The rapid growth of wireless communications is generating demand for integrated planar components to meet various needs, such as size reduction, performance, and cost requirement. The use of multilayer circuit configuration makes microwave and RF circuits more compact. This, in turn, opens more opportunity for their use in portable equipment, as well as offering design flexibility.

To that end, directional couplers and baluns have been implemented in multilayer circuits. This filter design, in multilayer configurations, is currently being applied to broadside-coupled symmetrical strip configuration.

There are many methods that can be used to analyze multilayer transmission lines, such as the boundary element method (BEM), finite element method (FEM), and spectrum domain method (SDM)^4,5.

Another method, called the modified Method of Line, is introduced here because it has the ability to simplify the deduction of formulas, making computation easier.

The design procedure of single-layer filter using symmetric coupled microstrip lines is well documented in literature and papers². However, one issue is that it is difficult to get tight coupling with this configuration. Multilayer broadside-coupled configurations overcame this restriction because they have inherent broadband characteristics. Furthermore, these configurations have additional desirable performance and functional properties not available with edge-coupled configurations¹.

In this article, a simple design method based on modified Method of Line combined EM simulator is presented. This method shows how to design multilayer dielectric, broadside-coupled, line band pass filters. The results of such a filter design at 5.7 GHz shows the method is suitable for designing multilayer coupled line filters.

A general configuration for a multilayer broadside-coupled lines bandpass filter made up of four coupled line sections (λ/4 each at the center frequency) is shown in figure 1.

The design procedure can be broken up into three steps:

1. Evaluate the desired normal mode parameters for various coupled line sections.

2. Determine the physical dimensions (width, spacing, etc.) of each coupled line section to obtain the required normal mode parameters as computed in step 1.

3. Simulate the physical structure obtained in step 2 to verify the design.

In step 1, using the filter specifications (bandwidth, insertion loss, ripple level, etc.), the “g” parameter's prototype lowpass filter elements are obtained. Considering the selected terminating impedances (Z₀₁, Z₀₂), the admittance is derived or the inverter model for each asymmetric coupled section as follows:

where Δ is the bandwidth, Z_0i is the impedance of the input line, Z_0o is the impedance of the output line, and J is the admittance of admittance inverter.

The normal mode parameters (c- and π-mode voltage ratios and impedances) are then obtained from this admittance inverter model:

Where R_c and R_π represent voltage ratios and Z_c1, Z_c2, Z_π1 and Z_π2 represent mode impedances for the n^th-coupled section.

For this step, the desired normal mode parameters obtained from step 1 are utilized to come up with physical dimensions which are later plugged into an EM simulator to verify the design. The capacitance and inductance matrices are determined by using Modified Method of Line³, and then compared with desired normal mode parameters via the optimization process. Approximate values of physical dimensions of each couple line section, based on capacitance, are used for an initial guess for the iterative process⁶.

Here, an ADS2001 was chosen to simulate the S-parameter response of the filter (whose physical dimensions were obtained from step 2), to verify the design.

The design method previously described was used to design two kinds of filters. One consists of two dielectric layers, the other has three dielectric layers. Each filter has four coupled line sections.

The dielectric used is RO4350 (ε_r = 3.48), and the thickness of each dielectric is 0.79 mm. The first filter (the two-layer filter) layout is shown in figure 2. The test and simulation results are shown in figure 3. The test results show that the center frequency is at 5.7 GHz, the bandwidth is 420 MHz, the maximum insertion loss is about 2.5 dB and the out-of-band rejection is greater than 30 dB at 4.78 GHz and 6.42 GHz. The physical dimensions of the filter are given in table 1.

The second filter (the three-layers filter) layout is shown in figure 4 with the test and simulation results shown in figure 5. The test results show that the center frequency is also at 5.7 GHz, the bandwidth is 510 MHz, the maximum insertion loss is about 2.5 dB and the out-of-band rejection is great than 30 dB at 4.12 GHz and 6.9 GHz. The physical dimensions of the filter are given in table 2.

There is good agreement between the simulation results and test results (from the data in figure 3 and figure 5). A slight difference between the two is because the center frequency of test response dropped slightly and the bandwidth expanded slightly when compared to the simulation response. These discrepancies can be attributed to the precision and technique of manufacturing, the discontinuity of coupled lines, the influence of metal walls and the tolerance of dielectric material.

This article has presented a novel and simple approach to designing multilayer dielectric broadside-coupled lines bandpass filters. Two kinds of filters were analyzed at 5.7 GHz. The results showed that the four coupled sections filter configuration on both two and three layers dielectric can be designed and perform reasonably well.

1. Choonsik Cho and K.C.Gupta, “Design Methodology for Multilayer Coupled Line Filters”, IEEE Trans on MTT, July, 1997.

2. Michael Tran and Cam Nguyen, “Modified Broadside-coupled Microstrip Lines Suitable for MIC and MMIC Applications and a New Class of Broadside-coupled Bandpass Filters” IEEE-MTT, vol 41. No. 8. pp. 1336-1342, August 1993.

3. Wei Hong, “Principle and Application of Method of Line,” (SEU Publishing House).

4. Vijai K.Tripathi, “Asymmetric-coupled Transmission Lines in an Inhomogeneous Medium,” IEEE Trans on MTT, pp. 734-739, September 1975.

5. Zhen_Hai Zhu et al., “Electromagnetic and Transient Simulation of Interconnects in High-speed VISI,” IEEE Module Chip Conference. pp. 93-98, 1994.

6. K.C. Gupta et al., “Computer-aided Design of Microwave Circuits” (Aretch House: 1981).

Xiaowei Zhu received his MSEE and Ph.D. degree from Southeast University, Nanjing P.R China. He is a professor at the department of radio engineering, Southeast University Nanjing (210096), P. R. China now. As a member of the State Key Laboratory of Millimeter Waves, his research interests include RF subsystem and circuit design, especially focus on the RF module design of wireless communication systems, such as WCDMA, cdma2000 and WLAN. He can be contacted at xwzhu@seu.edu.cn.

Yong Zou received his BSEE and MSEE degree from Southeast University, Nanjing, P.R China. He is a engineer at Nan Ning branch of China Mobile Company.

Wei Jiang received his MSEE degree from Nanjing University of Science and Technology, Nanjing P.R China. He is a lecturer at the Department of Radio Engineering, Southeast University Nanjing (210096), P. R. China. As a member of the State Key Laboratory of Millimeter Waves, his research interests include RF subsystem and circuit design, especially focus on the RF module design of wireless communication systems, such as W-CDMA and cdma2000. He can be contacted at weijiang@seu.edu.cn.

Ling Tian received her BSEE degree from Southeast University, Nanjing P.R China. She is a assistant at the Department of Radio Engineering, Southeast University Nanjing (210096), P. R. China. As a member of the State Key Laboratory of Millimeter Waves, her research interests include RF subsystem and circuit design, especially focus on the RF module design of wireless communication systems, such as W-CDMA and cdma2000. She can be contacted at ltian@seu.edu.cn.

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