IEEE MEiLECON 2002, May 7-9,2002, Cairo ,EGYPT.
Design and Synthesis of Microwave Coupled Resonator Diplexers for Cellular Base Stations *Amjad A. Omar, **Safieddin Safavi-Naeini, and **Sujeet K. Chaudhuri
*
Department of Electrical and Computer Engineering, Hashemite University, PO Box 150459,Zarqa 131 15, Jordan,
[email protected] , Tel(962) 5-3826613,Ext 4391,Fax: (962) 5-3826613 **Department of Electrical and Computer Engineering, University of Waterloo, Canada
Abstract An approximate circuit model is proposed for coupled coaxial cavity diplexers. The accuracy of this model is verified using available design software. A design strategy that will yield the parameters of this equivalent model for any set of diplexer design specifications is also proposed and verified using practical examples.
Keywords Microwave filters, Diplexer, Resonator. 1. INTRODUCTION A diplexer is a familiar device that separates a composite signal into its constituent parts to permit each part to be transmitted separately. The design of a diplexer is one of the basic problems in communications. Because of the mutual interaction effect of the two filters composing the diplexer, the characteristics of a diplexer are different from the responses of the individual two filters. The complexity of the interaction effect makes the design of a diplexer very complicated. This work provides a general design strategy for coupled coaxial resonator diplexers. These diplexers offer advantages in terms of low cost and good performance, and are therefore currently used in mobile base stations and GSM. The first step in designing any diplexer is the design of each of its two filter arms. For this purpose, Cameron [ I ] obtained the coupling matrix of general Chebyshev bandpass filters. Atia et a1 [2] and Kahrizi et al [3], suggested a proper optimization scheme that would yield the coupling matrix for any set of filter specifications. In addition, Borji [4] and Macchiarella [ 5 ] provided approximate formulas for converting the parameters of the coupling matrix of [2]-[3] into actual dimensions of the coupled resonator coaxial filter.
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2. APPROXIMATE CIRCUIT MODEL FOR A DIPLEXER The proposed approximate circuit model for a coupled cavity diplexer is shown in Figure ( I . 1). To verify the validity of this model, a coupled cavity diplexer was designed using the full wave commercial simulator HFSS-Ansoft. Since HFSS is very time consuming, especially for large circuits, the diplexer designed consisted only of a common cavity and one cavity for each filter arm, as shown in Figure (1.2). The circuit simulator and optimizer (HP-ADS) was then used to calculate the parameters of the proposed approximate circuit model for the diplexer that will fit the scattering parameters of HFSS. The results for the comparisons between the S-parameters obtained from the approximate model using ADS and those obtained using HFSS are shown in Figure (1.3). Very good agreement is obtained which validates the approximate model for a coupled cavity diplexer.
M.1
Figure 1.1: Proposed Approximate Circuit Model for A Coupled Resonator Diplexer.
transmission zeroes, and finally the probe inductance connected to each port of the filter. 2. Having calculated the parameters for each filter, what remains as unknowns are: the self inductance of the common cavity (Lc), the capacitance of the common cavity (C& the mutual inductance between the common cavity and the first cavity in filter 1 (Mc,), the mutual inductance between the common cavity and the first cavity in filter 2 (Mcz), the mutual inductance between the two apertures of the common cavity (Mco), the feed probe transformer ratici (n),the self inductance of the feed probe (Ls), the first self term in the coupling matrix
M
for each filter (i.e
for each filter).
3.The unknown parameters above are obtained using optimization which is carried out using a NAG library subroutine that employs the SQL method. The objective function is to minimize the scattering Figure 1.2: 3-D View of the Coupled Cavity Diplexer Designed Using HFSS (All dimensions are in mm)
parameter
Is, ,I
2
over the bands of both filter channels.
4 Numerical Examples For this application, the passband of filter 1 (port 2 of diplexer) is between 1890 MHz and 1910 MHz. The passband of filter 2 (port 3 of diplexer) is between 1920 MHz and 1940 MHz. Applying the design procedure explained in Section 3, results in the diplexer with response shown in Figure (4.1)
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Figure 1.3: Comparison Between the Full Wave Results of HFSS and the Approximate Model Results of ADS for the Diplexer of Figure (1.2)
3 Design Strategy for a Diplexer The next stage is to propose a general design strategy that will yield the parameters of the approximate model of Figure (1.1) given some general design specifications for the diplexer. The design stages are as follows: 1. Design the bandpass filter of channel 1 and 2 using the technique in [3]. At this stage, the required design specifications for the filter are: center frequency, bandwidth, retum loss in passband, number of
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Figure 4.1 : Scattering Parameters of the Diplexer . Each filter's Degree = 6, IRetum loss for each filter=30 dB, Transmission zeroes for filter 1 : 1885 MHz, 1915 MHz, Transmission zeroes for filter 2: 19 15 MHz, 1945 MHz.
5 Conclusion An accurate circuit model was proposed for coupled resonator diplexers. This model was verified numerically using available commercial software. A General design strategy was also proposed for this type of diplexers and has been tested for different design specifications. This strategy is simple and has been shown to yield accurate results. As for the computational efficiency, it takes about IO minutes to do the overall design for any set of practical design specifications.
References Cameron, Richard J.,” General coupling matrix synthesis methods for Chebyshev filtering functions”, ZEEE Trans. Microwave Theory Tech., Vol. MTT-47, no. 4, April 1999, , pp. 433- 442. Atia, Walid A, Zaki, Kawthar A., and Atia, Ali E., ” Synthesis of general topology multiple coupled resonator filters by optimization”, ZEEE MTT-S International Microwave Qmposium Digest., 1998, pp. 82 1-824. Kahrizi, M., Safavi-Naeini, S., and Chaudhuri, S.K.” Computer diagnosis and tuning of microwave filters using model-based parameter estimation and multi-level optimization”, ZEEE MTT-S International Microwave Symposium Digesr, Volume 3,2000, pp. 1641 -1644. Borji, A., “Approximate Models for Coaxial Cavity Filters”, Annual Report I, Center of Wireless Communications, University of Waterloo, Feb. 2000. Macchiarella, G.” An original approach to the design of bandpass cavity filters with multiple couplings”, IEEE Trans. Microwave Theory Tech., Vol. MTT-45, no. 2, February 1997, pp. 179-1 87.
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