The Design of New Structures of Planar Diplexers Using Microstrip Resonators

The Design of New Structures of Planar Diplexers Using Microstrip Resonators

Abdessamed Chinig (Hassan 1st University, Morocco), Ahmed Errkik (Hassan 1st University, Morocco), Abdelali Tajmouati (Hassan 1st University, Morocco), Hamid Bennis (Hassan 1st University, Morocco), Jamal Zbitou (Hassan 1st University, Morocco) and Mohamed Latrach (Microwave Group, ESEO, France)
DOI: 10.4018/978-1-5225-0773-4.ch006
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In this chapter, three planar diplexers based on microstrip resonators are presented. An overview on diplexers is introduced. Then, two designed and fabricated microstrip diplexers with compact size and good performances are exposed. The first diplexer is designed using open loop resonators while the second circuit is achieved based on triangular loop resonators. The third structure represents a compact diplexer designed by using a pair of H-shaped resonators coupled with a coupling patch and two Input/Output (I/O) feed lines. The incorporated slots in the ground plane of the proposed circuit allow the control of the resonant frequencies and enhance its electrical performances. Furthermore, the introduction of these slots represents an interesting solution to miniaturize the microwave filter and diplexer. An analytic method is applied to extract the equivalent LC model of the band pass filters and diplexer. A full wave electromagnetic Analysis is achieved to evaluate the electrical performances of the proposed diplexers by using ADS and CST-MWS.
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Microwave and RF filters and diplexers play an important role in the modern wireless communication applications. They are widely used in many microwave applications for selecting or confining the signals within specified spectral range. Microwave filters are two ports circuits that permit a good transmission of the required frequencies while rejecting the unwanted signal frequencies. Diplexer represents another application of the filters; this component is a three ports device that can separate different frequency bands. As shown in the Figure 1, this circuit is achieved by connecting two filters with different center frequencies (Matthaei, Young, & Jones, 1963). Thereby, it allows to a single antenna to be shared simultaneously by a transmitter and a receiver operating on different frequency bands (Pozar, 1998).

Figure 1.

A block-diagram of the diplexer

Diplexer is a key component in many microwave communication system, including wireless communications system, radar systems, cellular phones and satellite communication systems. Diplexers were widely studied in the early 1960’s by (Matthaei & Cristal, 1965; Matthaei, Young, & Jones, 1963; Wenzel, 1968). Many approaches are possible to design a diplexer based on different filter configurations such as low pass and high pass filters, two band pass filters or band pass and band stop filters (Yao et al., 1993).

The proliferation of the modern wireless communication systems, radar and satellite systems have boosted the demand of microwave and RF filters and diplexers. In this context, the development of highly selective planar filters and diplexers with low loss levels, compact size and low cost is currently a research area of fundamental interest. In recent years, the research activity was greatly devoted to the study of planar structures. Thanks to their low cost, lightweight and small size, these structures are capable of being fully integrated with the active circuits. However, their main drawback is a poor unloaded quality factor that results in poor selectivity and serious insertion losses.

It is obvious that resonators represent the basic components to design a filter. Thus, several microstrip resonators have been proposed to design (BPFs). One of these resonators is the U-shaped hairpin line resonator which was used by (Srisathit et al., 2005) to design diplexers with high isolation. Thanks to their compact and small size, the microstrip open loop resonators were introduced in order to design filters and diplexers (Hong, Shaman, & Chun, 2007; Konpang, 2009). The hybrid resonators proposed by (Yang, Chi, & Itoh, 2010) were also used to achieve diplexers. Besides, due to the possibilities they can offer to control the spurious response, to reduce the insertion losses and the overall size of the circuit, stepped impedance resonators (SIRs) are the most popular resonators utilized to achieve band pass filter and diplexers (Sheta, Coupez, Tanne, Toutain, & Blot, 1996).

The defected ground structure (DGS) which is one of the most promising planar technologies has become an interesting area of research. This technique was proposed in order to design filters and diplexers with good performances in terms of size and electrical response. Even the equivalent circuit models of DGS were studied and employed to design planar circuits. Recently, a great lot of band-pass filters and diplexers using DGS were proposed such as a double-H-shaped microstrip filter designed by (Xiao, Ma, & Li, 2005), the coupled line band pass filter using defected ground structure proposed by (Yun et al., 2000), or the microstrip diplexer using defected ground structure presented by (Liu et al., 2005).

Key Terms in this Chapter

Isolation: The relative attenuation provided between the two paths of the diplexer; transmitter to antenna path and antenna to receiver path.

Defected Ground Structure: When we intentionally modify the ground plane metal of a microstrip (or stripline, or coplanar waveguide) to enhance its performances.

Band-Pass Filter: A two-port frequency selective network used to reject the unwanted signal frequencies while permitting a good transmission for a specified frequency band.

Diplexer: A typical RF component used in several wireless communication systems to transmit and receive signals by a single antenna.

Insertion Loss: The loss in signal power resulting from the insertion of the diplexer in the path of the signal.

CST-MWS: A full-wave electromagnetic (EM) simulator used to calculate the electromagnetic behavior of a 3-D structure.

Coupled-Junction: A matching network used to combine two channel filters. This junction can replace the conventional T-junction and give better diplexer insertion loss and rejection performances.

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