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Elliptic Function Lowpass Filter with Sharp Roll-off by
树的故事
cascading Multiple Patch Resonators
Journal: IEEE Microwave and Wireless Components Letters
Manuscript ID: MWCL-14-0996 Manuscript Type: Original Manuscripts
Date Submitted by the Author: 24-Oct-2014
Complete List of Authors: PM, Raphika; Cochin University of Science and Technology, Division of
Electronics
Parambil, Abdulla; Cochin University of Science and Technology, Division of Electronics
Muhammed, Jasmine; Cochin University of Science and Technology, Division of Electronics
Keywords: Elliptic Function, Lowpass filter, Microstrip line万里明
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岳阳楼记作者简介
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Elliptic Function Lowpass Filter with Sharp Roll-off by Cascading Multiple Patch Resonators
Raphika P. M. Student Member IEEE , Abdulla P. Member IEEE , Jasmine P. M. Student Member IEEE
Abstract — A compact elliptic function lowpass filter with sharp roll-off and wide stopband is propod bad on cascading multiple patch resonators on high impedance transmission lines. The resonators are placed symmetrically about a high impedance central microstrip line. The filter has been designed and fabricated using a very low cost material. The experimental results shows a good agreement with the simulated results and demonstrate that, the sharp roll-off and wide stopband performance is obtained by the propod filter. The 3 dB cutoff frequency of the propod filter is at 2.28 GHz with a roll-off of 89 dB/GHz and has a wide stopband from 2.49 GHz to 11 GHz. The filter has a compact size of 17.6 mm x 12.2 mm.
Index Terms — Elliptic function, lowpass filter, microstrip line, polygonal patch resonator, stepped impedance.
I. I NTRODUCTION
教会学校
Compact planar lowpass filters with sharp roll-off are in great demand for modern communication systems to suppress harmonics and spurious signals. Microstrip filters offer low inrtion loss in the passband and infinite attenuation in the stopband together with compact size, low cost and ea of fabrication. The conventional lowpass filters using stepped-impedance and tuned stubs provides only Butterworth and Chebyshev characteristics with a gradual cutoff frequency respon. In order to have a sharp cutoff respon, the filters require more ctions. Besides, increasing the number of ctions also increas the size of the filter and inrtion loss in the passband [1]. Compactness and sharp roll-off will be obtained by introducing defective ground structure, which also exhibits wide harmonic suppression and slow wave characteristics [2]. But the filters have additional radiation due to the partially opened ground plane and cannot be fixed on a metal ba due to the inherent defects in the ground plane. Various methods have been propod to develop a high performance, compact microstrip lowpass filters by cascading multiple resonators [3] – [6]. A microstrip lowpass filter using the stepped impedance hairpin resonator was reported in [3]. Even though the size reduction was achieved in this configuration, the respon is not very sharp. Good stopband performance was achieved by multiple resonators and meander transmission line [4] - [5], the filter passband was too narrow and its transition characteristics were very poor. Besides, the above mentioned filters are designed using high cost, low loss substrate. Sharp roll-off lowpass filter with wi
de stopband using stub loaded coupled line hairpin unit fabricated on FR4 was reported in [6], but its passband is too narrow and there exist periodic fluctuation of more than 15 dB of return loss in the stopband.
In this letter, a new simple, compact elliptic function lowpass filter with low passband loss, wide stopband and a very sharp roll-off by cascading multiple patch resonators is prented. The filter is designed using microstrip polygonal patch resonators with stepped impedance and is placed symmetrically about the high impedance central microstrip line. The patch resonators are formed by low impedance polygonal patches with high impedance connecting stubs, which offer high power handling capability and lower conductor loss as compared with the narrow microstrip line resonators [1]. The filter is designed and fabricated using low cost FR4 substrate and provides a roll-off of 89 dB/GHz and a wide stopband with suppression level better than 23 dB from 2.49 GHz to 11 GHz.
II. F ILTER D ESIGN
Fig. 1. Layout of the propod filter, l 1 = 1.2, l 2 = 0.8, l 3 = 1.8, l 4 = 5.6, l 5 = 0.8, l 6 = 3.8, l 7 = 1.4, l 8 = 1.8, l 9 = 1.4, l 10 = 10, l 11 = 2.4, l 12 = 3.2, l 13 = 2, w 1 = 0.4, w 2 = 0.6, w 3 = 0.2, w 4 = 0.2, w 5 = 4.8, h 1 = 2, h 2 = 1, h 3 = 1.8, h 4 = 3.8, h 5 = 2.4, h 6, = 0.6, h 7 = 1, g 1 = 0.4, g 2 = 0.2, g 3 = g 4 = g 5 = g 6 = 0.4 (all in millimetres).
Fig. 1 shows the propod filter, which compod of three types of resonators, named as resonator 1, resonator 2 and resonator 3, symmetrically loaded on a low-high impedance main transmission line. The resonator 1 is a stepped-impedance polygonal patch resonator (SI-PPR), that consist of alternate high and low impedance stubs and a polygonal patch. Since the size of the components are much shorter than the associated guided wavelength, so as to act as mi-lumped elements [1]. The characteristic impedance of high impedance stub Z 0L1 is designed as 121 Ω and the low impedance Z 0C1 is of 48 Ω. The effects of stepped impedance introduce an alternate inductive and capacitive reactance that increas the effective reactance, which contribute a sharp
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transition characteristics. The characteristic impedance of the polygonal patch can be extracted through one port S-parameters [7]. Fig. 2 shows the layout and transmission characteristics of the propod SI-PPR. As shown in Fig. 2(a), by loading the stepped impedances of length θ1, θ2, and θ3, to the main transmission line, the transmission get attenuated after 3.8 GHz, and a transmission zero, Tz2, is formed at 7.23 GHz by adding the impedances of length θ4 and θ5. The characteristics can be modified to a great extend by the prence of the polygonal patch 1 together with stepped impedances, and exhibit a sharp transmission zero, Tz1 at 3.7 GHz with an attenuation of nearly 50 dB.血泪之路
(a) (b)
Fig. 2. Layout and transmission characteristics of propod filter with SI-PPR, (a) transmission characteristics, (b) layout of SI-PPR.
(a) (b)
Fig. 3. Layout and transmission characteristics of propod filter with resonator 2, (a) transmission characteristics, (b) layout of resonator 2.
Fig. 3 depicts the layout and transmission characteristics of propod filter with resonator 2. As shown in Fig. 3(a), the resonator 2 improves the stopband performance of the filter with two transmission zeroes, Tz3 at 4 GHz and Tz4 at 4.92 GHz, by the prence of two resonant components. The resonator 2 is designed with a high impedance inductive stub, Z 0L2 and the electrical length of θ1, same as that of resonator 1, and low impedance polygonal patch 2. Since lower impedance polygonal patch results in a better approximation of a lumped-element capacitor and a higher Z 0L2 leads to a better approximation of a lumped-element inductor, the resonator parameters are lected accordingly to get a complete elliptic function filter respon. The characteristic impedance of the inductive stub, Z 0L2 is designed as 146 Ω, and its position with respect to the patch forms an asymmetrical step discontinuity, that enhances the stopband bandwidth [8].
The characteristics of the filter are modified to a great extend by inrting resonator 1 into resonato
r 2, symmetrically to a high impedance central microstrip transmission line, as shown in Fig. 4. The filter achieves a better passband with cutoff frequency at 2.3 GHz and sharp
transmission zero, Tz5 at 2.52 GHz. The sharp transition near the cutoff frequency is achieved due to the prence of ries resonators formed by the inductive stub and patches which are in parallel with the high impedance central microstrip line, thus the propod filter have an elliptic function lowpass filter respon. The ries and parallel combination short out transmission at their resonant frequencies and thus gives three finite transmission zeros, Tz5, Tz6, and Tz7 within 8 GHz, so as to achieve the stopband bandwidth from 2.46 GHz to 7.75 GHz with a suppression level of 20 dB. Excluding the microstrip feed line, the filter has a compact size of about 17.6 mm x 12.2 mm which corresponds to the normalized circuit size of g g 0.24384 λx 0.16902 λ,where λg
is the guided wavelength at cutoff frequency.
Fig. 4. Simulated S - Parameters of the filter with resonator1 and resonator 2.
III. B ANDWIDTH E NHANCEMENT
Since each resonator contributes their own transmission zeroes, by loading one more resonator, resonator 3, to the main transmission line without change in the physical dimension, we can further enhance the stopband bandwidth of the filter. Resonator 3 is also designed using high impedance inductive stub of 146 Ω with the electrical length θ1, and a square patch. The resonators 3 provides the stopband bandwidth enhancement of 3.25 GHz, by short outing the transmission pole Tp2 at 7.9 GHz, and also provide the impedance matching at the passband, thereby minimize the inrtion loss to 0.5 dB in 80% of the passband.
Fig. 5 shows the simulated and measured S-parameters of the propod filter with all the three resonators. Cascading multiple resonators also introduce transmission zeroes that contribute wide stopband and also increas the effective inductance and capacitance, which makes the structure a slow-wave transmission line. The geometry and dimension of the filter is optimized to increa the effective coupling area between the resonators, so as to enhance the roll-off characteristics of the filt
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er. The tapered microstrip line at the input and output feed line reduces the discontinuity effect associated with the step junction, which provides effective impedance matching with 50 Ω feed line and 106 Ω central microstrip line.
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IV. M EASUREMENT A ND D ISCUSSION
The propod filter has been designed and fabricated on a low cost, FR4 substrate with relative permittivity 4.4, thickness 1.6 mm with a loss tangent of 0.02. The simulation has been accomplished
using Zeland IE3D and fabricated using photolithographic process. R&S ZVL 13 Vector Network Analyzer together with 50 Ω, 1.6 mm SMA connecters is ud for the measurement.
Fig. 5. Simulated and measured S- parameters of the propod filter.
As shown in Fig. 5, the measured results are in good agreement with the simulated results. Even though the propod filter consists of cascading multiple resonators, the measured inrtion loss of the filter is less than 0.5 dB in the passband up to 1.8 GHz with a 3 dB cutoff frequency, f c of 2.28 GHz. The filter achieves a wide stopband from 2.49 GHz to 11 GHz with a suppression level better than 23 dB and a sharp roll-off rate, ξ of 89 dB/GHz (attenuation: 3dB at 2.28 GHz and 20 dB at 2.47 GHz). The measured fractional bandwidth of the filter is 126%. The return loss is better than 16 dB for the entire passband and clo to 4 dB in the upper end of the stopband. The high value of inrtion loss in the passband and return loss in the stopband of the measured filter is due to the inherent dielectric loss associated with FR4 material.
Fig. 6. The photograph of the propod filter
Fig. 6 shows the photograph of the fabricated filter and Table I summarize the performance characteristics of published lowpass filters referred in this letter and the propod filter. From the table it is clear that the propod f ilter is made of low cost FR4 substrate and gives high roll-off rate among the quoted filters.
V. C ONCLUSION
In this letter, a novel compact microstrip lowpass filter by cascading polygonal patch with stepped impedance resonators using high impedance microstrip transmission line is prented. The filter has a cutoff frequency of 2.28 GHz with a sharp roll-off of 89 dB/GHz and a wide stop band from 2.49 GHz to 11 GHz with a suppression level of 23 dB. The low inrtion loss, wide stopband, sharp roll-off, low cost and compact size of the propod elliptic function lowpass filter, is very uful for applications in modern communication systems. Several degrees of freedom exist for tuning the respon of the propod filter.
R EFERENCES
[1] J. S. Hong and M. J. Lancaster, “Microstrip Filters for RF/
Microwave Applications,” John Wiley, New York, 2001.
[2] C. J. Wang and C. H. Lin, “Compact lowpass filter with sharp
transition knee by utilizing a quasi-π-slot resonator and open stubs,” IET Microwaves Antennas & Propagation , vol. 4, no. 4, pp. 512 – 517, April 2010.
[3] M. H. Yang and J. Xu, “Design of compact broad-stopband
lowpass filters using modified stepped impedance hairpin resonators,” IET Electron. Lett., vol. 44, no. 20, pp. 1198 – 1200, September 2008.
厂场
[4] H. Cui, J. Wang and G. Zhang, “Design of microstrip low pass
filter with compact size and ultra-wide stopband,” IET Electron. Lett., vol. 48, no. 14, pp. 856 – 857, July 2012.大学学习目标
[5] L. Ge, J. P. Wang and Y-X Guo, “Compact microstrip lowpass filter with ultra-wide stopband,” IET Electron. Lett ., vol. 46,
no. 10, pp. 689 – 691, May 2010.
[6] V. K. Velidi and S. Sanyal, “Sharp roll-off lowpass filter with
wide stopband using stub-loaded coupled-line hairpin unit,” IEEE Microw. Wireless Compon. Lett ., vol. 21, no. 6, pp. 301-303, June 2011.
[7] K. Ma and K. S. Yeo, “New ultra-wide stopband low-pass filter
using transformed radial stubs,” IEEE Trans. Microw. Theory Tech., Vol. 59, No. 3, pp. 604-611, March 2011.
[8] L. Zhu and K. Wu, “Short–open calibration technique for field
theory-bad parameter extraction of lumped elements of planar integrated circuits,” IEEE Trans. Microw. Theory Tech., Vol. 50, No. 8, pp. 1861-1869, August 2002.
TABLE I
C OMPARISON WITH P REVIOUS W ORK
Ref. ξ
Stopband (GH Z ) SL
(dB)
Dielectric Material f c
(GHz) [3] 18 2.0 - 15.018 10 RT/duroid 5880 1.5 [4] 18 1.57 -12.6 15 Rogers RO4003 0.85 [5] 20 2.0 -14.60 17 RT/duroid 5870 1.3 [6] 63 0.8 - 4.60 20 FR4 0.53 This work 89 2.49 – 11 23 FR4 2.28
ξ = roll-off rate, SL = Suppression Level
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