Design and simulation of an Edge-coupled Band Pass Filter at X Band

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Design and simulation of an Edge-coupled Band Pass Filter at X Band

Md Rasheduzzaman Al-Amin, Sourav Sundar Chowdhury Dept. of Electronic and Telecommunications Engineering

International Islamic University Chittagong Bangladesh

Khondoker Ziaul Islam

Dept. of Electrical and Electronic Engineering Bangladesh University of Business and Technology

Abstract— This paper presents the design and simulation of band pass filter (BPF) which is developed with the help of Richards-Kuroda transformation method on the basis of Chebyshev low pass filter approximation. The designed filter is operated at X band segment (8 – 12) GHz of the microwave frequency spectrum. The filter is designed at a center frequency of 10 GHz and with 4 GHz bandwidth. The circuit is simulated using Rogers R06006 substrate having thickness of 0.035 mm, dielectric constant of 6.45 and substrate height of 0.787 mm. Due to resource limitation, the work is confined to simulation that is carried out by Ansoft Designer v2.2 for accurate in-depth analysis and arrival at the final layout. Simulation results show that the designed filter has flat and uniform response over the entire pass band. The obtained results are verified with the result obtained from a full EM wave professional simulator HFSS. The results are excellent and the final achieved layout of the 3rd order edge-coupled strip line band pass filter is suitable for installation with different systems, sub-systems at X band.

Index Terms—Band Pass Filter, Chebyshev, Edge-coupled, X Band, Stripline.

I. INTRODUCTION

Wireless communication is developing and flourishing very fast that pacifies the need of RF/Microwave filters with higher selectivity and low insertion loss. One possible option for fulfilling the desired characteristics is elliptic function response filter. This type of band pass filter is designed and simulated using waveguide cavities or dielectric-resonator-loaded cavities [1-2]. Due to the advancement of high temperature super- conducting and machined circuit technologies, researchers are showing their keen interest in microstrip, stripline filter structure [3].

In RF and microwave applications, Stripline filters play vital role. With the advancement of technology, more specific requirements of filters are needed [4, 5]. At high frequencies like that of RF and Microwave, lumped inductors and capacitors lack their intrinsic characteristics. For this reason, using distributed elements for example transmission line

methods are used for designing required filter at higher frequencies [6]. Now-a-days, waveguides are used for fabricating filters extensively. But fabrications of filters with these waveguides are expensive and bulky. Stripline and microstrip technologies are cheaper and low power alternatives. In this paper an edge coupled filter is designed and simulated using strip line technology. Because this does not suffer from dispersion and its propagation mode is pure TEM mode where as microstrip has quasi-TEM propagation mode.

Therefore it is the ultimate choice among many other alternatives [6, 7]. Our objective is to design an edge-coupled band pass filter for 8 to 12 GHz application using Ansoft designer V2.2 Software and verify the results with the result of HFSS design.

II. METHODOLOGY

Various techniques are prevalent for manufacturing and fabricating these essential microwave filters. Among them, waveguide cavities and resonator based filters are the most widely used. The specifications of dielectric material are obtained from Rogers Corporation [8]. It is depicted in Table I while the design specifications of the filter are shown in Table II. The steps followed for designing an edge-coupled band pass filter are: determination of the order of filter and type of approximation functions, determination of the corresponding low-pass prototype, transformation of the low-pass prototype into a band pass configuration, impedance and frequency scaling of the band pass configuration, transformation of the lumped circuit element into distributed elements.

A good band pass filter has minimal signal loss in its pass band, as well as a narrow pass band with as much out of band attenuation as possible. Chebyshev filter is selected because this filter has steeper initial descent in stop band unlike other filters. Chebyshev filters have narrower pass band response in

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trade for more ripples in the pass band section [9]. With the help of the attenuation characteristics for a Chebyshev filter with 0.5dB ripple shown in Fig.1 the order of the filter is determined. Third order filter is chosen in this work in order to satisfy the design specification with the help of equation (1) [10, 11].

TABLE I. SPECIFICATIONS OF DIELECTRIC MATERIAL

S No. Name Values

1. Substrate: Rogers R06006

2. Conductor thickness: 0.035 mm 3. Dielectric constant: 6.45

4. Loss tangent: 0.0027

5. Substrate height: .787 mm

TABLE II. DESIGN SPECIFICATIONS OF THE FILTER

S No. Name Values

1. Input and output impedance: Z = 50 Ohms

2. Pass band ripple 0.5 dB

3. Filter order: n= 3

4. Pass band centre frequency: 10 GHz

5. Ripple bandwidth: 2 GHz

=

cosh 10 − 1

− 1 cosh

(1)

Normalized element values for equal ripple band-pass filter prototypes are taken from Table III. [10]. Normalized element values for 0.5 dB ripple low-pass Chebyshev filter are g1 = g3 =1.5963, g2 =1.0967, g4 = 1.0000. Fig.1 Characteristics for a Chebyshev filter with 0.5dB ripple TABLE III. NORMALIZED CHEBYSHEV ELEMENT VALUES FOR 0.5 DB RIPPLE Order g 1 g 2 g 3 g 4 g 5 g 6 g 7 g 8 g 9 2 1.4029 0.70710.5040 3 1.5963 1.09671.5963 4 1.6704 1.19262.3662 0.8419.05040 5 1.7058 1.22962.5409 1.22961.7058 6 1.7254 1.24782.6064 1.31362.4759 0.86960.5040 7 1.7373 1.25822.6383 1.34432.6383 1.25821.7373 8 1.7451 1.26472.6565 1.35902.6965 1.33892.5093 0.87950.5040 9 1.7505 1.26902.6678 1.36732.7240 1.36732.6678 1.26901.7505 With these values the low pass prototype is drawn as shown in the Fig.2. The next step is transforming the low pass prototype into band pass configuration. This transformation process is carried out by resonating each low pass element with element of the opposite type but having same value. Thus every series elements of the low pass prototype are transformed into series- resonator circuit and every shunt elements are transformed into parallel –resonator circuit [11]. The complete band pass transformation of low pass prototype for third –order Chebyshev filter is depicted in Fig.3 using the equations (2-5). Fig.2. Third order low pass prototype After transforming the low pass prototype into band pass network, the filter is then frequency-scaled and impedance- scaled [11]. The transformations of the third –order band pass filter are completed using Richard’s transformations and Kuroda’s identities [10-11]. = 1

∗ ∗ . (2) 

= 1

∗ . (3)

(3)

= . (4)

= 1

∗ . (5)

Fig.3. Band Pass Prototype The coupled line structure of the edge-coupled filter supports two quasi- TEM modes namely even and odd modes. The three parameters of the stripline filters namely Width (W), diameter or the height of the substrate (d) and separation (S) of the edge-coupled filter are determined from the even (ZOE) and odd (ZOO) mode impedances of the coupled line structure [11]. The even and odd mode impedances are dependent on the gain of the admittance inverter J as shown in equations (6- 11). = 1 ∆

2 (6)

= 1

2 ∆ . (7)

= 1 ∆

2 . (8)

= (1 + + ( ) ). (9)

= (1 − + ( ) ). (10)

∆= . (11)

Insertion loss and return loss are the two parameters for analyzing the performance of filter. A good filter has higher return loss and smaller insertion loss ripple in the pass band. Return loss measurement is used to evaluate the impedance match of a filter. As the match between the characteristic impedance (Zo) and the load impedance improves the reflected wave becomes smaller. The related equations are shown in equations (12-14). Thus, reflection co-efficient as shown in the equation no (12) gets lowered. = 10 log . (12)

Γ = . (13)

= 10 log( ) . (14)

When the impedances match completely there remains no reflected wave and the reflection co-efficient becomes zero.

When reflection co-efficient becomes equal to 1, complete mismatch is said to exist. Thus, the range for reflection co- efficient is between zeros to one.

III. RESULTS AND DISCUSSION

The simulated results from Ansoft Designer Student Version 2.2 and HFSS software are presented in this section for analyzing the performance of the designed and simulated band pass filter. The Software result of odd and even impedances response of 3rd order edge-coupled strip line band pass filter is presented in Fig.4. The Physical model configuration of 3rd order edge-coupled strip line band pass filter is depicted in Fig.5.The Electrical model configuration of 3rd order edge- coupled strip line band pass filter is depicted in fig.6. The Simulated result for insertion loss and return loss response of 3rd order edge-coupled stripline band pass filter is shown in Fig.7 and Fig.8 respectively. The simulated results from a full EM wave professional simulator HFSS is shown in Fig.09. The Layout and the 3D-Layout of 3rd order edge-coupled stripline band pass filter are shown in the Fig.10 and Fig.11 respectively.

Fig.4. Software result of odd and even impedances response of 3rd order edge-coupled strip line band pass filter.

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The three parameters of the stripline filters namely Width (W), diameter or the height of the substrate (d) and separation (S) of the edge-coupled filter were determined from the even (ZOE) and odd (ZOO) mode impedances of the coupled line structure. From the figure, it is seen that the insertion loss is less than 0.02 (Fig.7.) dB in pass band which is the characteristics of a good filter. Like a good filter, over the entire pass band the response is flat and uniform (Fig.8). A return loss of almost -80dB (Fig.8) is seen at the desired band where as this paper [11] reports the achievement of -50dB of return loss at their desired band. Thus the calculated reflection co-efficient of our work is smaller in comparison to that paper [11].

Fig.5. Physical model configuration of 3rd order edge-coupled strip line band pass filter

As results of other research paper of Edge coupled filter design for different band such as Ka [11] and U[12] band were observed, it is seen that similar and better simulation results are achieved in this paper as depicted in the figures. Therefore, from the figures it can be concluded that perfect matching is achieved in the desired band. Moreover, it is seen that obtained performance of the designed 3rd order edge-coupled stripline band pass filter is as desired and expected.

Fig.6. Electrical model configuration of 3rd order edge-coupled strip line band pass filter

Fig.7. Simulated result for insertion loss and return loss response of 3rd order edge-coupled stripline band pass filter

Fig.8. Simulated insertion loss and return loss at X band

Fig.9. HFSS simulated insertion loss and return loss at X band

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Fig.10. Layout of 3rd order edge-coupled stripline band pass filter

Fig.11. 3D-Layout of 3rd order edge-coupled stripline band pass filter

The comparative studies among the figures 7, 8 and 9 clearly denote that the results are having flat response over the entire pass band both from Ansoft Designer SV and HFSS simulators. Moreover, the insertion losses in both the cases are very small. Though the figures of the Ansoft Designer SV has a return loss of -80 dB, HFSS reports to show around -40 dB which could happen due to impossibility of perfect matching of the substrate’s details for both the simulators. The last two figures show the layout for the designed and simulated filter.

This layout then used for fabricating filter on the hardware and after proper manufacturing can be integrated at different systems, sub-systems at X band.

IV. CONCLUSION

This paper describes an effective procedure for designing RF and Microwave filters. It describes the in detail procedure of designing an edge coupled strip line band pass filter particularly at X band. The presented process includes the estimation of filter parameters using analytical formulas, the simulation of stripline transmission line models in EM simulator, and the final 3D layout of the 3rd order edge-

coupled stripline band pass filter. Experimental and hardware implementation of this work involves the Roger R06006 substrate with dielectric constant of 6.45 dielectric characterizations at the desired microwave frequencies. Due to resource limitation, the work is confined to simulation that is carried out by Ansoft Designer v2.2 for accurate in-depth analysis and arrival at the final layout. Finally, the obtained results are verified with the result obtained from a full EM wave simulator HFSS.

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