].
Filters may consist of passive circuit elements such
as resistors, capacitors, and inductors, or may contain
amplifying elements such as transistors. Filters called
passive filters, which consist only of passive circuit
elements, do not require any power supply and can
operate properly even at very high frequencies. They
also have very low noise compared to active filters.
Their disadvantage is that they cannot provide signal
gain, [3], [4].
Microwave filters are of great importance in RF
and microwave applications. They combine or
separate different frequencies from each other. Filter
structures are frequently used in microwave systems,
especially in satellite and mobile communication
systems. In general, in devices such as oscillators and
mixers; to block unwanted signals, band-stop filters
are added to the structures. Many microwave systems
such as these include band-stop filter structures. To
meet the requirements in this context, filters are
designed as lumped element and discrete element
circuits, [5], [6], [7], [8], [9].
In an ideal band-stop filter, the attenuation in the
pass band is zero, the attenuation in the stop band is
infinite, and the transition from the pass band to the
stop band is extremely sharp. Although such ideal
filters are theoretically possible, they are not possible
in practice. Passband insertion losses are desired to
be as small as possible and stop band attenuations are
as high as possible, [10].
The authors designed a bandstop filter using L-
shaped resonator structures to be used in WLAN
applications. They made their design at 2.45 GHz
frequency and achieved -60 dB attenuation. They
simulated their designs in the ADS program using
FR4 material, [11].
They designed a wideband band-stop filter to be
used in the X band in their study. They created their
design by making 5 pairs of L-shaped studs on,
microstrip line. The operating frequency of the
created filter was controlled by changing the
dimensions of the L-shaped studs. Simulation results
and experimental results showed excellent
agreement, [12].
The authors designed a microstrip band-stop
filter in their study. They used L and T -shaped studs
together in the design and connected the T-shaped
resonator to the transmission line between two
identical L-shaped resonators. They calculated the
structure according to ABCD matrix analysis. With
this method, they designed two different filters and
worked in the X and Ku bands, [13].
In their study, they designed a narrow-band
Received: August 15, 2023. Revised: April 21, 2024. Accepted: May 11, 2024. Published: August 6, 2024.
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band-stop filter based on microstrip resonators
connected to the transmission line in an L-shape.
Three loosely coupled microstrip resonators are
cascaded to form a bandstop filter circuit. They
produced the designed circuit on Rogers RO4350B
substrate material. As a result, a narrow-band
absorber band-stop filter with a three-stage resonator
was designed for the 2.23 GHz center frequency.
They measured a return loss of better than 30 dB at
the center frequency and in the 20 MHz band, [14].
In this study, narrow-band, band-stop filter designs
with different numbers of L-resonators will be made.
2 Narrow-Band, Band-Stop Filter
Designs with Different Numbers of
L-resonators
FR4 material was chosen as the substrate in the
designed circuits. The purpose of choosing FR4
material is that it is easily accessible and has low
cost. The relative permeability coefficient of this
material at 1 MHz is 4.5, the loss tangent is 0.022
and the material height is 1.6 mm. In these designs,
the center frequency was chosen as 2.45 GHz. f1 and
f2 frequencies, band-stop filter response frequency
points were selected as 2.40 GHz and 2.50 GHz, and
-3 dB partial bandwidth was calculated according to
these values using the equation
Transmission lines are λ/4 long. Scattering
parameters, center frequencies and bandwidths were
calculated in the simulation environment using the
ADS (Advanced Design System) program, which
can perform two-dimensional microwave circuit
analysis. Values for Chebyshev low pass filters are
given in Table 1. In the designs, x/Z0 slope parameter
values were calculated using
Ω according to the values of Table
1.
Table 1. Values for Chebyshev low pass filters
( g0=1.0, Ωc=1)
3 Narrow-Band, Band-stop Filter
Design with Single L-Resonator
The circuit with a single resonator is designed and
shown in Figure 1. S21 values of the design are
shown in Figure 2.
Fig. 1: Single resonator narrow-band band-stop filter
design
Fig. 2: Simulation result of single resonator narrow-
band band-stop filter S21 value
By changing the gap value between the two
transmission lines shown in Figure 1 between 0.05
mm and 0.5 mm, the X/Z0 slope parameter curve was
optimized according to the gap value and is given in
Figure 3. Considering this curve in the designs, gap
values were calculated according to the x/Z0 slope
parameter value.
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Fig. 3: x/Z0 slope parameter curve according to gap
values
4 Narrow-band, Band-stop Filter
Design with Five L-Resonators
First, a circuit with five resonators was designed. The
design center frequency was selected as 2.45 GHz
and the -3 dB partial bandwidth was calculated as
0.0408 using
. While designing,
firstly the slope parameters for each resonator were
calculated using equation
Ω here, Table 1 is looked at for the
element values of the low-pass prototype. Then, the
gap values corresponding to the calculated slope
parameter values were obtained according to Figure
3. Design values for the five-resonator circuit are
given in Table 2.
Table 2. Design values for five-resonator circuit
Considering the design values for the five-
resonator circuit given in Table 2, the circuit given in
Figure 4 was designed. The designed circuit was
simulated, scattering parameter values were
calculated and shown in Figure 5.
Fig. 4: Narrow-band bandstop filter design with five
resonators
Fig. 5: Narrow-band band-stop filter with five
resonators S-parameter values simulation result
When the simulation result of the circuit design
with five resonators is examined in Figure 5, the S21
value is observed to be -60 dB with a center
frequency of 2.45 GHz. In the design, -3 dB
bandwidth is calculated as 13.9%. Then, after the
design shown in Figure 4 was produced, the
scattering parameters of the design were measured
using the Rohde & Schwarz ZVA 24 vector network
analyzer as shown in Figure 6. The measurement
results are given in Figure 7.
Fig. 6: Narrow-band L-resonator band-stop filter
measurement setup
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Fig. 7: Narrow-band L-resonator band-stop filter
measurement results
The measurement results shown in Figure 7 were
compared with the simulation results shown in
Figure 5. When this comparison was examined, it
was seen that the scattering parameters were quite
compatible with each other in the simulation and
measurement results. As a result of the simulation, it
was seen that the center frequency was exactly 2.45
GHz, while the center frequency of the measurement
result was 2.35 GHz.
According to these results, the deviation between
the center frequencies was calculated to be 4%. This
frequency deviation; It is due to the fact that the
relative permeability coefficient of the dielectric
material used depends on the frequency; this is a
very acceptable value considering the connector,
cable and solder losses.
5 Narrow-band, Band-stop Filter
Design with Nine L-Resonators
A circuit with nine resonators has been designed.
The design center frequency is 2.45 GHz and the -3
dB partial bandwidth is calculated as 0.0408 using
equation
while designing, firstly the
slope parameters for each resonator were calculated
using equation
Ω here, Table 1 is used for the
element values of the low-pass prototype. Then, the
gap values corresponding to the calculated slope
parameter values were obtained according to Figure
3. Design values for the nine-resonator circuit are
given in Table 3.
Considering the design values for the nine-
resonator circuit given in Table 3, the circuit design
given in Figure 8 was made. The designed circuit is
simulated and the scattering parameter values are
shown in Figure 9.
Table 3. Design values for a nine-resonator circuit
Fig. 8: Narrow-band bandstop filter design with nine
resonators
Fig. 9: Narrow-band band-stop filter with nine
resonators
S-parameter values simulation result. When the
circuit design with nine resonators is examined in
Figure 9, the S21 value is observed to be -123.3 dB,
with a center frequency of 2.45 GHz. In the design, -
3 dB bandwidth is calculated as 27.8%.
6 Conclusions
In this study, three different band-stop filter
structures were examined at the 2.45 GHz center
frequency. While designing the narrow-band band-
stop filter, L-shaped resonators were magnetically
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connected to the main transmission line. The
dimensions of the gaps between the main
transmission line and each L-shaped resonator had to
be calculated individually. Then, the gap value
between the two was optimized, starting from 0.01
mm and increasing by 0.01 mm steps, up to 0.5 mm.
After this, designs with 1, 5 and 9 L-shape resonator
connections were made and the slope parameter
values for each L-shape were calculated one by one.
From the previously drawn curve, the necessary gaps
between the transmission line and the L-shaped
resonator were found, the designs were created in
this way and the results were interpreted. Then, a
circuit was randomly selected among narrow-band
band-stopping designs and produced. The
measurement results of the produced circuit were
compared with the results of the design simulated in
the computer environment, and the results were
interpreted.
When the design results by adding L-resonators
one by one are examined, an improvement in the S21
value is observed as the number of L-resonators
increases, while the bandwidth increased between
13.9% and 27.8% between the five-resonator design
and the nine-resonator design. While bandwidth is
generally preferred depending on the place of use; It
should be noted that each time an L-resonator is
added to the circuit, the length of the circuit
increases. Considering these results, the desired
narrow-band band-stop filter design can be selected
according to the place of use, the length of the design
and the desired bandwidth.
Even though the filters used today meet the
current needs, the gain and return loss in filter
characteristics are not at the desired level. This
requires that the filters available are always better. In
this way, it is aimed to advance the rapidly
developing sector at the national and international
level by contributing to the studies in this field.
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Progress in Electromagnetics Research
Symposium - Spring (PIERS), 22-25 May, St.
Petersburg, Russia, 2017.
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Conflicts of Interest
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publication of this article.
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