DESIGN OF PLANAR FILTERS USING FRACTAL GEOMETRY AND EBG STRUCTURES

Transcription

1 DESIGN OF PLANAR FILTERS USING FRACTAL GEOMETRY AND EBG STRUCTURES Abstract submitted to The Faculty of Technology, University of Delhi For the award of Degree of Doctor of Philosophy in Electronics and Communication Engineering Submitted by DHIRENDRA KUMAR Under the Supervision of Prof. ASOK DE Electronics and Communication Engineering Delhi College of Engineering, Bawana Road, Delhi 42 Year 2012

2 ABSTRACT 1.1 PROBLEM IDENTIFICATION The compact size filters with spurious free responses are always desirable for the modern wireless communication systems. There are number of techniques have been reported in the literatures to achieve these stringent requirements. The attention of the reported research in the filter design using fractal structures are less in number. In this thesis more emphasis are given on the fractal geometry to design the planar filters to reduce the lengths of filter circuit with easy design methodology. More over the filters with compact size and free of spurious pass bands can be designed using electromagnetic band gap (EBG) structures and defective ground structures (DGS). To exploit these structures for the filter design of desired specifications, researches are needed. The effects of different patterns on the ground plane of microstrip create a lot of interests for research in filter design. The fractal structure and electromagnetic band gap or defective ground structures are very interestingly applied to design the low pass, band pass and band stop filters in this thesis ORGANIZATION OF THESIS In this thesis a research work has been carried out to design the microstrip filters using fractal geometry and electromagnetic band gap structures. The first chapter consists of the basic information about the fractal structure, electromagnetic band gap structures, and defective ground structures along with the historical back ground of microstrip filter research. A complete literature survey of microstrip low pass filters, band pass filters, and band reject filters are given. The second chapter starts with the synthesis of low pass filter using insertion loss (IL) method. A microstrip low pass filter has been designed using stepped

3 impedance technique using Chebyshev prototype technique. The size reduction technique is then applied by using the Kotch fractal structure. The length reduction using this technique is compared with the length reduction by considering the end and T-junction effects of stepped impedance resonators. The filter designed using fractal technique is observed to be more compact. A low pass filter is designed using non periodic defective ground plane. This filter has a wide band characteristic. The slots on the ground plane are elliptical in shape. A very wide stop band characteristic has been obtained. Another low pass filter using the periodic defects on the ground plane of the microstrip line has been designed. Here the length and width of the slots are determined using the formula derived in this section. A compact microstrip low pass filter is designed using open stubs for L-band applications. In this filter the input and output lines are taped from the open stubs. All the design equations are derived and explained. The triangular patch resonators are also used to design the low pass filter. This filter is compact and gives good pass band and stop band characteristics. All these low pass filters are fabricated and the measured results are compared with the simulated results of the designed structures. The design of band pass filter designs are discussed in the third chapter. In this thesis the band pass filter design is divided in to two different parts. One is narrow band pass filter which is considered in chapter-3 and other is the wide band pass filters which has been elaborated in the next chapter. With the fractional band width (FBW) of 20% or less may be considered as narrow band pass filter and the wideband pass filter has FBW much higher than the 20%. For ultra wide band pass filter the requirement of FBW is more than the 100%. The first section of chapter-3 deals with the introductory part of microstrip coupled lines to design the narrow band microstrip filters. Problems in the response of microstrip parallel coupled line band pass filter are also discussed. Few suggestions to resolve these problems already given in the literatures are reviewed critically. A parallel coupled microstrip band pass filter with minimized second harmonic has been proposed using a modified structure of Kotch curve.

4 A new concept has been used to design the spurious free parallel coupled microstrip filter by applying middle stage over coupled resonator on Koch fractal shape resonators. This new design incorporates both the advantages of compact size and minimization of second harmonic. To minimize the second harmonic of a parallel coupled microstrip filter an electromagnetic band gap structure is proposed using the periodic defect on the ground plane. A narrow band pass filter with 3% of FBW has been designed using the end coupling. The structure used here is commonly referred as tubular structure. On the ground of this filter rectangular slots are etched which makes the filter more compact. The structures of the slots made on the ground plane are symmetrical about the horizontal axis. A microstrip line loaded with a periodic structure with periodic slots on the ground plane is used to design the narrow band pass filter. The modern wireless systems and high data-rate communication systems require ultra wideband band pass filters (UWB BPFs). The conventional edge-coupled BPFs for wideband applications require strong coupling between adjacent resonators, which leads to fabrication difficulty because of the close spacing. Stronger coupling is enabled by broadside coupled structures and can be used to design the wide band pass filter. The techniques such as three-line microstrips, multimode resonators, the cascade of lowpass and high pass filters, and the new coupling schemes are also used to design wideband BPFs. Using the steppedimpedance resonator (SIR) is another structure to design a wide band pass filters. In chapter-4 the two different filter structures have been proposed to design wide band filters for ultra wide band applications. The filter designed here is based on the concept of cascading the high pass and low pass structures. The cascaded structures are embedded into each other a very compact size of the filter achieved. The first filter is based on the periodic triangular patch resonators where as in the second technique a fractal structure has been applied on the ground to design a wide low pass filter for upper cut off. The high pass filter is

5 designed using the two shorted stubs. The simulated and measured values of the structures are depicted with a good agreement. Band stop filters (BSFs) are commonly used to eliminate particular frequencies of noise. And BSF is one of the important devices in modern communication systems because a radio frequency (RF) signal has undesired spurious components, such as harmonic, sub harmonic, parasitic effect, and intermodulation distortion through devices or the system. To suppress spurious bands, a number of BSFs have been developed. Three different structure of BSFs are designed in chaper-5. The BSF is designed using the fractal structures for the size reduction of band stop filters using open stub lines. Koch fractal curves with first iteration have been implemented on the inductive lines of the conventional open stub line band stop filter resulting in compact size for the same response. A fractal structure is used to design another microstrip band reject filter with a very good response. Microstrip triangular patch resonators have been applied to design a band stop filter. All these three structures are designed, simulated and fabricated. In the recent years the attention of the microwave researchers towards dual-band microwave filters have been increased due to the advancement in modern wireless communication systems operating at different frequency. In chapter-6 two different structures with dual band operations are presented. The first structure is a dual-band pass filter which has been designed using perturbed square patch resonator. A small rectangular slot has been made on the ground plane due to which insertion loss characteristic of the pass bands enhanced. The proposed design method is simple and efficient. There are very good agreement in simulated and measured results. The second filter in this chapter is designed for the dual band reject operations. These two stop bands are close to each other with narrow fractal widths. This filter is designed using the combination of U-shape and L-shape resonators in one structure. A compact filter is achieved through this design.

6 At the last in seventh chapter the conclusions of the complete research is concisely explained. The future scope of the research is given in the last section of chapter seven.