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Duplexer, Microstrip Filter, Fractional Bandwidth, Resonators International Journal of Electromagnetics and Applications 2016, 6(1): 7-12 DOI: 10.5923/j.ijea.20160601.02 Design of Duplexers for Microwave Communication Systems Using Open-loop Square Microstrip Resonators Charles U. Ndujiuba1,*, Samuel N. John1, Taofeek O. Bello2 1Electrical & Information Engineering, Covenant University, Nigeria 2Electrical & Electronics Engineering, University of Lagos, Nigeria Abstract This paper proposes a duplexer design with two microstrip bandpass filters which are designed using Open Loop Square Resonator method and matching between these two bandpass filters using T-junction matching devices. Both seven-pole band-pass filters for the receiver at 1710-1785 MHz and transmitter at 1805-1880 MHz, with fractional bandwidth of 4.29% and 4.07% respectively, of the center frequencies were implemented by resonators and admittance inverters with a 0.1dB ripple in the pass band. Agilent Advance Design Simulation (ADS) EM simulator was used in obtaining the dimensions of the resonators, extraction of coupling coefficients and external quality factors associated with the microstrip design. Results obtained showed a return loss of about 8dB, insertion loss less than 1dB and isolation between transmit and receive filters of about 32dB. The choice of the square open-loop resonators against parallel coupled lines is to avoid the inherent shift in the center frequency of the parallel coupled technique. The designed microstrip duplexer filter showed good agreement with theoretical results. Keywords Duplexer, Microstrip filter, Fractional bandwidth, Resonators defects are particularly serious in duplexers where isolation 1. Introduction between the transmit and receive paths are required to be high, and multiplexers where adjacent channels must be Microwave communication systems require, especially in sharply and accurately defined. satellite and mobile communications, high-performance Duplexer designed for the base station antenna allows the narrow-band bandpass filters having low insertion loss and combination and separation of the signals in band1 and high selectivity together with linear phase or flat group delay band2 wireless bands. To minimize band inter-reaction, the in the pass-band. These filters form an integral part of power inputs are all isolated and have minimal insertion loss over amplifiers at the transmitter and receiver in radio systems. their respective frequency bands. The design ensures The miniaturization of electronic components has low-passive intermodulation. An efficient duplexer needs received a lot of attention in the last decades due to the rapid high isolation between two passband frequencies to avoid development of the telecommunication industry. Traditional interference of signal from one port to another. high performance waveguide and dielectric resonator filters The implementation of the duplexer filter on a microstrip are usually too heavy and bulky for most applications like structure will help to achieve a filter with a compact size and tower-top mounting in base stations [1]. high performance. The duplexer consists of two bandpass In radar and radio communications systems, a duplexer filters which could either be realized using lumped allows for a bi-directional (duplex) communication over a components or distributed components. Lumped single path. This device isolates the receiver from the components consists of discrete elements like inductors, transmitter while permitting them to share a common capacitors etc. Distributed elements consist of transmission antenna. One major problem associated with this design is line sections which simulate various inductance and the degradation in filter selectivity. Practical RF and capacitance values. It is hard to realize filters with lumped Microwave filters realized with low-Q reactive elements not elements at high frequencies in the GHz range, the only have high insertion loss, but also have rounded dimensions of the electronic components are comparable passband corners, the positions of the poles and zeros with the wavelength of the signal as a result of which there displaced, and the desired centre frequency shifted. These could be distributed effects. However transmission line filters are easy to implement and are compact at these * Corresponding author: [email protected] (Charles U. Ndujiuba) frequencies [2]. A bandpass filter allows transmission of Published online at http://journal.sapub.org/ijea frequencies in the passband and attenuates frequencies in the Copyright © 2016 Scientific & Academic Publishing. All Rights Reserved stopband. The circuit synthesis and microstrip realization of 8 Charles U. Ndujiuba et al.: Design of Duplexers for Microwave Communication Systems Using Open-loop Square Microstrip Resonators the duplexer filter is presented in this paper 1Ω and a cut-off frequency of 1rad/s. For a 7th order (n = 7) Chebyshev filter where the first element is an inductor. The element values for the low-pass circuit are given in table 1 2. Filter Design Using Lumped and represented in figure 1. Components B. Conversion to shunt only using J inverters The synthesis of bandpass filter often starts with a lowpass Immittance inverters play a very important role in filter filter which is achieved by frequency transformation. The design. They are used to transform a filter circuit into an design in this paper is two band pass filters with the same equivalent form that can be easily implemented using characteristics but different band responses of various microwave structures [4]. Immittance inverters are 1710-1785MHz and 1805-1880MHz, for the receive and either impedance or admittance inverters. In this design the transmit path, respectively. Chebyshev lowpass filter J-inverters (an admittance inverter) is used to convert the prototype is adopted in this paper because the attenuation of lumped element low-pass filter to a design that eliminates the the Chebyshev filter is steeper in amplitude versus frequency inductors and contains shunt only capacitors as shown in curve [3] and hence requires only fewer stages for the same figure 2. attenuation demanded. A further transformation to make all the values of the A. Design of Prototype Low Pass Filter shunt capacitor to be equal in figure 3 was carried out using the equation 1.0 to obtain values of J as shown in the table 2 Prototype filter acts as template to provide a modified filter design for particular application. A normalized lowpass = (1) , 2 filter prototype in which the elements involved are 푔1 normalized so as to make the source resistance to be equal to where k = 1, 2 , 3, ...6 퐽′푘 푘+1 �푔푘푔푘+1 Table 1. g values of a 7th order chebyshev normalised lowpass filter order Rs g1 g2 g3 g4 g5 g6 g7 g8 7 1 1.1812 1.4228 2.0967 1.5734 2.0967 1.4228 1.1812 1 Figure 1. Prototype Lumped Element Low pass Filter 01 g0 J g1 J12 g2 J23 g3 J34 g4 J45 g5 J56 g6 J67 g7 J78 g8 Figure 2. Shunt only normalised lowpass filter Table 2. J inverter values with equal g J values for equal g , J01 , , , , , , , 퐽′푘k푘 +1 1 0.9111퐽′1 2 0.6839퐽′2 3 0.6503퐽′3 4 0.6503퐽′4 5 0.6839퐽′5 6 0.9111퐽′6 7 퐽′17 8 ‘ ‘ ‘ ‘ ‘ ‘ g0 J01 g1 J12 g1 J23 g1 J34 g1 J45 g1 J56 g1 J67 g1 J78 g8 Figure 3. Lowpass filter with equal shunt value of International Journal of Electromagnetics and Applications 2016, 6(1): 7-12 9 C. Impedance and Frequency scaling coupling occurring between resonators [5]. The capacitances The low-pass filter prototypes are circuit designed for representing the J inverter were calculated using the equation unity cutoff frequency and unity source and load impedance. 4.0 and presented in table 5. The circuit is desired to resonate at the bandpass cutoff = (4) frequency and have a source and load resistance of 50Ω, an 퐽 푗 impedance and frequency scaling is carried out using Table 5. Values퐶 of 휔the0 -capacitor equation 2.0, new values of g1 and J are obtained. It can be seen from the transformation that the J inverters are only Rx (pF) 흅 Tx (pF) affected by impedance scaling and not affected at all by the Cj01 1.8219 1.7280 frequency scaling. Cj12 1.6601 1.5744 , Cj23 1.2460 1.1817 = , = (2) 1 푘 푘+1 푔 퐽′ Cj34 1.1849 1.1237 where R is 50Ω and휔 푐푅0 = 푘.푘 +1 푅0 0 퐶 퐽′′ C 1.1849 1.1237 Table 3 presents the scaled value of the J inverter j45 푐 1 2 휔 √휔 휔 Cj56 1.2460 1.1817 Table 3. Values of scaled J inverter Cj67 1.6601 1.5744 Values of scaled J inverter Cj78 1.8219 1.7280 Rx Tx Each filter was simulated to give a response shown in , 0.02 0.02 Figures 9 and 10. 0 1 퐽′ , 0.0182 0.0182 The duplexer formed from these two bandpass filter was 1 2 obtained by using a T-junction between the filters. A 퐽′′ , 0.0137 0.0137 combination of the Rx and Tx filter was carried out using an 2 3 퐽′′ , 0.0130 0.0130 input ideal transmission line of 50Ω connected to the J 3 4 퐽′′ , 0.0130 0.0130 inverter (pi capacitors) for the Rx and for the Tx filter. The two 50Ω transmission lines from the Rx and Tx filter were 퐽′′4 5 0.0137 0.0137 both connected together to another 50 Ω transmission line , 0.0182 0.0182 forming a T junction network which forms the duplexer 6 7 퐽′′ , 0.02 0.02 circuit shown in the figure 5. The response obtained after tuning in ADS is given below 퐽′7 8 D. Band pass transformation in figure 11. Low-pass prototype filter designs can also be transformed E. Design of Matching Circuit to an equivalent bandpass filter. During the transformation process, the shunt capacitor of the low-pass prototype is The Matching network and combining circuit ensure that converted to series LC circuits having element value given both filters match the antenna and have good isolation by equation 3.0.
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