The Journal of Engineering Research (TJER), Vol. 15, No. 2 (2018) 124-128
DOI: 10.24200/tjer.vol15iss2p124-128
Tuning the Resonance Frequency and Miniaturization of a
Novel Microstrip Bandpass Filter
A. Rezaeia,* and L. Noorib
a Department of Electrical Engineering, Kermanshah University of Technology, Kermanshah, Iran
b Young Researchers and Elite Club, Kermanshah Branch, Islamic Azad University, Kermanshah, Iran
Received 8 June 2016; Accepted 24 December 2017
Abstract: In this paper, a compact microstrip bandpass filter is designed using two open loop
resonators. In order to obtain a tunable bandpass response with low insertion loss, two stubs are
loaded inside them. The design process is based on obtaining the input admittance. Then, using the
input admittance, a method is presented to control the resonance frequency and miniaturization
simultaneously. The obtained insertion loss and the return loss at the resonance frequency are 0.1 dB
and 19.7 dB respectively. To verify the design method, the proposed filter is fabricated and measured.
The measured results are in good agreement with the simulated results.
Keywords: Microstrip filter; Tunable; Miniaturization; Insertion loss.
نطاق ترددي مضغوط باستخدام شرحية جديدة متناهية الصغري
بوليلى نوري * ، أ عباس رضائي
ذواتا حلقة مفتوحة . و من أجل ني باستخدام رناناتمرشح نطاق ترددي مضغوط ، مت تصميم البحثية : يف هذه الورقة امللخص
وتعتمد عملية .احلصول على استجابة نطاق قابل للضباط ذو خبسارة منخفضة عند اإلدراج ، يتم حتميل شرحيتني بداخله
ا بتقديم طريقة للتحكم يف تردد الرنني والتصغري يف الستخدام مدخل االدراج ، قمنو .التصميم على معرفة حاصل اإلدخال
دي 19,7. دي بي و ,اوتبلغ نسبة اخلسارة يف اإلدراج اليت مت احلصول عليها واخلسارة عند رجوع تردد الرنني: .نفس الوقت
قياس توافقا بشكل جيد و اظهرت نتائج ال .وللتحقق من طريقة التصميم ، يتم تصنيع الفلرت املقرتح وقياسه .على التواليبي
.مع نتائج احملاكة
اخلسارة يف اإلدراج ،التصغري ،قابل لالنضباط ،رغشرحية متناهية الص ،مرشح : الكلمات املفتاحية
* Corresponding author’s e-mail: unrezaei@yahoo.com
mailto:unrezaei@yahoo.com
A. Rezaei and L. Noori
125
1. Introduction
High performance and compact microstrip
filters play an important role in developing
modern microwave/RF communication circuits
and systems. To design microstrip filters several
types of resonators such as: open loops (Hayati
et. al. 2012; Salehi et. al. 2014; Hayati et. al. 2014;
Zhang et. al. 2012), parallel-coupled lines
(Moradian et. al. 2009; Othman et. al. 2013;
Fathelbab et. al. 2005; Kuan et. al. 2010) and
quarter/half wavelength resonators (Li et. al.
2010; Deng et. al. 2010) have been used. Also,
several approaches on suppressing harmonics
have been proposed (Kang et. al. 2010; Wang et.
al. 2008; Chen et. al. 2009; Cheng et. al. 2014; Wu
et. al. 2017). In Zhang et. al. (2012), three open
loop resonators are used to design a bandpass
filter with a high selectivity. However, it has an
undesired fractional bandwidth. The common
weakness of these reported filters is their large
sizes and large insertion losses. In Moradian et.
al. (2009) a third-order bandpass filter is
proposed to attenuate the harmonics. In Kuan
et. al. (2010), the parallel coupled lines and step
impedance resonators are used to design a
microstrip bandpass filter. A microstrip
bandpass filter operating at 1.78 GHz with
suppressed harmonics up to 6.2 GHz is
presented in Li et. al. (2010). But this filter has a
large return loss. In Kang et. al. (2010), a ring-
balun bandpass filter has been proposed to
attenuate the harmonics up to 12 GHz. In Wang
et. al. (2008), coupled ring resonators have been
utilized to design a bandpass filter with a
complex structure, while using a complex
structure leads to hard fabrication. In Chen et.
al. (2009), various types of bandpass filters have
been introduced utilizing cross coupled
resonators to suppress the second harmonic.
Nevertheless, their stop bands are narrow. In
Cheng et. al. (2014), the inductive-coupled
stepped-impedance quarter-wavelength resona-
tors are used to design a microstrip bandpass
filter with an undesired insertion loss. A dual-
mode microstrip bandpass filter operating at
1.67 GHz has been introduced in Lu et. al.
(2017), which has a narrow fractional
bandwidth and large area. A microstrip
bandpass filter with high return loss has been
designed in Guan et. al. (2017). In this filter, a
triangular cell has been coupled to the step
impedance feed structures that results in
improving the bandwidth.
In this paper, first a simple resonator (open
loop) is chosen similar to Hayati et. al. (2012);
Salehi et. al. (2014); and Hayati et. al. (2014).
Then, a method is proposed for miniaturizing
and tuning the resonance frequency
simultaneously. Using this method, the shapes
and dimensions of the internal stubs can be
determined as well as the resonance frequency.
Next, a simple tunable bandpass filter is
presented using two proposed resonators to
solve the problems of previous works in terms
of large implementation area and large insertion
loss. In addition, the harmonics are attenuated
reasonably and fractional bandwidth and
selectivity are improved. Finally, the effect of
changing the dimensions on the frequency
response is investigated.
2. Filter Design
An open loop resonator is shown in Fig. 1a. A
large size simple step impedance cell is loaded
inside the proposed resonator to control the
resonance frequency. The electrical lengths θi
are used to obtain the input admittance, where
i=1, 2, 3, s. An LC equivalent circuit of the
proposed resonator is shown in Fig. 1b, where C
and L are the capacitors and inductors of the
bends respectively. The parameters Cg and Cp
present the gap capacitors. The parameters of
La, Lb, Lc, Le, Lf and Lg are the inductors of the
stubs with the physical lengths la, lb, lc, le, lf, and lg
respectively. Co is the capacitor of the open end,
which its position is subsequent the loaded step.
Cs, LS1 and LS2 are the capacitance and
inductance of the step impedance cell. The input
admittance from the open end of θ1 can be
written as follows:
)](tan)(tan)()[tan(tan1
)(tan)(tan)(tan)(tan
jYY
s321
s321
in
(1)
where θs is the total electrical length of the step
impedance cell, Y is the admittance of the step
impedance cell and open loop resonator. For the
even mode, when, Yin=0, the resonance
condition is obtained from:
0)(tan)(tan)(tan)(tan s321 (2)
So the resonance condition can be tuned by
adjusting θ1, θ2, and θ3 while θs is fixed. The
electrical length has a direct relation with the
physical length. Therefore, the resonance
frequency can be controlled by adjusting the
loop and step impedance open stub dimensions.
Tuning the Resonance Frequency and Miniaturization of a Novel Microstrip Bandpass Filter
126
When the electrical length θs is maximum,
then from θs= ls β (where β is propagation
constant) ls must be maximum. Therefore, by
choosing a maximum value for ls, (from
Equation 2 and also from θs= ls β) tan(θs) is
increased and the total of tan(θ1)+ tan(θ2)+
tan(θ3) is decreased (tan(θ1)+ tan(θ2)+ tan(θ3) is
minimum). Under this condition θ1, θ2, and θ3
are small values. Therefore, (from θ3=(la+ lg+ lf)
β, θ2= lcβ and θ1= leβ) the open loop dimensions
consisting of la, lb, lc, le, lf, and lg are decreased.
This is a method to decrease the resonator size
and adjust the resonance frequency
simultaneously.
As a total result of above discussion, a
method to control the resonance frequency and
miniaturization is obtained by the following
steps: In the first step, a resonator is selected
which some stubs are loaded inside it. In the
second step, the main resonator size is
decreased and the dimensions of stubs are
increased, so that the desired resonance
frequency can be obtained. In the proposed
resonator (Fig. 1a), the physical lengths la, lb, lc,
le, lg and lf must be smaller while the internal
stub length must be larger. Therefore, the
inductors La, Lb, Lc, Le, Lg and Lf can be smaller
and LS2 or/and LS1 can be larger.
According to Equation (2), in some cases,
there is a degree of freedom to control the
resonance frequency so that we have to use the
optimization method. Therefore, in order to
obtain a compact size at the target resonance
frequency, the additional optimization is
performed.
In order to design a bandpass filter as shown
in Fig. 2a, two open loop resonators consisting
of different step impedance cells are used. The
loops are connected together using mix
coupling. The coupling structure consists of
three coupled lines with different widths, which
are used to attenuate the harmonics. The feed
structures with the step impedance forms are
added to the input and output ports to decrease
the insertion loss without size increment. The
simulated and measured frequency responses of
the propose filter are shown in Fig. 2b.
According to the above discussion, the
resonance frequency can be controlled by
adjusting the physical lengths of open loops
while the internal stubs have a maximum size.
Therefore, a method to control the resonance
frequency is changing of the lengths (L3, L9)
or/and (L4 , L11).
The frequency response as a function of L3,
L9, L4 and L11 are shown in Fig. 3a and Fig. 3b.
Figures 3a and 3b depict the loop size changing
effect on the resonance frequency. When the
loops are large, the resonance frequency is
shifted to the left. When the loops are small, the
resonance frequency moves to the right. A
photograph of the fabricated filter is shown in
Fig. 3c.
Figure 1. Proposed resonator (a) layout, (b) LC
equivalent circuit.
Figure 2. Proposed filter (a) layout, (b)
frequency response.
A. Rezaei and L. Noori
127
3. Results
The proposed filter is simulated by Advanced
Design System (ADS) full wave EM simulator.
The proposed filter is designed and fabricated
on a RT_Duroid_5880 substrate with 15 mil-
dielectric thickness, εr=2.22 and loss tangent
0.0009. The dimensions of the proposed
structure shown in Fig. 2a are presented in
Table 1. This filter has the cutoff frequencies at
1.68GHz, 1.93GHz and the resonance frequency
is 1.8GHz. The harmonics are attenuated from
2.16 up to 7.1GHz with a maximum level of -
19.5dB. Therefore, the harmonics are attenuated
up to 3.94fo where fo is the resonance frequency.
The obtained insertion loss at 1.8GHz is better
than 0.1dB, while the return loss is better than
19.7dB. The filter size is 0.23λg × 0.1λg
(26.7×12mm²). In comparison to the previous
works, the filter size is small and the best
insertion loss is obtained. The achieved
fractional bandwidth (FBW) is 14%. The
insertion loss, return loss, fractional bandwidth,
and filter size are compared to the previous
works in Table 2. In the Table 2, IL, RL, and
FBW are the insertion loss, return loss, and
fractional bandwidth respectively.
4. Conclusion
A simple compact low loss bandpass filter is
designed, fabricated and measured for RF
systems. In this structure, two open loop
resonators are loaded by two large stubs. By
using the additional large size microstrip cells
inside the open loops, the resonance frequency
is decreased without size increment. The overall
size of the proposed filter is 320mm². In
comparison to all of the references, the lowest
Table 1. The dimensions of proposed structure (in mm).
Parameters L1 L2 L3 L4 L5 L6 L7 L8
Value 0.3 1 6.8 3.5 3 3.5 9.7 4.5
Parameters L9 L10 L11 L12 L13 W1 W2 W3
Value 1 4.5 4.8 0.2 1 3 1.2 1
Parameters W4 W5 W6 W7 S g
Value 5 0.6 0.5 1 0.1 0.4
Table 2. Comparison between the proposed filter and previous works.
Reference IL (dB) RL (dB) FBW Size (mm2) fo(GHz)
This work 0.1 19.7 14% 320 1.8
(Hayati et. al. 2012) 0.53, 0.59 10, 13.4 --- 320 2.4, 5.2
(Salehi et. al. 2014) 0.35 , 0.25 13.6, 18.2 17%, 13% 363 2.4, 5.7
(Hayati et. al. 2014) 0.2, 0.4 15, 12.7 13%, 11% 233 2.4, 5.2
(Zhang et. al. 2012) 2.2 --- 6% 11595 0.95
(Moradian et.al. 2009) 3.85 --- 13% 9000 1
(Kuan et.al. 2010) 2.5 15 12% 2034 2
(Li et.al. 2010) 1. 8 12 16.5% 488 1.78
(Deng et.al. 2010) 1.66 --- 9.5% 606 2.41
(Kang et.al. 2010) 1.34 21.6 4.08% 1564 2.45
(Chen et.al. 2009) 0.83 --- 11.2 2300 2
(Cheng et.al. 2014) 0.85 --- 11.5% 220 2.35
Tuning the Resonance Frequency and Miniaturization of a Novel Microstrip Bandpass Filter
Figure 3. (a) Frequency response as a function of
L3 and L9 , (b) Frequency response as a
function of L4 and L11 , (c) a
photograph of the fabricated filter.
insertion loss is obtained, while the harmonics
are attenuated from 2.16 to 7.1GHz with a
maximum attenuation level of -19.5dB. In
addition, the resonance frequency is tuned by
calculation of the input admittance
Conflict of Interest
The authors declare no conflicts of interest.
Funding
No funding was received for this research.
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