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Ultrawide stopband low-pass filter
using triangular resonators defected
ground
Hany Taher
ab
a
Electrical Engineering Department, Faculty of Engineering and
Islamic Architecture, Um Alqura University, Alabdia, PO (5555),
21955 Makkah, KSA
b
Microstrip Department, Electronic Research Institute, Giza,
Egypt
Published online: 09 Jan 2014.
To cite this article: Hany Taher (2014) Ultrawide stopband low-pass filter using triangular
resonators defected ground, Journal of Electromagnetic Waves and Applications, 28:5, 542-550,
DOI: 10.1080/09205071.2013.879048
To link to this article: http://dx.doi.org/10.1080/09205071.2013.879048
PLEASE SCROLL DOWN FOR ARTICLE
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“Content”) contained in the publications on our platform. However, Taylor & Francis,
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the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
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Downloaded by [Kyungpook National University] at 00:40 09 March 2014
Conditions of access and use can be found at http://www.tandfonline.com/page/termsand-conditions
Journal of Electromagnetic Waves and Applications, 2014
Vol. 28, No. 5, 542–550, http://dx.doi.org/10.1080/09205071.2013.879048
Ultrawide stopband low-pass filter using triangular resonators
defected ground
Downloaded by [Kyungpook National University] at 00:40 09 March 2014
Hany Tahera,b*
a
Electrical Engineering Department, Faculty of Engineering and Islamic Architecture,
Um Alqura University, Alabdia, PO (5555), 21955 Makkah, KSA; bMicrostrip Department,
Electronic Research Institute, Giza, Egypt
(Received 10 July 2013; accepted 22 December 2013)
In this article, a novel complementary triangular split ring resonator (CTSRR)
using defected ground structure (DGS) is introduced. The unit is implemented in
microstrip technology. The CTSRR has unique filter characteristics, including six
dimension-dependent finite attenuation poles with wide bandgap, very high
attenuation rate, and very low level of passband ripples. CTSRR units with different
dimensions CTSRR units are cascaded together to design a high-performance
low-pass filter (LPF) with a cutoff frequency ( fc) of 1.80 GHz. The designed LPF
exhibits an attenuation rate of 250 dB/GHz and passband ripples of less than 0.30
dB. Moreover, it has a wide 20-dB stopband at up to 16.5 times fc. To our
knowledge, this is the best obtained stopband value among the published results
until now. The designed filter has been fabricated and validated using small signal
parameter measurements. Excellent agreement is noticed between transmission
coefficient measurements and its simulated counterpart using electromagnetic
simulator.
Keywords: low pass filters; microstrip; defected ground structure; split ring resonators
1. Introduction
Modern microwave communication systems require LPFs to suppress unwanted
harmonics generated by nonlinear devices. A conventional LPF uses a stepped
impedance structure. Although its design procedure is simple, it suffers from serious
disadvantages such as a low attenuation rate and narrow stopband.[1] LPFs can be
realized using a periodic transmission line structure. Defected ground structure (DGS)
has recently been used to implement this periodic form. DGS periodic structures have
attractive properties including their cutoff and bandgap characteristics. These properties
can be controlled by selecting the physical dimensions of the DGS units.
In the past, periodic DGS LPFs were realized with DGS units of uniform
dimensions.[2] Later, DGS LPFs of nonuniform dimensions were investigated and
reported. They were implemented using different shapes, such as square,[3] crossshaped,[4] double equilateral U-shaped,[5] bow tie resonators,[6] and complementary
square split ring resonators (CSSRR).[7] Moreover, LPF is implemented with stepped
impedance combined with shunt open stubs.[8] DGS is utilized with defected
microstrip lines to improve frequency response of LPF.[9] All these structures offer
*Email: [email protected], [email protected]
© 2014 Taylor & Francis
Downloaded by [Kyungpook National University] at 00:40 09 March 2014
Journal of Electromagnetic Waves and Applications
543
lower passband ripples and a wider stopband than uniform-dimension DGS structures.
It is noticed that all these different structures can provide up to 5fc 20-dB stopband.
However, there is still room to improve the performance, especially increasing the
stopband, through investigation of new shapes of DGS units.
The performance of the used DGS unit determines the global behavior of
the designed LPF. Consequently, the solution is to find a DGS unit that fulfills all
high-performance criteria of LPF simultaneously. It is very clear that sharp attenuation
rate is the main disadvantage of the nonuniform DGS LPFs. To obtain very sharp attenuation rate, a DGS with elliptic response has to be used. Square Split Ring Resonator
(SRR) DGS unit has been modeled with elliptic function from third order.[10] SSRR
DGS exhibits high attenuation rate with one attenuation pole and very small passband
ripples. In a different context, a double equilateral U-shaped DGS [5] unit has been
utilized to produce two attenuation poles.
To exploit the advantages of the two concepts, they have been merged. In other
words, double SSR DGS units with elliptic function have been combined to form a
single unit. This unit is considered the Complementary (negative image) of SSR
Resonator (CSSRR).[7] Two advantages have been obtained from using this approach.
Firstly, every DGS unit has the ability to nullify the transmission characteristics twice
on predetermined locations instead of only one. Secondly, the bandgap transmission
characteristics of double units are wider than the one that can be provided by the single
unit. It is arguable that DGS unit with more attenuation poles results in improved
attenuation characteristics of LPF.
In this work, CTSRR unit is investigated. It is found that the unit can provide six
attenuation poles, i.e. with four additional poles more than double equilateral U-shaped
[45] and (CSSRR) [7] supply. Therefore, CTSRR unit enhances the rejection
performance compared with other shapes. A spurious response in the stopband of a
LPF is effectively suppressed utilizing a proper dimension of cascaded CTSRR units.
A microstrip LPF prototype with fc of 1.8 GHz and 20-dB stopband up-to 7.5 times fc
is designed, fabricated, and experimentally measured. The stopband width value is
obtained using the available limited measurement range vector network analyzer
(VNA). However, simulations results indicate that the 20-dB stopband extends up to
16.5 times fc.
This article is organized as follows: Section 2 investigates the suitability of the
proposed CTSRR unit for designing a high-performance LPF. Moreover, parametric
design study is shown in this study. The design procedure of the LPF is demonstrated
in Section 3. Enhancement of design methodology is presented in Section 4. The
validation of the design with fabricated circuit measurements is presented in Section 5.
Conclusions are drawn in Section 6.
2. Performance of CTSRR unit
The CTSRR is the complementary form of the TSRR. The CTSRR is excited by an
electric field which is normally incident unlike the TSRR which is excited by a
perpendicular magnetic field. Since the CTSRRs require electrical coupling, they are
loaded in the ground plane of the microstrip line underneath the signal line. The
structure inhibits the signal propagation at a resonance frequency and within a narrow
band. This phenomenon reflects the negative effective permittivity of the medium.
Therefore, this unit is a perfect candidate to build frequency-selective structures such as
filters and antennas.
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544
Figure 1.
H. Taher
Geometric dimensions of the CTSRR unit.
The proposed CTSRR unit is depicted in Figure 1 with its geometric dimensions. It
has a 50-Ω microstrip line of width (d) on top and an etched CTSRR section in the
ground plane. The triangular unit is equilateral and has an outer side length (L). To
decrease the number of design parameters, thicknesses of the two slots and widths of
the splits, as well as the distance between the two slots, have the same dimension (g).
The frequency response of the proposed unit is investigated with the method of
moments (MOM) using the Zeland IE3D software package.[11]
The dimensions of the simulated CTSRR unit are L = 12 mm and g = 0.5 mm. The
substrate used in all the simulations is Teflon Duroid with a dielectric constant (r ) of
10.20 and thickness of 0.635 mm. The width d of the 50-Ω microstrip line is 0.58 mm.
Figure 2 clearly shows the superiority of the filtering performance of the CTSRR unit
in rejection behavior. The insertion loss behavior of the CTSRR unit has six attenuation
poles at different frequencies, in which three of them reach more than 20-dB level.
Therefore, CTSRR unit has six bandgaps which are four more than those provided by
CSSRR and U-shaped DGS units.
The effect of L on the frequency response of the CTSRR unit is studied by keeping
g constant at 0.50 mm and varying L (12, 11, and 10 mm). The simulation results, as
depicted in Figure 3, show that the fc and attenuation poles locations fon are inversely
Figure 2.
Insertion loss of CTSRR.
Downloaded by [Kyungpook National University] at 00:40 09 March 2014
Journal of Electromagnetic Waves and Applications
Figure 3.
545
Insertion loss of CTSRR, with different L.
proportional to L. The reason is that as L decreases, the effective series inductance of
the line decreases, and consequently values of fc and attenuation pole locations are
increased.
3. Design and optimization of CTSRR LPF
To demonstrate the effectiveness of the proposed CTSRR unit, an LPF is built. The
filter is designed for GSM applications, with an fc of 1.8 GHz. A 50-Ω microstrip line
is placed on top of a Teflon Duroid substrate having the specifications listed in the
previous section. A nonuniform CTSRR DGS periodic structure with N units is etched
at the bottom of the substrate as shown in Figure 4. Constant g equals to 0.5 mm for
all CTSRR units is assumed to decrease the number of design variables.
The overall performance of the filter is considered a linear resultant of individual
behavior of all units. fc and the attenuation rate of the LPF are determined mainly from
the performance of the unit that has the lowest fc. As discussed earlier, the unit with
largest L has lowest fc. The concerned unit is the first one from the left side of the
periodic structure, as depicted in Figure 4. The first step in the design procedure is to
find the proper dimensions of this unit. As shown in Figure 2, the CTSRR unit with
Figure 4.
CTSRR LPF with same g and different L.
Downloaded by [Kyungpook National University] at 00:40 09 March 2014
546
Figure 5.
H. Taher
Insertion loss of CTSRR unit with L = 7.6 mm.
L = 12 mm has fc equals 1.8 GHz. Therefore, this unit is chosen to be the first unit in
the designed LPF.
Attenuation region width is mainly dependent on the CTSRR unit that has the
highest f0. The unit with the smallest L has the highest f0. The concerned unit is the last
unit from the right side as shown in Figure 4. The target attenuation width is at least
eight times fc, i.e. f0 must be >14.5 GHz. It is found that CTSRR DGS with L equals
7.8 mm has an attenuation pole at 14.80 GHz, as depicted in Figure 5. Consequently,
L = 7.8 mm is chosen as the last CTSRR DGS unit.
The values of N, L, and the space between the units (W) are the design parameters
that have to be determined. The initial values of WS have been fixed at 0.50 mm; these
values are optimized during the design procedure to improve the final behavior of the
LPF. Consequently, N, Ls, and Ws are tuned until the target attenuation level, >20 dB,
is achieved. It is found that N equals 12, with Ls and Ws that are listed in Table 1,
yields to a LPF with performance depicted in Figure 6.
The designed LPF exhibits an attenuation rate of 200 dB/GHz and passband ripples
of less than 0.30 dB. Moreover, it has a wide 20-dB stopband at up to 8.5 times fc. It is
noticed that the insertion loss exceed 20-dB level at 4.2 and 11.4 GHz. In the next
Table 1.
Dimensions of the designed LPF.
N
L (mm)
W (mm)
1
2
3
4
5
6
7
8
9
10
11
12
12.1
11.78
11.3
11
10.63
10.26
9.87
9.5
9.12
8.36
8
7.6
11.9
11.7
8.5
7.5
7.6
7.1
6.7
6.6
6.7
6.5
5.7
–
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Journal of Electromagnetic Waves and Applications
Figure 6.
547
Insertion loss of LPF composed of 12 CTSRR units.
section, it is explained how the performance of the same design, without changing the
LPF dimensions, is enhanced. Substrate with lower r and wider microstrip line is used
to achieve this target.
4. Enhancing the performance of the designed LPF
Performance of individual used DGS unit determines the global behavior of the
designed LPF. It is noticed that using lower r substrate enhances the bandgap characteristics of the CTSRR unit. The CTSRR unit with L = 9.12 mm which lies under the
substrate with a r value of 6.15 is investigated. As depicted in Figure 7, the first two
5-dB bandgaps have 0.45 and 0.63 GHz width, respectively. Consequently, increments
of 32 and 110%, respectively, are obtained compared to the performance of the same
CTSRR unit that utilizes substrates with r equals 10.2. It is noticed from Figure 7 that
fc of the CTSRR unit, with substrate has r equals 6.15, is shifted upward with 0.4
GHz. To enhance the performance of the current design of LPF, a substrate with lower
r has to be used. However, in this case, the fc will degrade and move upward. The
problem is how to utilize the beforehand-designed LPF and substrate with lower r
simultaneously without changing the value of fc.
Since the effect of decreasing r is lowering slow wave factor (SWF) of the
structure, it is then proper to cancel this effect using a different mechanism. Utilizing
wider microstrip line, characteristic impedance less than 50 Ω increases SWF due to the
increase in the capacitance of the line. In other words, SWF of the initial design, on
substrates with high r , will not change noticeably if a suitable wide microstrip line is
used in conjunction with substrates with lower r . Therefore, fc is still fixed, and moreover, the wide bandgap width privilege still appears in the behavior of the structure.
For example, the CTSRR unit with L = 9.12 mm is tested in two different setups. The
first, on substrate that has r equals 10.2 and a microstrip line with 0.58 mm width and
50 Ω. The second case, the same CTSRR unit lies on substrate that has r equals 6.15
and utilizes 2.2 mm microstrip line. In this case, the characteristic impedance equals
29 Ω. As shown in Figure 8, the same fc is obtained in both cases. Furthermore, the
enhancement of the bandgap width is still preserved.
From the previous discussion, it is concluded that the physical dimensions of the
beforehand-designed LPF can be used without changing the substrate which has lower
Downloaded by [Kyungpook National University] at 00:40 09 March 2014
548
H. Taher
Figure 7. Insertion loss of CTSRR unit with L = 9.12 mm, and (a) r = 10.2 (
r = 6.15 (
), the microstrip line characteristic impedance is 50 Ω.
) and (b)
r and a wide microstrip line to enhance its performance. Therefore, the only value
needed to be modified is d of the microstrip line. The filter is simulated when d equals
2.2 mm.
Comparisons between the performance of the designed LPF and some of the
available previously published works in [12]–[15] are listed in Table 2. CTSRR LPF
exhibits the highest performance among these works on different aspects of the comparison, especially for obtained stopband value and rejection rate. Since the CTSRR LPF
has the lowest fc, it has the largest area among others. This design is adopted as a final
design, fabricated and measured, as it is shown in the next section.
5. Validation of the enhanced design of CTSRR LPF
The enhanced design of CTSRR DGS LPF is fabricated, as shown in Figure 9, on the
Teflon Duroid substrate with r equals to 6.15 and a thickness of 0.635 mm. The
Figure 8. Insertion loss of CTSRR unit with L = 9.12 mm and (a) r = 10.2 and d = 0.58 mm
(
), and (b) r = 6.15 and d = 2.2 mm (
).
Journal of Electromagnetic Waves and Applications
549
Table 2. Comparisons between the performance of the designed CSSRR LPF and some of the
previously published works.
Performance
Work
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This work
[12]
[13]
[14]
[15]
Rejection rate
fc
(dB/GHz)
(GHz)
250
120
106
40
43.21
1.8
2.8
3.68
2.4
3.5
20-dB isolation
bandwidth/fc
Passband
ripples level
(dB)
Physical
dimensions
(mm × mm)
r
16.5
2.5
4.4
0.8
4.6
0.30
0.30
0.39
0.41
0.44
89 × 11
20 × 19
48 × 8.5
55 × 11.3
50 × 30
6.15
3.38
2.2
4.4
6.15
Figure 9. Insertion loss of LPF, r = 6.15 and d = 2.2 mm, simulated (
(b) measured up to 13.5 GHz (
).
) up to 30 GHz and
fabricated circuit has same dimensions listed in Table 1 with d equals 2.2 mm. The
simulation results show that the 20-dB bandwidth extends to 30 GHz, 16.5 times fc. As
it is mentioned in the introduction, due to the measurement capabilities of VNA, the
fabricated LPF is measured in the range of 0.05–13.5 GHz. The results of S parameter
measurements agree well with the simulation results, as shown in Figure 9.
6. Conclusion
In this article, a new DGS shape, CTSRR unit, is investigated and used to design a
periodic LPF. The CTSRR unit exhibits interesting filtering properties such as six finite
attenuation poles in its stopband in addition to a very sharp attenuation rate. These
characteristics can be controlled by proper selection of the geometric dimensions of the
unit. The designed filter demonstrates superiority in various features compared with
other previously published works, particularly in offering very wide rejection band.
Very good agreement is noticed between simulations of the designed LPF and
experimental measurements of the fabricated circuit.
550
H. Taher
Downloaded by [Kyungpook National University] at 00:40 09 March 2014
References
[1] Hong JS, Lancaster MJ. Microstrip filters for RF/microwave applications. New York (NY):
John Wiley and Sons; 2001.
[2] Kim CS, Park JS, Ahn D, Lim JB. A novel 1-D periodic defected ground structure for
planar circuits. IEEE Microw. Guided Wave Lett. 2000;10:131–133.
[3] Liu HW, Li ZF, Sun XW, Mao JF. An improved 1D periodic defected ground structure for
microstrip line. IEEE Microw. Wireless Compon. Lett. 2004;14:180–182.
[4] Chen HJ, Huang TH, Chang CS, Chen LS, Wang NF, Wang YH, Houng MP. A novel
cross-shape DGS applied to design ultra-wide stopband low-pass filters. IEEE Microw.
Wireless Compon. Lett. 2006;16:252–254.
[5] Ting SW, Tam KW, Martins RP. Miniaturized microstrip lowpass filter with wide stopband
using double equilateral U-shaped defected ground structure. IEEE Microw. Wireless
Compon. Lett. 2006;16:240–242.
[6] Xie H-H, Jiao Y-C, Huang J-X, and Zhang L. Ultra-wide stopband low-pass filter with
Bowtie defected ground structure. Proc. of Microwave and Millimeter Wave Technology
(ICMMT); 2010 May 8–11:1655–1657.
[7] Taher H. High performance low pass filter using complementary square split ring resonators
defected ground structure. IET, Microw. Anten. Propag. 2011;5: 771–775.
[8] Yang R-Y, Lin Y-L, Hung C-Y, Lin C-C. Design of a compact and sharp-rejection low-pass
filter with a wide stopband. J. Electromagn. Waves Appl. 2012;26:2284–2290.
[9] Tirado-Mendez A, Jardon-Aguilar H, Flores-Leal R. Improving frequency response of
microstrip filters using defected ground structures and defected microstrip structures. Prog.
Electromagnet. Res. Lett. 2010;13:77–90.
[10] Parui SK, Das S. Modeling of split-ring type defected ground structure and its filtering
applications. J. Microw. Opt. Electrom. Appl. 2009;8:6–12.
[11] IE3D User’s Manual. Release 11. Freemont, CA: Zeland Software Inc; 2004.
[12] Abdulhadi AAA, Ashoka A, Jeberson W. Combined shaped microstrip line and DGS
techniques for compact low pass filter design. Int. J. Eng. Technol. 2013;13:74–77.
[13] Wu GC, Wang GM, Wang YW, Hu LZ. A compact microstrip low-pass filter using
D-CRLH transmission line with ultra-wide stopband and high selectivity. Radio Eng.
2013;22:734–738.
[14] Tiwary AK, Gupta N. Performance of microstrip low-pass filter on electromagnetic band
gap ground plane. IETE J. Res. 2010;56:230–234.
[15] Martin K, Zbyněk R. Comparison of planar fractal filters on defected ground substrate.
Radio Eng. 2012;21:1019–1024.
On: 09 March 2014, At: 00:40
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered
office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH, UK
Journal of Electromagnetic Waves and
Applications
Publication details, including instructions for authors and
subscription information:
http://www.tandfonline.com/loi/tewa20
Ultrawide stopband low-pass filter
using triangular resonators defected
ground
Hany Taher
ab
a
Electrical Engineering Department, Faculty of Engineering and
Islamic Architecture, Um Alqura University, Alabdia, PO (5555),
21955 Makkah, KSA
b
Microstrip Department, Electronic Research Institute, Giza,
Egypt
Published online: 09 Jan 2014.
To cite this article: Hany Taher (2014) Ultrawide stopband low-pass filter using triangular
resonators defected ground, Journal of Electromagnetic Waves and Applications, 28:5, 542-550,
DOI: 10.1080/09205071.2013.879048
To link to this article: http://dx.doi.org/10.1080/09205071.2013.879048
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the
“Content”) contained in the publications on our platform. However, Taylor & Francis,
our agents, and our licensors make no representations or warranties whatsoever as to
the accuracy, completeness, or suitability for any purpose of the Content. Any opinions
and views expressed in this publication are the opinions and views of the authors,
and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content
should not be relied upon and should be independently verified with primary sources
of information. Taylor and Francis shall not be liable for any losses, actions, claims,
proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or
howsoever caused arising directly or indirectly in connection with, in relation to or arising
out of the use of the Content.
This article may be used for research, teaching, and private study purposes. Any
substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing,
systematic supply, or distribution in any form to anyone is expressly forbidden. Terms &
Downloaded by [Kyungpook National University] at 00:40 09 March 2014
Conditions of access and use can be found at http://www.tandfonline.com/page/termsand-conditions
Journal of Electromagnetic Waves and Applications, 2014
Vol. 28, No. 5, 542–550, http://dx.doi.org/10.1080/09205071.2013.879048
Ultrawide stopband low-pass filter using triangular resonators
defected ground
Downloaded by [Kyungpook National University] at 00:40 09 March 2014
Hany Tahera,b*
a
Electrical Engineering Department, Faculty of Engineering and Islamic Architecture,
Um Alqura University, Alabdia, PO (5555), 21955 Makkah, KSA; bMicrostrip Department,
Electronic Research Institute, Giza, Egypt
(Received 10 July 2013; accepted 22 December 2013)
In this article, a novel complementary triangular split ring resonator (CTSRR)
using defected ground structure (DGS) is introduced. The unit is implemented in
microstrip technology. The CTSRR has unique filter characteristics, including six
dimension-dependent finite attenuation poles with wide bandgap, very high
attenuation rate, and very low level of passband ripples. CTSRR units with different
dimensions CTSRR units are cascaded together to design a high-performance
low-pass filter (LPF) with a cutoff frequency ( fc) of 1.80 GHz. The designed LPF
exhibits an attenuation rate of 250 dB/GHz and passband ripples of less than 0.30
dB. Moreover, it has a wide 20-dB stopband at up to 16.5 times fc. To our
knowledge, this is the best obtained stopband value among the published results
until now. The designed filter has been fabricated and validated using small signal
parameter measurements. Excellent agreement is noticed between transmission
coefficient measurements and its simulated counterpart using electromagnetic
simulator.
Keywords: low pass filters; microstrip; defected ground structure; split ring resonators
1. Introduction
Modern microwave communication systems require LPFs to suppress unwanted
harmonics generated by nonlinear devices. A conventional LPF uses a stepped
impedance structure. Although its design procedure is simple, it suffers from serious
disadvantages such as a low attenuation rate and narrow stopband.[1] LPFs can be
realized using a periodic transmission line structure. Defected ground structure (DGS)
has recently been used to implement this periodic form. DGS periodic structures have
attractive properties including their cutoff and bandgap characteristics. These properties
can be controlled by selecting the physical dimensions of the DGS units.
In the past, periodic DGS LPFs were realized with DGS units of uniform
dimensions.[2] Later, DGS LPFs of nonuniform dimensions were investigated and
reported. They were implemented using different shapes, such as square,[3] crossshaped,[4] double equilateral U-shaped,[5] bow tie resonators,[6] and complementary
square split ring resonators (CSSRR).[7] Moreover, LPF is implemented with stepped
impedance combined with shunt open stubs.[8] DGS is utilized with defected
microstrip lines to improve frequency response of LPF.[9] All these structures offer
*Email: [email protected], [email protected]
© 2014 Taylor & Francis
Downloaded by [Kyungpook National University] at 00:40 09 March 2014
Journal of Electromagnetic Waves and Applications
543
lower passband ripples and a wider stopband than uniform-dimension DGS structures.
It is noticed that all these different structures can provide up to 5fc 20-dB stopband.
However, there is still room to improve the performance, especially increasing the
stopband, through investigation of new shapes of DGS units.
The performance of the used DGS unit determines the global behavior of
the designed LPF. Consequently, the solution is to find a DGS unit that fulfills all
high-performance criteria of LPF simultaneously. It is very clear that sharp attenuation
rate is the main disadvantage of the nonuniform DGS LPFs. To obtain very sharp attenuation rate, a DGS with elliptic response has to be used. Square Split Ring Resonator
(SRR) DGS unit has been modeled with elliptic function from third order.[10] SSRR
DGS exhibits high attenuation rate with one attenuation pole and very small passband
ripples. In a different context, a double equilateral U-shaped DGS [5] unit has been
utilized to produce two attenuation poles.
To exploit the advantages of the two concepts, they have been merged. In other
words, double SSR DGS units with elliptic function have been combined to form a
single unit. This unit is considered the Complementary (negative image) of SSR
Resonator (CSSRR).[7] Two advantages have been obtained from using this approach.
Firstly, every DGS unit has the ability to nullify the transmission characteristics twice
on predetermined locations instead of only one. Secondly, the bandgap transmission
characteristics of double units are wider than the one that can be provided by the single
unit. It is arguable that DGS unit with more attenuation poles results in improved
attenuation characteristics of LPF.
In this work, CTSRR unit is investigated. It is found that the unit can provide six
attenuation poles, i.e. with four additional poles more than double equilateral U-shaped
[45] and (CSSRR) [7] supply. Therefore, CTSRR unit enhances the rejection
performance compared with other shapes. A spurious response in the stopband of a
LPF is effectively suppressed utilizing a proper dimension of cascaded CTSRR units.
A microstrip LPF prototype with fc of 1.8 GHz and 20-dB stopband up-to 7.5 times fc
is designed, fabricated, and experimentally measured. The stopband width value is
obtained using the available limited measurement range vector network analyzer
(VNA). However, simulations results indicate that the 20-dB stopband extends up to
16.5 times fc.
This article is organized as follows: Section 2 investigates the suitability of the
proposed CTSRR unit for designing a high-performance LPF. Moreover, parametric
design study is shown in this study. The design procedure of the LPF is demonstrated
in Section 3. Enhancement of design methodology is presented in Section 4. The
validation of the design with fabricated circuit measurements is presented in Section 5.
Conclusions are drawn in Section 6.
2. Performance of CTSRR unit
The CTSRR is the complementary form of the TSRR. The CTSRR is excited by an
electric field which is normally incident unlike the TSRR which is excited by a
perpendicular magnetic field. Since the CTSRRs require electrical coupling, they are
loaded in the ground plane of the microstrip line underneath the signal line. The
structure inhibits the signal propagation at a resonance frequency and within a narrow
band. This phenomenon reflects the negative effective permittivity of the medium.
Therefore, this unit is a perfect candidate to build frequency-selective structures such as
filters and antennas.
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544
Figure 1.
H. Taher
Geometric dimensions of the CTSRR unit.
The proposed CTSRR unit is depicted in Figure 1 with its geometric dimensions. It
has a 50-Ω microstrip line of width (d) on top and an etched CTSRR section in the
ground plane. The triangular unit is equilateral and has an outer side length (L). To
decrease the number of design parameters, thicknesses of the two slots and widths of
the splits, as well as the distance between the two slots, have the same dimension (g).
The frequency response of the proposed unit is investigated with the method of
moments (MOM) using the Zeland IE3D software package.[11]
The dimensions of the simulated CTSRR unit are L = 12 mm and g = 0.5 mm. The
substrate used in all the simulations is Teflon Duroid with a dielectric constant (r ) of
10.20 and thickness of 0.635 mm. The width d of the 50-Ω microstrip line is 0.58 mm.
Figure 2 clearly shows the superiority of the filtering performance of the CTSRR unit
in rejection behavior. The insertion loss behavior of the CTSRR unit has six attenuation
poles at different frequencies, in which three of them reach more than 20-dB level.
Therefore, CTSRR unit has six bandgaps which are four more than those provided by
CSSRR and U-shaped DGS units.
The effect of L on the frequency response of the CTSRR unit is studied by keeping
g constant at 0.50 mm and varying L (12, 11, and 10 mm). The simulation results, as
depicted in Figure 3, show that the fc and attenuation poles locations fon are inversely
Figure 2.
Insertion loss of CTSRR.
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Journal of Electromagnetic Waves and Applications
Figure 3.
545
Insertion loss of CTSRR, with different L.
proportional to L. The reason is that as L decreases, the effective series inductance of
the line decreases, and consequently values of fc and attenuation pole locations are
increased.
3. Design and optimization of CTSRR LPF
To demonstrate the effectiveness of the proposed CTSRR unit, an LPF is built. The
filter is designed for GSM applications, with an fc of 1.8 GHz. A 50-Ω microstrip line
is placed on top of a Teflon Duroid substrate having the specifications listed in the
previous section. A nonuniform CTSRR DGS periodic structure with N units is etched
at the bottom of the substrate as shown in Figure 4. Constant g equals to 0.5 mm for
all CTSRR units is assumed to decrease the number of design variables.
The overall performance of the filter is considered a linear resultant of individual
behavior of all units. fc and the attenuation rate of the LPF are determined mainly from
the performance of the unit that has the lowest fc. As discussed earlier, the unit with
largest L has lowest fc. The concerned unit is the first one from the left side of the
periodic structure, as depicted in Figure 4. The first step in the design procedure is to
find the proper dimensions of this unit. As shown in Figure 2, the CTSRR unit with
Figure 4.
CTSRR LPF with same g and different L.
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546
Figure 5.
H. Taher
Insertion loss of CTSRR unit with L = 7.6 mm.
L = 12 mm has fc equals 1.8 GHz. Therefore, this unit is chosen to be the first unit in
the designed LPF.
Attenuation region width is mainly dependent on the CTSRR unit that has the
highest f0. The unit with the smallest L has the highest f0. The concerned unit is the last
unit from the right side as shown in Figure 4. The target attenuation width is at least
eight times fc, i.e. f0 must be >14.5 GHz. It is found that CTSRR DGS with L equals
7.8 mm has an attenuation pole at 14.80 GHz, as depicted in Figure 5. Consequently,
L = 7.8 mm is chosen as the last CTSRR DGS unit.
The values of N, L, and the space between the units (W) are the design parameters
that have to be determined. The initial values of WS have been fixed at 0.50 mm; these
values are optimized during the design procedure to improve the final behavior of the
LPF. Consequently, N, Ls, and Ws are tuned until the target attenuation level, >20 dB,
is achieved. It is found that N equals 12, with Ls and Ws that are listed in Table 1,
yields to a LPF with performance depicted in Figure 6.
The designed LPF exhibits an attenuation rate of 200 dB/GHz and passband ripples
of less than 0.30 dB. Moreover, it has a wide 20-dB stopband at up to 8.5 times fc. It is
noticed that the insertion loss exceed 20-dB level at 4.2 and 11.4 GHz. In the next
Table 1.
Dimensions of the designed LPF.
N
L (mm)
W (mm)
1
2
3
4
5
6
7
8
9
10
11
12
12.1
11.78
11.3
11
10.63
10.26
9.87
9.5
9.12
8.36
8
7.6
11.9
11.7
8.5
7.5
7.6
7.1
6.7
6.6
6.7
6.5
5.7
–
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Journal of Electromagnetic Waves and Applications
Figure 6.
547
Insertion loss of LPF composed of 12 CTSRR units.
section, it is explained how the performance of the same design, without changing the
LPF dimensions, is enhanced. Substrate with lower r and wider microstrip line is used
to achieve this target.
4. Enhancing the performance of the designed LPF
Performance of individual used DGS unit determines the global behavior of the
designed LPF. It is noticed that using lower r substrate enhances the bandgap characteristics of the CTSRR unit. The CTSRR unit with L = 9.12 mm which lies under the
substrate with a r value of 6.15 is investigated. As depicted in Figure 7, the first two
5-dB bandgaps have 0.45 and 0.63 GHz width, respectively. Consequently, increments
of 32 and 110%, respectively, are obtained compared to the performance of the same
CTSRR unit that utilizes substrates with r equals 10.2. It is noticed from Figure 7 that
fc of the CTSRR unit, with substrate has r equals 6.15, is shifted upward with 0.4
GHz. To enhance the performance of the current design of LPF, a substrate with lower
r has to be used. However, in this case, the fc will degrade and move upward. The
problem is how to utilize the beforehand-designed LPF and substrate with lower r
simultaneously without changing the value of fc.
Since the effect of decreasing r is lowering slow wave factor (SWF) of the
structure, it is then proper to cancel this effect using a different mechanism. Utilizing
wider microstrip line, characteristic impedance less than 50 Ω increases SWF due to the
increase in the capacitance of the line. In other words, SWF of the initial design, on
substrates with high r , will not change noticeably if a suitable wide microstrip line is
used in conjunction with substrates with lower r . Therefore, fc is still fixed, and moreover, the wide bandgap width privilege still appears in the behavior of the structure.
For example, the CTSRR unit with L = 9.12 mm is tested in two different setups. The
first, on substrate that has r equals 10.2 and a microstrip line with 0.58 mm width and
50 Ω. The second case, the same CTSRR unit lies on substrate that has r equals 6.15
and utilizes 2.2 mm microstrip line. In this case, the characteristic impedance equals
29 Ω. As shown in Figure 8, the same fc is obtained in both cases. Furthermore, the
enhancement of the bandgap width is still preserved.
From the previous discussion, it is concluded that the physical dimensions of the
beforehand-designed LPF can be used without changing the substrate which has lower
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548
H. Taher
Figure 7. Insertion loss of CTSRR unit with L = 9.12 mm, and (a) r = 10.2 (
r = 6.15 (
), the microstrip line characteristic impedance is 50 Ω.
) and (b)
r and a wide microstrip line to enhance its performance. Therefore, the only value
needed to be modified is d of the microstrip line. The filter is simulated when d equals
2.2 mm.
Comparisons between the performance of the designed LPF and some of the
available previously published works in [12]–[15] are listed in Table 2. CTSRR LPF
exhibits the highest performance among these works on different aspects of the comparison, especially for obtained stopband value and rejection rate. Since the CTSRR LPF
has the lowest fc, it has the largest area among others. This design is adopted as a final
design, fabricated and measured, as it is shown in the next section.
5. Validation of the enhanced design of CTSRR LPF
The enhanced design of CTSRR DGS LPF is fabricated, as shown in Figure 9, on the
Teflon Duroid substrate with r equals to 6.15 and a thickness of 0.635 mm. The
Figure 8. Insertion loss of CTSRR unit with L = 9.12 mm and (a) r = 10.2 and d = 0.58 mm
(
), and (b) r = 6.15 and d = 2.2 mm (
).
Journal of Electromagnetic Waves and Applications
549
Table 2. Comparisons between the performance of the designed CSSRR LPF and some of the
previously published works.
Performance
Work
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This work
[12]
[13]
[14]
[15]
Rejection rate
fc
(dB/GHz)
(GHz)
250
120
106
40
43.21
1.8
2.8
3.68
2.4
3.5
20-dB isolation
bandwidth/fc
Passband
ripples level
(dB)
Physical
dimensions
(mm × mm)
r
16.5
2.5
4.4
0.8
4.6
0.30
0.30
0.39
0.41
0.44
89 × 11
20 × 19
48 × 8.5
55 × 11.3
50 × 30
6.15
3.38
2.2
4.4
6.15
Figure 9. Insertion loss of LPF, r = 6.15 and d = 2.2 mm, simulated (
(b) measured up to 13.5 GHz (
).
) up to 30 GHz and
fabricated circuit has same dimensions listed in Table 1 with d equals 2.2 mm. The
simulation results show that the 20-dB bandwidth extends to 30 GHz, 16.5 times fc. As
it is mentioned in the introduction, due to the measurement capabilities of VNA, the
fabricated LPF is measured in the range of 0.05–13.5 GHz. The results of S parameter
measurements agree well with the simulation results, as shown in Figure 9.
6. Conclusion
In this article, a new DGS shape, CTSRR unit, is investigated and used to design a
periodic LPF. The CTSRR unit exhibits interesting filtering properties such as six finite
attenuation poles in its stopband in addition to a very sharp attenuation rate. These
characteristics can be controlled by proper selection of the geometric dimensions of the
unit. The designed filter demonstrates superiority in various features compared with
other previously published works, particularly in offering very wide rejection band.
Very good agreement is noticed between simulations of the designed LPF and
experimental measurements of the fabricated circuit.
550
H. Taher
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