International Journal of Research in Engineering and Technology (IJRET) Vol. 1, No. 3, 2012 ISSN 2277 – 4378
Abstract — In this paper, band pass filter (BPF) development with the assistance of the Richards-Kuroda Transformation method, on the basis of the known Chebyshev-Lowpass Filter, is presented.
This suggested filter consists of four edge-coupled striplines. The filter is operated at Ka-band downlink frequency segment of 19.7
GHz – 20.2 GHz for satellite application. The proposed circuit is simulated using Roger 5870 substrate with dielectric constant of 2.33, substrate height of 0.508 mm and thickness of 0.035 mm. The simulation results are excellent and the filter is suitable for integration within various microwave subsystems.
Keywords— Band Pass Filter, Chebyshev, Edge-coupled, Ka
Band, Stripline
I.
I NTRODUCTION
S TRIPLINE filters play an important role in many RF applications. As technologies advances, more stringent requirements of filters are required. One of the requirements is the compactness of filters [1]–[2]. At very high frequencies, the practical inductors and capacitors loses their intrinsic characteristics. Also a limited range of component values are available from the manufacturer [3].
Therefore for microwave frequencies (>3GHz), passive filter is usually realized using distributed circuit elements such as transmission line sections [4].
Many works have been reported that use waveguides for transmission line filter. However, waveguides systems are bulky and expensive. Low-power and cheaper alternatives are stripline and microstrip. These transmission lines are compact
[5]. Edge-coupled stripline is used instead of microstrip line as stripline does not suffer from dispersion and its propagation mode is pure TEM mode. Hence it is the preferred structured for coupled-line filters [4].
Therefore, a third order chebyshev edge-coupled stripline filter is designed in the research. The band pass filter is simulated by using Ansoft Designer software.
II.
M ETHODOLOGY
The design specification of the filter is shown in Table I.
The specification of dielectric material is obtained from
Rogers Corporation. It is shown in Table II.
The proposed filter is designed by following the five steps.
First step: Determining the order and type of approximation functions to be used. Second step: Finding the corresponding low-pass prototype. Third step: Transforming the low-pass network into a bandpass configuration. Fourth step: Scaling the bandpass configuration in both impedance and frequency.
Fifth step: Transforming the lumped circuit element into distributed realization.
TABLE I
S PECIFICATIONS OF B AND P ASS F ILTER
Upper cut-off frequency 19.7 GHz
Lower cut-off frequency
3-dB Bandwidth
Passband ripple
Attenuation at center frequency
20.2 GHz
0.5 GHz
0.5 dB
≥ 30 dB
TABLE II
S PECIFICATIONS OF D IELECTRIC M ATERIAL FROM R OGERS C ORPORATION
Dielectric Material Used Roger 5870 from Rogers
Corporation
Dielectric Constant
Loss tanget, tanδ
Substrate Height, h
Copper Thickness, t
Metal Roughness
2.33
0.0012
0.508 mm
0.035mm
2.4e
-3
mm
Hnin Yu Wai is with Department of Electronic Engineering, Mandalay
Technological University, Ministry of Science and Technology, Myanmar (email: powerlay@gmail.com).
Zaw Min Naing is with Technological University (Maubin), Myanmar (email: zawminnaing@pmail.ntu.edu.sg).
Kyaw Soe Lwin is with Department of Electronic Engineering, Mandalay
Technological University, Ministry of Science and Technology, Myanmar (email: kyawsoelwin007@gmail.com).
Hla Myo Tun is with Department of Electronic Engineering, Mandalay
Technological University, Ministry of Science and Technology, Myanmar (email: hmyotun@myanmar.com.mm).
Fig. 1 Characteristics for a chebyshev filter with 0.5dB ripple
125

International Journal of Research in Engineering and Technology (IJRET) Vol. 1, No. 3, 2012 ISSN 2277 – 4378
Since Cyebyshev filter has steeper initial descent into the stopband than other filter types, this type of filter is chosen in this research work. Order of filter can be determined by using the attenuation characteristics shown in Fig. 1 for a Chebyshev filter with 0.5dB ripple. Based on design specification, thirdorder filter is chosen using the proposed edge-coupled structure.
Order
1
2 st nd
3 rd
TABLE III
L OW P ASS P ROTOTYPE E LEMENT V ALUES g o
0.6986
1.4029
1.5963 g
1
1.0000
0.7071
1.0967 g
2
1.9841
1.5963 g
3
1.000
The element values for the third-order are taken from Table
III of normalized values of 0.5dB equal ripple lowpass prototype. The lowpass prototype is presented in Fig. 2.
1.5963H
1.5963H
L
1 f = 0.159Hz
1.0967F
C
2
L
3
Fig. 2 Third-order low pass prototype
After getting a lowpass filter prototype values, it is transformed into a bandpass design. Although, lowpass filter is transformed into bandpass filter design, the attenuation bandwidth ratios remain the same such that:
BW
BW c
=
(1) f f c
Where, BW = bandwidth at the required value of attenuation BW c
= 3dB bandwith of bandpass filter.
The actual transformation from lowpass to bandpass configuration is accomplished by resonating each lowpass element with an element of the opposite type and the same value. All shunt elements of lowpass prototype circuit become parallel-resonant circuits, and all series elements become series-resonant circuit. T he corresponding transformation of lowpass network into bandpass configuration for third-order
Chevbyshev filter is shown in Fig. 3.
1.5963H1.5963F
1.5963H 1.5963F
L
1
C
1
L
3 C
3
1.0967H
L
2
1.0967F
C
2 f = 0.159Hz
Fig. 3 Low pass to band pass transformation
The transformed filter is then frequency-scaled and impedance-scaled using the following formulas. For parallelresonant branches,
C
L
=
π
C
π n
(2)
2 RB
=
2
RB
2 f o
L n
And, for the series-resonant branches,
(3)
C
=
B
2
π f o
2
C n
R
(4)
L
=
RL n
2
π
B
(5)
T he frequency and impedance scaling of thirdorder bandpass filter with a cut-off frequency of 19.95GHz with
3dB bandwidth of 500MHz and 50Ω impedance rating for both ports is presented in Fig. 4 [6].
Fig. 4 BPF prototype for designed frequency
To source network
J
1
-90 degree
λo/4
L
1
25.41nH
f = 19.95 GHz
J
2
-90 degree
Section 1
0.00251pF
C
0.00912nH
1
C
2
L
2
Section 2
Section 3
50
J
3
-90 degree
L
3
0.00251pF
25.41nH
6.982pF
C
3
J
4
-90 degree
Section 4
Fig. 5 3 rd
order edge-coupled stripline filter
Richard’s transformation and Kuroda’s identities are used to
Ω accomplish the conversion from the lumped and distributed circuit designs. The explanation of Richard’s transformation and Kuroda’s identities are in [2], [4] and [5]. An equivalent circuit model for 3 rd
order edge-coupled transmission lines with open circuit at 2 ends and each section of coupled stripline are shown in Fig. 5.
Each section of coupled stripline contains three parameters:
S, W, and d. Two striplines of width W are in the parallel- or edge-coupled configuration with a separation S and d is the diameter or height of the substrate. The coupled line structure supports two quasi-TEM modes, i.e., the even mode and the odd mode. The three parameters are determined from the odd and even mode impedance (Zoo & Zoe) of each coupled line.
Zoo and Zoe are in turn depends on the gain of the corresponding admittance inverter J.
J
1
J n
=
=
1
Zo
1
2 Zo
2
π ∆
(6) g
1
π ∆
(7) g n
−
1 g n
J
N
+
1
=
1
Zo
π ∆
2 g
N g
N
+
1
(8)
126

International Journal of Research in Engineering and Technology (IJRET) Vol. 1, No. 3, 2012 ISSN 2277 – 4378
∆
Z oe
Z oo
=
= f
2
Z f o
( 1
+ o
− f
1
(9)
JZ o
+
( JZ o
)
2
) (10)
=
Z o
( 1
−
JZ o
+
( JZ o
) 2 ) (11)
220
200
180
160
140
120
100
80
60
Zo
40
20
20 40 60 80 160 r
Fig. 6 Characteristic curve of odd and even mode design data for edge-coupled striplines r
100 120 140
Return loss measurement is used to evaluate the impedance match of a filter. As the match between the characteristic impedance of the transmission line and the terminating impedance improves, the reflected wave becomes smaller.
Therefore, the reflection coefficient in “(8)” decreases. When a perfect match exists, there is no reflected wave and the reflection coefficient is zero. If the reflection coefficient is equal to 1, a perfect mismatch exists. Thus, the normal range of values for the magnitude of the reflection coefficient is between zero and one.
Insertion loss (S
21
) is the difference in dB power between the signals at the filter input and output [7].
III.
S IMULATION RESULTS
The BPF circuit is simulated with Ansoft Designer Student
Version 2.2 software in order to predict the performance of the filter. Few parameters in the circuit are analyzed and have a good relationship to the microwave theory. An optimization process has been introduced along the simulation procedure focusing on the filter dimension in order to improve the response of the filter.
The configurations of the software for Ka-band edgecoupled stripline filter are shown in Fig.8. The software results of odd and even mode impedances for each section are presented in Fig.9. The software results are nearly identical with the calculated results. The simulated response of insertion loss (S
21
) and return loss (S
11
) is shown in Fig.10.
Fig. 7 Photograph of edge-coupled stripline BPF
From the characteristic curve in Fig.6, the parameters W, S and d for each section are obtained. For section 1 and 4;
S/b=0.2, W/b=0.7. Therefore, b=d=h=0.508mm, W=0.355mm and s=0.1mm. For Section 2 and 3; S/b = 0.8, W/b = 0.75.
Therefore, s= 0.4064mm and W = 0.381mm. The photograph of edge-coupled stripline band pass filter is shown in Fig.7.
The two parameters (insertion loss and return loss) are crucial to analyze to obtain a good performance of filter. A good filter will be high return loss and small insertion loss ripple in passband. Return loss (S
11
) is a ratio of reflected power to incident power at the test port of a filter and is expressed in decibels (dB).
S
11
=
10 log


P reflected

 (12)
P incident
The ratio of the reflected wave to the incident wave is known as the reflection coefficient and is simply a measure of the quality of the match between the transmission line and the terminating impedances.
Γ
S
11
=
V reflected
(13)
V incident
=
10 log
( )
(14)
127
Fig. 8 Configuration for filter specifications
Fig. 9 Software results of odd and even impedances

International Journal of Research in Engineering and Technology (IJRET) Vol. 1, No. 3, 2012 ISSN 2277 – 4378
0
Insertion loss (dB)
20
40
Return loss (dB)
0
10
20 frequencies, lumped element filter is converted into distributed stripline filter by using Richard’s transformation and Kuroda’s identities. The filter is simulated with Ansoft Designer software to predict the performance of filter. The simulated insertion loss is less than 0.01dB in the desired passband and the simulated return loss is greater than 50 dB at center frequency. All simulated results are nearly identical with the calculated results and also they are good agreement to the design specifications.
60 30
80
100
17.45 18.075
18.7
19.325
19.95
Freq (GHz)
20.575
40
50
21.2
21.825
22.45
A
CKNOWLEDGMENT
The author is greatly indebted to her parents and all of her teachers who have taught her during the whole life. The author also would like to thank Dr. Chaw Myat Nwe, Associate
Professor at Electronic Engineering Department, Mandalay
Technological University, for her suggestions and comments in doing this research.
Fig. 10 Simulated insertion loss and return loss
The simulated insertion loss is less than 0.01 dB in passband. Also the response is flat and uniform over the entire pass-band. In addition, the simulated response has the attenuation of more than 50 dB at 19.95 GHz. For S
11
of 50 dB, reflectio n coefficient is 0.00001 which is nearly equal to
0 and a perfect match exists. Therefore, all simulated results are nearly identical to the design specifications.
The simulated group delay for the designed filter is depicted in Fig.11. It is clear from the result that the group delay reaches a peak value close to the cut-off point. Moreover, the simulated group delay has a very low peak-to-peak variation of less than 3ns across the passband of the filter. Therefore, the designed filter shows attractive characteristics for BPF applications.
R EFERENCES
[1] Y. M. Yan, Y. T. Chang, H. Wang, R. B. Wu, a nd C. H. Chen,
"Highly selective microstrip bandpass filters in Ka- band, " in 32th
Eur. Microwave Conf. Proc., 2002, pp. 1137-1140.
[2] Design and Optimization of Low Pass Filter Using Microstrip Lines
[3] A. A. Sulaiman, M.F.Ain, S.I.S. Hassan, Design of Hairpin Band
Pass Filters for K-band Applications, 2008 IEEE I nternational
RF and Microwave Conference, 2008.
[4] Fabian Kung Wai Lee, RF/Microwave Filters, August 2007.
[5] W.Lars, Analog Filters Using MATLAB, 2009.
[6] B.Chris, RF Circuit Design, 1 ed
., John Wiley&Son, 2000.
[7] Rohde, Interfacing ZVA/ZVB/ZVT with AWR software for Filter
Design and Optimization Application Note, 2009.
4
3
5
Delay (ns)
2
1
0
17.45
18.075
18.7
19.325
19.95
20.575
21.2
21.825
Freq (GHz)
Fig. 11 Simulated group delay result
22.45
IV.
C ONCLUSION
This paper describes a procedure for designing edgecoupled stripline band pass filter. It also rejects image signal appearing at the frequency range between 19.7GHz to
20.2GHz. Third-order edge-coupled stripline filter is used in order to realize these objectives. Since the practical inductors and capacitors loses their intrinsic characteristics at high
128