International Journal of Electronics, Communication & Instrumentation Engineering Research and Development (IJECIERD) ISSN 2249-684X Vol. 2 Issue 4 Dec 2012 89-95 © TJPRC Pvt. Ltd.,
THE MINIATURIZED BULB SHAPE MICROSTRIP LOW-PASS FILTER USING PARALLEL COUPLED STRIP LINES 1
INDER PAL SINGH, 2MANISH GUPTA & 2N KUMAR SWAMY 1
Shinas College of Technology, Shinas, P.O. Box 77, PC 324,Oman 2
Materials Science Research Lab ITM University, Gurgaon, India
ABSTRACT In this paper, two reduced size of microstrip low-pass filter using different shape and size patches with broad stopband and sharp skirt characteristic are simulated and compared . The idea of the structure behind this microstrip lowpass filter is simple as it is composed of a pair of parallel coupled-line and an open-stub. To further improve the design performance, high impedance lines are magnetically coupled, resulting an attenuation pole near −3 dB cut off point of the filter and open-stub helps to achieve the sharp skirt response. In order to validate the the proposed design method, a 3order Chebyshev low-pass filter with 0.01-dB ripple is designed. With this configuration, a finite attenuation pole near the stopband cutoff frequency is available, and the notch frequency can be well controlled by adjusting the circuit parameters.
KEYWORDS: Microstrip Low Pass filter, High–Low Impedance, Bulb Type Microstrip filter, Skirt Characteristic INTRODUCTION The low-pass filters are often employed in many communication systems to suppress harmonics and spurious signals with demand of compact size, low pass band insertion loss, and high attenuation. Due to increasing constraint of reduction in size with optimum performance, design of many low pass filters are available in the literature with different methods. The con-ventional filter design method such as open-stub low-pass filters and stepped impedance low-pass filters can not meet the requirements for modern communication systems because of some drawbacks [3]. In literature, many design approaches have been proposed to improve the low-pass filter’s performance is normally not used now a days due to inherited deficiencies like slow roll off in the stop band, poor frequency response in the pass band, narrow stop band, large size [1,2]. A number of methods are available in liter-ature to design the low pass filter with the The step discontinuity at the junction of two transmission lines with different characteristic impedance on a uniform substrate occurs frequently . The symmetrical step shown in fig: 1, Where
Fig.1: The Symmetrical Microstrip Discontinuity with Fringing Electric Fields and Current Flow
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Inder Pal Singh , Manish Gupta & N Kumar Swamy
suitably selection of stub. . The low-pass filters using PBG and DGS structures [4], [5] can improve the skirt characteristics and provide wide and deep stopband compared with the conventional low-pass filters. Filters using inter digital ca-pacitances based on even and odd mode analysis are also reported. Performances of this kind of filters are compatible with modern filter requirements of compact and integrated size [6], but needs extraction of capacitance and inductances, which is a cumbersome task.
EQUIVALENT CIRCUIT MODEL ANALYSIS
Fig. 2: The Equivalent Circuit of Step Impedance Line A high impedance line with characteristic impedance Z01, joins one of the lower impedance line Z02. The step is symmetrical with respect to the centre line of the strip conductor. In the vicinity of the step there will be transition region, assumed small compared with wavelength, where the current flow from one line to another is no longer that of either uniform lines. This effect is modelled by a series inductance for the step Ls. The electric field will be distorted at the corners of the step are approached and in particular there will be fringing electric fields from the transition edge. The excess charge stored in the region will be modelled by step capacitance. Thus an equivalent circuit for step is formed, fig: 2, with inductance component splits into two parts. For symmetrical step, the capacitances and inductances of the equivalent circuit indicated in fig. 2 are approximated by the following formulation
ε re1 W2 ε re1 + 0.3 W1 h + 0.264 1 − Z 01 W1 ε re1 − 0.258 W1 h + 0.8 Capacitance in picofarad (pF) C = 0.00137h
L1 =
L w1 L , L w1 + L w 2
L1 =
Lw 2 L L w1 + L w 2
(1)
(2)
With L wi = Z ci
ε rei c 2
ε re1 Z L = 0.000987 h1 − c1 (nH ) Z c 2 ε re 2 Where Lwi for i= 1,2 are the inductances per unit length of the appropriate microstrips, having widths W1 and W 2 ,
respectively. While Z ci and ε rei denote characteristic impedance and effective dielectric constant corresponding to width
Wi , c is the speed of light in free space, and h is the substrate thickness in micrometer.[1]
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The Miniaturized Bulb Shape Microstrip Low-Pass Filter Using Parallel Coupled Strip Lines
Parallel Coupled Line: Coupled microstrip lines are widely used for implementing microstrip filters. The cross section of
a pair of coupled microstrip lines under consideration in this section, where the two microstrip lines of width W are in the parallel- or edge-coupled configuration with a separation s. This coupled line structure supports two quasi-TEM modes, i.e., the even mode and the odd mode. For an even-mode excitation, both microstrip lines have the same voltage potentials or carry the same sign charges, say the positive ones, resulting in a magnetic wall at the symmetry plane. In general, these two modes will be ex-cited at the same time. Therefore, the coupled microstrip lines are characterized by the characteristic impedances as well as the effective dielectric constants for the two modes. However, they propagate with different phase velocities because they are not pure TEM modes. This means that they experience different permittivities.
Fig. 3: Equivalent Circuit of Parallel Couple Line CP =
Yl sin (β l )
(3)
ω
and
LP =
βl Z l tan 2
(4)
ω
Fig. 4: Combined Equivalent Circuit of Fig.1 & Fig.3 Design Analysis
A 3rd order Chebyshev filter is considered for designing Purpose. Method of analysis begins with the calculation of inductive and capacitive stubs with the help of traditional high–low filter design [10]. Even though filter realization using [10] shows a reduction in elements’ size, circuit shows a poor response. The technique reported in [9] improves the performance of the filter. However, size of the filter and performance can be further improved if the capacitive stub is modeled as the parallel capacitances. C = CP + Cf
where C is the total capacitance extracted from the design table. CP is the parallel plate capacitance between metallic plate and ground plate and Cf is the fringing field capaci-tance associated with electric field lines passing through air via dielectric to the ground. Since parallel plate capacitance is directly proportional to the surface area, geometry of the plate may be changed while keeping CPconstant. With this logic, filter can be realized with the help of any kind of geometry.
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Inder Pal Singh , Manish Gupta & N Kumar Swamy
Existing Filters Design
A 3rd order Chebyshev filter with −3 dB cut off frequency at 2 GHz and attenuation of 0.1 dB in pass band with port impedance 50 is analyzed using the substrate of dielectric constant 3.2, thickness 0.762 mm. Prototype and real values of inductances and capacitance are calculated using [10]. Conventional Microstrip Low-Pass Filter
Prototype values of inductance and capacitance are con-verted in to the microstrip structure using existing 2
technique. A number of iterations are made to reduce parasitic com-ponents and this filter requires 134.21 mm effective are on the substrate. This kind of filter is not suitable for the modern communication system due to very poor stop band characteristic and wide effective are on the substrate. Skirt Type Low Pass Filter
Fig. 5: Skirt Microstrip Low Pass Filter Feed Line Width W1 =1.8, W2 = 0.32, L = 6.78, B = 5.76, C= 1.92 H = 6.94, G=0.55(Dimensions In Mm)
Here filter is designed using even and odd mode analysis[9]. Response of the filter is compatible with the modern communication system. The value of inductance and capaci-tance are also extractable from the conventional filter design technique. Resulting structure using the conventional filter design technique and its simulated response are shown in fig: 7
Fig. 6: Equivalent Circuit of Impedances of Skirt Microstrip Low Pass Filter
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The Miniaturized Bulb Shape Microstrip Low-Pass Filter Using Parallel Coupled Strip Lines
Fig.7: Response of Skirt Filter 2
Total effective area of this filter is 80.32 mm and single attenuation pole occurs at the frequency 3.85 GHz. Although along with sharp cut off frequency and wide stop band, the filter shows a reduction of 42 .11% of area as compared to conventional filter, yet there is a possibility of further reduc-tion in the size by using arbitrary shape of the capacitive patch and folding the high impedance lines. Proposed Bulb Type Microstrip Low Pass filter
Here filter analysis has been done with the help of the classical filter design technique with the added feature of magnetic coupling as discussed earlier. The capacitive rect-angular stub is replaced by a bulb type patch with area of 39.90 2
mm as shown in fig: 8. The proposed filter shows multiple attenuation poles in the stop band. To investigate the exact cause of generation of multiple attenuation pole in the stop band as shown in fig: 9 , the structure is simulated for different value of ‘s’ while keeping the total length of high impedance lines constant. For low value of ‘s’ attenuation poles are generated due to the coupling between parallel high impedance lines, bending of impedance lines and coupling between left upper edge of capacitive stub and right side high impedance line. The first attenuation pole near cut off frequency is due to coupling between parallel high impedance lines whereas the second attenuation pole is due to the discontinuity at the right angle bend of high impedance lines. The Middle attenuation pole is generated by coupling between high and low impedance lines. When the value of ‘s’ is less, the coupling between right side high impedance line and capacitive patch is experienced and for extreme high value ‘s’ the coupling is between high impedance lines and feed lines. When’s’ is
Fig. 8: Bulb Type Microstrip Low Pass Feed Line Width w1 = 1.8, w2= 0.32, l1 = 5.37, l2= 2.46, l3 = 4.13, g1 = 0.55, g2 = 0.64, (dimensions in mm)
g3 = 0.70, d = 8.05, s = 0.277, r= 4.025
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Inder Pal Singh , Manish Gupta & N Kumar Swamy
approximately half of the length of high impedance line, the middle attenuation pole disappears. In the present design s = 0.277 mm is selected to improve the stop band characteristics and reduce the effective area of proposed filter The results of the proposed structure is simulated using software IE3D . The results of simulation are shown in fig: 9
Fig. 9: Response of Proposed Filter Table 1: Simulated Result
Filter Structures
Capacitive Patch Area 2 (In Mm )
Effective Area on Substrate 2 (In Mm )
Conventional low pass filter
39.97
Skirt type low pass filter Bulb type low pass filter
39.97
80.32
39.97
75.23
134.21
RESULTS AND CONCLUSIONS Results are Summarized in Table 1. 2
It is observed from the above table that proposed filter design requires 75.23 mm effective area, giving an 13.57% improvement over existing skirt filter area. First attenuation pole −56.7 dB at 4.2 GHz is achieved in the proposed design. Simulated results by IE3D software as shown in fig: 9 are in the close agreement in the pass band of the filter. Compact Low-Pass Filter Demonstrated Many Desirable Features
Low pass band insertion loss, sharp skirt characteristic and very broad stopband. Also, our design can be further extended and used as the unit element in higher order design process. The low-pass filter proposed in this paper is quite useful for applications in modern communication systems In the present letter a new structure of microstrip low pass filter has been analyzed and results have been verified. This technique may be extended to miniaturise the other capacitive structure topology by making the trade off between separation parameter ‘s’ and total effective area.
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The Miniaturized Bulb Shape Microstrip Low-Pass Filter Using Parallel Coupled Strip Lines
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