A Compact Microstrip Quad-Channel Diplexer With High-Selectivity and High-Isolation Performances

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coupled-resonator,
Fig. l. Coupling structures of the proposed quad-channel diplexer with an Nth-order Chebyshev bandpass response.
I. INTRODUCTION In multi-service and multi-band communication systems, diplexer is an important and essential component in the radio frequency (RF) front ends of both the receiver and transmitter. In general, diplexers require high compactness, low cost, high isolation, high selectivity, flexible channel frequencies, and ease to realize. With the rapid growth of modem communication systems, diplexers and multiplexers with multi-band operation are always necessary. Recently, several advanced techniques for the diplexer and multiplexer design have been widely studied [1]-[8]. In [1], diplexer based on the stepped-impedance coupled­ line resonator can offer a compact size and high isolation performance by equalizing velocities of the odd- and even­ mode waves in an inhomogeneous medium. In order to reduce circuit size, diplexers designed with the common-resonator technique are proposed [2], [3]. In [4], a compact diplexer is implemented by using slotline stepped-impedance resonators in the ground plane. In [5], a systematic design procedure for the matching circuits of triplexers is presented. In [6] and [7], a six-channel multiplexer composed of parallel-coupled bandpass filters and a quadplexer based on the tri-mode net­ type resonators have been developed to operate in multiple separated frequency bands. In [8], a quad-channel diplexer formed by two dual-band bandpass filters (BPFs) is proposed. However, these techniques mentioned above are difficult to further realize a diplexerlmultiplexer with higher frequency selectivity and more operating channels. In this work, an effective design method to realize compact, high-selectivity, and high-isolation quad-channel diplexers has been proposed. By introducing multiple coupling paths between input and output ports, the degrees of freedom in the design parameters at all channel passbands are, then, increased. Thus, the diplexer design can be easily extended to achieve better frequency selectivity with higher filter order. In addition, based on the common-resonator technique [9], the resonator number and/or circuit size can be reduced. Moreover, no extra matching network is needed for this design. For a demonstration, a quad-channel diplexer with third-order bandpass response has been designed and implemented to verify the proposed concept. II. DIPLEXER DESIGN Fig. 1 shows the proposed coupling scheme a quad-channel diplexer with an N-order Chebyshev bandpass response, where each node represents a resonator and the lines between nodes represent the direct coupling paths; S and L denote the input and output ports, respectively. As can be seen, the diplexer is formed by two dual-band filters (BPFs I and 11), where the BPF I operates atfOl andf02, i.e., the center frequencies of the channels 1 and 2, respectively and the BPF II operates at f03 and f04, i.e., the center frequencies of the channels 3 and 4, respectively. Here, the common-resonator technique in [9] is employed to design the two dual-band BPFs. For the BPF I design, the odd-number resonators with superscript I are designed to resonant simultaneously at fOl and f02; the even­ number resonators with superscript I and subscript x are designed to resonant at fOl while the other resonators with subscript yare designed to resonant atf02. Similarly, the same manner can be applied to the BPF II design. Note that since the four channels of the diplexer are designed to operate at different frequency bands, the loading effect between bands can be neglected. In other words, each channel can be designed individually to yield the desired passband before
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The measured results are in good agreement with the
simulated predictions, showing that an excellent isolation of better than 55 dB between channels is achieved. Index Terms - Bandpass filter coupling, diplexer, microstrip.
A Compact Microstrip Quad-Channel Diplexer With High-Selectivity
and High-Isolation Performances
Chi-Feng Chen, Cheng-Yu Lin, Bo-Hao Tseng, and Sheng-Fa Chang Department of Electrical Engineering, Tunghai University, Taichung, Taiwan 40704
978·1-4799-3869-8/14/$31.00 ®2014 IEEE
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