INVESTIGATIONS ON SPECIFIC MICROSTRIP FILTERS WITH DEFECTED GROUND STRUCTURE

Abstract

Microwave filters are very essential components of modern wireless communication systems [1]. A filter is a two port network that provides perfect transmission of signals with frequencies in certain passband and infinite attenuation in the stop band regions [2]. Microwave system often requires a means for suppressing unwanted signals and/or separating signals having different frequencies. These functions are performed by filters. Filters are generally categorized in terms of their frequency characteristics, such as, low-pass, high-pass, bandpass, and bandstop. A microstrip filter consists of a conducting strip line on one side of a dielectric substrate and a ground plane on the other side. The performance of microwave filters can be analyzed in terms of sharpness or roll-off factor, insertion loss, return loss, suppression of higher harmonics, bandwidth and selectivity. Generally, a microstrip filter can be designed by using two methods. One is insertion loss method and another one is image parameter method. Both the image parameter and insertion loss methods of filter design provide lumped element circuits. These lumped element circuits are converted into distributed circuit elements using Richard’s transformations [2]. However, these techniques for designing microwave filters make the circuits bulky. Since the late 1980’s, techniques employing the electromagnetic bandgap (EBG) or photonic bandgap (PBG) structures and defected ground structures (DGS) attracted the attention of the researchers, to achieve miniaturization, reducing surface wave probi lems and removing an arbitrary stopband in microwave components. Recently, various other techniques, such as, metamaterials or complementary split ring resonators (CSRR), low temperature co-fired ceramics (LTCC), low temperature co-fired ferrites (LTCF), defected microstrip (DMS), ground plane aperture (GPA) and substrate integrated waveguides (SIW) are also exploited for size reduction and improving the performance of microwave circuits. But DGS is a simple, popular and efficient method among all these techniques of miniaturization of microwave components, particularly, filters [3–10]. The concept of DGS is originated from the electromagnetic/photonic bandgap structures. Basically, the DGS is a truncated version of an EBG/PBG structure with one or two unit cells. The EBG/PBGs are periodic structures. These techniques have an attractive property to stop a particular band of frequency and size reduction in filter design [11–14]. It is complex to use EBG/PBG structures for the design of the microwave or millimeter wave components due to the difficulties of modeling. The other difficulty in using the EBG/PBG circuit is caused by the radiation from the periodically etched defects but in DGS radiation loss is less due to the employment of limited number of the unit cells of EBG/PBG (one or two only). DGS is a simple shape, which is etched in the ground plane of microstrip line. The dimensions of defect disturb the current distribution in the ground plane of microstrip line which controls the excitation and propagation of electromagnetic waves through substrate layer [15–20]. The shape of the defect may vary from a simple to a complex geometry [11]. The defect in the ground plane of planar transmission lines also changes the characteristics of the transmission line, such as, inductance and capacitance [7]. The DGSs have been used or can be used for the design of microwave components such as power amplifiers [21, 22], microwave oscillators [23], dividers [24], couplers [25], combiners [26], antennas [27–31] and filters [15–20] [32–36] in radar systems, optical and wireless ii communication systems [37–59]. A lot of literature has been reported on microstrip filters with defected ground structure to improve the performance of filters [5–29] [32–36, 60–64]. However, there is still a scope for improving the performance of microstrip filters with DGS . Various aspects of the DGS application in planar microstrip filters have not yet fully been explored, although it has been developed to a great extent for other microwave components. The basic topologies of DGS were not explored to achieve sharpness in microstrip filters. Fractal geometries are also not investigated and discussed in detail with different positions of fractals in dumbbell-shaped (conventional) DGS [65]. The conventional CSRR-DGSs (square CSRR and circular CSRR) are available for filter design [5,6,66–69], but these CSRR-DGSs are not extensively explored and investigated for filter design. Very less has been reported on the design of THz filters with DGS. This has provided the basic impetus to carry out the design and realization of microwave filters with DGS. Most of the above mentioned problems are discussed in this work which forms the basic embodiment of the thesis. This thesis presents a detailed analysis of microstrip filters with various DGS structures with their enhanced properties. The first chapter of this thesis presents an introduction, motivation and scope of the work and organization of the thesis. In chapter two of the thesis, a comprehensive overview of the relevant literature survey is given [16]. Chapter three presents the design and realization of microstrip filters with a new defected ground structure (DGS). Various conventional DGSs (dumbbellshaped) are available in literature but these DGSs have some limitations. The main limitation of conventional DGS filters is the DGS itself which is responsible for losses due to its large area. The sharpness of filter depends on effective capacitance and this effective capacitance depends on the slot gap which can be adjusted upto certain extent to increase the sharpness of the filter. So, in this chapter a new DGS is proposed and compared with conventional DGS. This new iii DGS provides an extra degree of freedom to improve the performance of filters. On the basis of comparative studies, bandstop, bandpass and low pass filters are designed with the proposed new DGS topology [70]. The proposed filters are designed and fabricated using 50Ω, =4 microstrip line which is very compact when compared with conventional microstrip filters. A two step validation procedure is adapted. Design and co-design are carried out using HFSS electromagnetic( EM) simulator and ADS circuit simulator respectively, and these results are compared with the experimental results. For this purpose, all the designed filters were fabricated and S21=S11 responses were measured using Rohde & Schwarz Vector Network Analyzer 1127.8500. Chapter four presents the detailed design studies of defected ground structures with fractal geometries [71]. The conventional DGSs (dumbbell-shaped) have a flat rejection property. To design and realize low pass filters with steep rejection property, many of these conventional DGSs are needed to be cascaded, which means higher insertion loss and larger size. Fractal DGS has two important properties: one is self-similarity for multiband application and other one is space-filling for compactness [72]. The design studies of microwave filters with fractal DGSs are thus presented in this chapter. These fractal geometries have been employed at different positions of dumbbell-shaped DGS and compared. Minkowski and Peano fractals are employed for the design of low pass filters at the cut-off frequencies 5.4 GHz and 1.9 GHz respectively [73–79]. All the designed filters are fabricated and tested in VNA and the measured results are in good agreement with the simulated results. Chapter five focuses on microstrip filters with complementary split ring resonators (CSRR) as defected ground structure [80–85]. In this chapter various CSRR-DGSs (including conventional DGS) are compared. On the basis of these comparison a new CSRR-DGS is proposed with enhanced filter properties. This new CSRR-DGS topolgy is used to design filter (bandpass, lowpass, and dualiv band bandpass). All the designed filters with DGS, discussed above are fabricated and tested and the measured results are amiable with those obtained using the full wave simulators. Chapter six deals with design studies of microstrip bandstop filters with the defected ground structure at terahertz frequency. In this chapter, a planar microstrip terahertz (THz) bandstop filter has been proposed with defected ground structure with high insertion loss (S21) in stopband of -25.8 dB at 1.436 THz. A dielectric substrate of Benzocyclobuten (BCB) is used to realize a compact bandstop filter using modified hexagonal dumbbell shaped defected ground structure (DB-DGS) in a =4, 50Ω microstrip line at THz frequency range for compactness [86–89]. The proposed filter can be used for sensing and detection in biomedical instruments in DNA testing [90–93]. Finally, in chapter seven, the contributions made in the thesis are given and scope for the future work is outlined. In summary, the thesis contributes towards the improvement of filter performance.