CHAPTER 3 DEVELOPMENT OF UWB BANDPASS FILTERS

Transcription

1 33 CHAPTER 3 DEVELOPMENT OF UWB BANDPASS FILTERS 3.1 INTRODUCTION As discussed in the first chapter under the sub-section literature review, development of Bandpass Filters (BPFs) for UWB systems have been attempted in many directions by the researchers. CPW, microstrip, SSL, LCP, SAW and LTCC are the notable methods competing with each other to give small sized high performance BPFs for UWB systems. Most of the filters have been developed based on multilayers with parallel plates, which produce undesired bulk modes in the operation (Wang et al 2005, 2006, Chen et al 2008, Nedil et al 2008 and Mao et al 2009). Some of these methods experience high heat dissipation due to the lack of grounding (Tsai et al 2004, Li et al 2005; 2006, Wang et al 2005; 2006, Kuo et al 2006, Li et al 2008, Haimeng et al 2008 and Luo et al 2008). A few of the filters reported in the literature are having very complex geometry and are expensive (Cruz et al 2007, Jorge et al 2007, Chen et al 2008, and Oshima et al 2010). Therefore, in this thesis an attempt is made to develop filters with enhanced performance while reducing the size and cost. Here, the microstrip line technology is experimented to design almost all of the filters since it is planar, low cost, compact and easy for integration with other modules in the UWB systems (Mattaei 1980 and Pozar 1998).

2 34 The choice of thick dielectric substrate with lower dielectric constant value results larger bandwidth. On the other hand, the requirement of substrate with higher dielectric constant value for compact size leads to narrow bandwidth (Hong et al 2001). Hence, a trade-off must be arrived between filter dimension and their performance. There are numerous substrates that can be used for the design of filters based on mictrostrip line and their dielectric constant lies in the range, 2.2 ε r 12 (Hunter 2006). In this research, commercially available substrate FR4 with dielectric constant of 4.4 and thickness of 1.6 mm is considered for developing all of the proposed filters to ensure compactness and better performance. To construct the filter on the substrate, microstrip MMRs and coupling schemes are required for generating modes and obtaining flat passband for wider range, respectively (Hong et al 2001). The microstrip MMRs may be a square, rectangle, thin strip, circular, elliptical, triangle or any other geometry. The MMR is generally made up of conducting material such as copper or gold and can take any possible shape (Hunter 2006). There are different coupling schemes available in the literature, namely, direct coupling, interdigital coupling, dual line coupling, parallel coupling, and broadside coupling. The microstrip MMR and coupling are combinedly photo etched (used Cu) on the dielectric substrate for 35 µm or 70 µm in the fabrication process. In this chapter, four microstrip line filters with interdigital coupling using different shape of resonators, namely, discoid (Type-I), hexagonal (Type-II), ring (Type-III), and ellipsoid (Type-IV) are proposed, developed and tested. In addition, another three filters developed based on dual line coupling, parallel coupling, composite of lowpass and highpass configuration using direct coupling. Then, the design of BPF using parallel and broadside coupling based on LTCC structure is also presented in this chapter.

3 35 For all designed filters their functional parameters, namely, insertion loss, return loss, group delay, phase, passband, and percentage FBW are simulated using electromagnetic simulation tool, IE3D. They are also fabricated using Printed Circuit Board (PCB) technology with FR4 as substrate and their characteristics, namely, insertion loss, return loss, FBW, and group delay are experimentally verified. It is found that the measured results show good agreement with the simulated results. Parametric analysis is carried out for all filters by varying one of the physical parameters (coupling gap, strip length and strip width) and keeping the others constant. This analysis is repeated for all filters and their qualitative and quantitative results are discussed in the subsequent sections. 3.2 DISCOID MMR BASED BANDPASS FILTER : Type-I Narrowband filters using square, rectangular and triangular MMRs are attempted in the literature (Hong et al 2001, Hsu et al 2007 and oraizi et al 2010). Here, the Type-I filter using discoid MMR and interdigital coupling is developed for UWB application. It will be easily matched with microstrip line along the circumference even if there is a variation in input impedance. The radius of the discoid is the only degree of freedom to control the modes. In order to widen the passband and tight coupling, interdigital coupling is used instead of direct coupling Discoid Bandpass Filter Geometry The UWB BPF presented in this section consists of a discoid MMR, which is kept at center and two identical coupled lines located both sides (right and left) of the resonator as shown in Figure 3.1. The discoid resonator and the symmetrical interdigital feed lines are working together to constitute a BPF with the desired passband. The parameters considered for the development of this filter are L 1 = 8, L 2 = L 3 = 9.6, g 0 = g 1 = g 2 = g 3 = 0.3, d = 4, W = 3.2, L = All dimensions are represented in mm.

4 36 The signal power is coupled into and out of the resonator through feed lines and coupling gaps. If the distance between the feed lines and the resonator is large then the coupling gaps does not affect the resonant frequencies of the discoid. The coupling gap is adjusted to control the bandwidth Simulation Results and Discussion Simulation S parameters of the proposed UWB BPF on microstrip line using discoid MMR is shown in Figure 3.2. It is clear from the response that it has a low insertion loss of db at 4 GHz, -0.7 db at 6 GHz and a minimum return loss of -38 db. For wideband applications, the examination of the flat group delay is essentially required. The simulated group delay for the filter is shown in Figure 3.3, which exhibits a flat response less than 0.1 ns over the passband. Figure 3.4 shows the simulation result of phase of S 21 for UWB BPF, which is acceptably linear. Figure 3.1 Geometry of the discoid MMR based bandpass filter

5 37 Figure 3.2 Simulated S parameters of the discoid MMR based bandpass filter Figure 3.3 Simulated group delay of discoid MMR based bandpass filter

6 38 Figure 3.4 Simulated phase of S 21 of discoid MMR based bandpass filter Experimental Verification The multiple coupled line structure is incorporated with MMR to provide wide passband and to enhance other functional parameters of the filter. The interdigital coupling is used here to enhance the coupling degree between the discoid resonator and feed lines. The filter is fabricated using FR4 substrate with dielectric constant of 4.4, loss tangent of and thickness of 1.6 mm. The prototype of the proposed filter is fabricated with its optimal values and tested by using SNA-HP8757D. The insertion loss, return loss and group delay are measured. The schematic diagram of the experimental setup used for the measurement is shown in Figure 3.5. A snapshot of the fabricated filter is shown in Figure 3.6. As shown in Figure 3.6, the two 50 Ω transmission lines are extended to accommodate the Sub Miniature version A (SMA) connector to the SNA for measurement.

7 39 Figure 3.5 Schematic diagram of the experimental setup for insertion and return losses measurement Figure 3.6 Snapshot of the fabricated discoid MMR bandpass filter The simulated and measured S parameters of the developed filter are given in Figure 3.7 (a) and (b). The measured results show an insertion loss of db at 4 GHz and a minimum return loss of -38 db. The fabricated filter has a wide passband between 2.25 GHz and GHz at -10 db transmission level and the FBW computed from the response is %. The simulated and measured group delay for the filter is shown in Figure 3.8, which exhibits a flat response less than 0.1 ns over the entire passband. It implies that this proposed UWB filter has a very good linearity of signal transfer and minimum distortion to the input pulse when implemented in the real time UWB system.

8 40 (a) (b) Figure 3.7 Simulated and measured S parameters of the discoid MMR bandpass filter (a) insertion loss and (b) return loss

9 41 Figure 3.8 Simulated and measured group delay of the discoid MMR bandpass filter The overall dimension of the filter is 39 (L) 6 (W) mm 2. The S parameters of the filter obtained prove that the developed filter exhibits optimal performance in terms of insertion and return losses. There is a small discrepancy between them that varies from 11% to 13%, which is negligible. This might be due to the effect of soldering the SMA connector or fabrication tolerance. The simulation results are obtained by assuming microstrip as input port, but for the prototype SMA connector is used. The imperfect transition between SMA feed to microstrip may also introduce loss. The capacitance or soldering position of the feed point may also be the reason for the shift (Ma et al 1999 and Raj et al 2006).

10 HEXAGONAL MMR BASED BANDPASS FILTER : Type-II To improve the passband and other functional parameters better than the previous filter, the hexagonal MMR based filter is designed with small size and developed. The -10 db passband of this filter using hexagonal MMR is 8.3 GHz, which is higher than the filter developed with discoid MMR Hexagonal Bandpass Filter Geometry The UWB BPF filter presented in this section consists of feed lines, coupling gaps, and hexagonal structure, which is shown in Figure 3.9. This hexagonal geometry is extracted from the discoid geometry, which widens the passband further as it has tighter coupling than discoid. The parameter values for the developed filter are L 1 = 8, L 2 = 9.8, L 3 = 9.2, g 0 = g 1 = g 2 = g 3 = 0.3, W = 3.2, d 1 = 2, d 2 = 4. All dimensions are with the unit of mm Simulation Results and Discussion The S parameters of the proposed UWB BPF on microstrip line with hexagonal MMR by simulation are shown in Figure The plot shows that the proposed filter has low insertion loss and minimum return loss of -2 db and -35 db, respectively. Figure 3.9 Geometry of the hexagonal MMR based bandpass filter

11 43 Figure 3.10 Simulated S parameters of the hexagonal MMR bandpass filter Figure 3.11 Simulated group delay of hexagonal MMR based band pass filter

12 44 Figure 3.12 Simulated phase of S 21 of the hexagonal MMR based bandpass filter The simulated group delay for the filter is below 0.2 ns which is constant over the passband as shown in Figure Figure 3.12 shows the simulation result of phase of S 21 for UWB BPF. The response shows that the phase of S 21 throughout the passband of the BPF is linear for UWB applications Experimental Verification The proposed filter is fabricated using PCB technology. Snapshot of the fabricated filter is shown in Figure Simulated and measured S parameters of the filter are illustrated in Figure The fabricated filter has wide passband from 2.1 GHz to 10.4 GHz at -10 db transmission level and FBW computed from the response is about %.

13 45 Figure 3.13 Snapshot of the fabricated hexagonal MMR based bandpass filter 3.4 RING MMR BASED BANDPASS FILTER : Type-III This structure is also derived from the discoid structure (Type-I) by perturbation method (Chang et al 2004). Even though previously discussed filters provide a large bandwidth that covers more than FCC mask, they could not able to maintain a high flatness in the passband. To achieve low scattering losses, a ring filter is designed using the symmetric structure. From this simple structure, many more complicated circuits can be devised by making a slit, adding a notch, cascading two or more rings, implementing some solid-state devices, integrating with multiple input and output lines, and so on (Hong et al 2001 and Chang et al 2004). These filters can be used for various UWB applications Ring Bandpass Filter Geometry The geometry of ring resonator based UWB BPF is shown in Figure This filter consists of a single ring structure and interdigital coupling for wide coupling. The structure would only support waves that have an integral multiple of the guide wavelength equal to the mean circumference. The optimized parameter values of the filter are, r = 2.5 mm, R = 3 mm, L = 8 mm, W = 3.2 mm, L 1 = 7.9 mm, L 2 = 7.7 mm, d 0 = 0.3 mm, d 1 = 0.5 mm, d = 1.9 mm. The return loss, insertion loss and group delay of the developed filter are simulated and its performance has been studied.

14 46 (a) (b) Figure 3.14 Simulated and measured responses of the hexagonal MMR bandpass filter (a) insertion loss and (b) return loss

15 47 Figure 3.15 Geometry of the ring MMR based bandpass filter Simulation Results and Discussion The simulated S parameters of the developed UWB BPF using ring MMR are shown in Figure It is known from the response that the filter has an insertion loss of -2 db (-0.1 db at center frequency) and lower return loss of -45 db. The computed FBW from the response at -10 db transmission level is %. The simulated group delay for the filter is shown in Figure 3.17, which exhibits a flat group delay response below 0.05 ns over the passband. Figure 3.18 shows the simulation result of phase of S 21 for the proposed UWB BPF which is linear in the passband Experimental Verification The snapshot of the fabricated filter and measured results of the developed filter are shown in Figures 3.19 (a) and (b), respectively. Comparative results of simulated and measured parameters are given in Figures 3.20 (a) and (b). The measured results have shown an insertion loss below -1.3 db, return loss of about -45 db. With the above feature total size of the filter is (L) 6 (W) mm 2. The S parameter characteristics obtained for the UWB BPF ensures optimal performance in terms of insertion loss, return loss and group delay.

16 48 Figure 3.16 Simulated S parameters of the ring MMR based bandpass filter Figure 3.17 Simulated group delay of the ring MMR based bandpass filter

17 49 Figure 3.18 Simulated phase of S 21 of the ring MMR based bandpass filter (a) (b) Figure 3.19 Ring MMR based bandpass filter (a) snapshot of the fabricated filter and (b) measured response

18 50 (a) (b) Figure 3.20 Simulated and measured responses of ring MMR based bandpass filter (a) insertion and (b) return loss 3.5 ELLIPSOID MMR BASED BANDPASS FILTER : Type-IV The performance of the BPF using prolate MMR in microstrip line is studied in this section. The ellipsoid MMR is also derived from the basic structure, discoid (Type-I), by removing some outer portions. The ellipsoid geometry will be useful for dual mode operation. So far, there is no attempt made to design UWB BPF using elliptical patch. This filter gives better performance in terms of filter functional parameters such as insertion loss, return loss, group delay, and FBW Ellipsoid Bandpass Filter Geometry In this filter, prolate ellipsoid MMR is introduced in the middle section of the geometry. The multiple coupled line structure is incorporated with ellipsoid MMR to provide wide transmission band. The interdigital coupling is used to enhance the coupling degree between the ellipsoid resonator and the feed lines. The interdigital coupled line structure and MMR are designed and implemented to provide bandpass filtering over GHz as shown in Figure The parameter values considered for the development of the filter are L 0 = 8, L 1 = 8.6, L 2 = 8.9, L = 39.4, d 1 = d 2 = 0.3, W = 3.2, L = 39.2, A = 7, B = 5. All dimensions are in mm.

19 51 Figure 3.21 Geometry of the UWB bandpass filter using prolate ellipsoid resonator Figure 3.22 Simulated S parameters of the ellipsoid MMR based bandpass filter

20 52 Figure 3.23 Simulated phase of S 21 of the prolate ellipsoid MMR based bandpass filter Simulation Results and Discussion The S parameters of the proposed UWB BPF using ellipsoid MMR by simulation are shown in Figure From the response, it is noted that filter s insertion loss has the variation from -1 db to -3 db in the passband and it has minimum return loss of -25 db. Figure 3.23 shows the simulation result of phase of S 21 for UWB BPF, which is acceptably linear. Physical parameters of the filters such as coupling gap, strip width and resonator sizes are different for each of them. Optimal values for these parameters are obtained by parametric analysis to design the filters with improved performance.

21 53 As discussed in earlier, other than interdigital coupling, there are many couplings are available. In following sections, in order to further compact the size, filters are attempted using different coupling schemes, namely, dual line coupling, parallel coupling, direct coupling, and broadside coupling. 3.6 WIRED RING RESONATOR BASED BANDPASS FILTER BPFs using dual line coupling with wired ring resonator are discussed in this section. The filters using ring resonator and dual line coupling structure are already available in the literature (Song et al 2009). They used substrate with dielectric constant of 3.2 with the thickness of mm. coupling are: The distinctive features of the proposed filters based on dual line Wired ring resonator is introduced for strong coupling and to obtain a large passband Rectangular slot is introduced in the ring resonator in order to achieve a flat passband. It is possible to get multiple resonances in the passband by introducing more slots in the geometry The proposed filter is designed based on commercially available substrate FR4

22 Wired Ring Bandpass Filter Geometries The filter design comprises of two main components, the wired ring resonator and parallel coupled dual-line structure as shown in Figures 3.24 (a) and (b), respectively. (a) (b) Figure 3.24 Geometrical parameters of proposed filter (a) with wired ring resonator and (b) with wired ring resonator using rectangular slot A ring resonator formed by microstrip line results low radiation loss and a structural advantage that is enabling the elimination of parasitic components which are usually induced at the open and short circuited ends of

23 55 conventional half or quarter wavelength resonators. The ring resonator used in this UWB is a dual mode resonator consisting of one ring section. Four identical high impedance line sections at the two sides act as the parallel coupled dual line structure. It is similar to the conventional coupled line resonator, while the dual mode resonator may be viewed as a Stepped Impedance Resonator (SIR). The parallel coupled dual line structure is used to achieve much tighter coupling between input and output port than the conventional parallel coupled line, which can increase the S 21 magnitude and widen the passband of the filter Results and Discussion The filter with wired ring resonator and the filter with two rectangular slots are named as BPF-1 and BPF-2, respectively. The insertion loss along with return loss performance of the simulated filters is shown in Figure 3.25 (a) and (b), respectively. In general, good flatness is achieved throughout the passband of the filters. However, some occasional slight ripple is achieved for BPF-1. The BPF-1 has passband of 16 GHz from 5-21 GHz, minimum insertion loss of approximately, -3 db, return loss of around -38 db and -10 db FBW of % while for the BPF-2, these are 7 GHz from 5-12 GHz, db, -33 db and %, respectively. Figure 3.26 depicts the simulation of the group delay for BPF-2. Group delay variation in the passband for BPF-2 is ± 0.03 ns. Figure 3.27 shows the simulation result of phase of S 21 for BPF-2 which is linear in the operating band.

24 56 (a) (b) Figure 3.25 Simulated S parameters of UWB bandpass filters (a) with wired ring resonator and (b) with wired ring resonator using rectangular slot

25 57 Figure 3.26 Simulated group delay of bandpass filters using wired ring resonators with rectangular slot Figure 3.27 Simulated phase of S 21 of wired ring resonator based bandpass filter with rectangular slot

26 PARALLEL COUPLED BANDPASS FILTERS Parallel coupled line microstrip filters have been found as one of the most commonly used microwave filters in many practical wireless systems for several decades (Hong et al 2001, Azhumada et al 2005 and Chen et al 2007). The major advantage of parallel coupled BPF is planar structure, wide bandwidth and relatively simple design procedure. Based on the insertion loss method, the filter functions of maximally flat and Chebyshev type can be easily synthesized. Moreover, the filter performance can be improved in a straightforward manner by increasing the order of the filter. When these filters are to be realized by parallel coupled microstrip lines, one of the major limitations is the small gap size of the first and the last coupling stages. Obviously, shrinking the gap size is not the only way to increase coupling of coupled lines. Higher FBW and smaller gap size is also required to increase the coupling Parallel Coupled Bandpass Filter Geometries The proposed BPFs discussed in this section are based on parallel coupled line microstrip structure. The filter with coupling gaps of 0.7 mm and 1.4 mm covers only the upper band of UWB range which is named as BPF-1. Also the filter with coupling gaps of 0.5 mm and 0.1 mm is named as BPF-2. Figure 3.28 (a) and (b) show the possible circuit arrangements of BPF using parallel coupled line microstrip structure for UWB range. They consist of transmission line sections having the length of half wavelength at the corresponding center frequency. Half wavelength line resonators are positioned so that adjacent resonators are parallel to each other along half of their length. This parallel arrangement gives relatively large coupling for the

27 59 given spacing between the resonators, and thus, these structures are particularly convenient for constructing filters having larger bandwidth as compared to the other structure. The gaps between the resonators introduce a capacitive coupling between the resonators, which can be represented by a series capacitance. BPF-1 dimensions are denoted in the geometry itself. The gap of coupled lines and the widths are optimized to have fixed values i.e., 0.7 mm, 1.4 mm, 1 mm and 0.9 mm. The physical parameters of the BPF-2 are optimized to the following values, L = 5.8 mm; G 1 = 0.05 mm; G 2 = 0.1 mm; l 1 = 1.5 mm; l 2 = 0.5 mm; l 3 = 0.6; l 4 = 3 mm; W = 4 mm and H = 31 mm to cover the entire UWB range between 3.1 GHz and 10.6 GHz. Using this configuration, higher coupling in turn wider bandwidth is achieved. These structures are used to generate a wide passband and expected to achieve a tight coupling, and lower insertion by reducing both strip and slot width. (a) All dimensions are in mm (b) Figure 3.28 Geometry of the parallel coupled UWB bandpass filters (a) BPF-1 and (b) BPF-2

28 Results and Discussion The simulated S parameters of the BPF-1 and BPF-2 using parallel coupled line are shown in Figure 3.29 (a) and (b), respectively. It is clear from the response that the BPF-1 has insertion loss of -1.2 db and lower return loss of about db. The -10 db FBW computed from the response is about 52.17%. It is clear from the response that the BPF-2 has a better insertion loss of -0.5 db and lower return loss of about db at 6.3 GHz. The -10 db FBW computed from the response is about 86%. The Simulated group delay for the BPF-1 and BPF-2 are shown in Figure 3.30 (a) and (b), which exhibits a flat group delay response below 0.15 ns and 0.02 ns over the whole passband of the filters, respectively. It implies that this proposed UWB filter has a very good linearity of signal transfer as well ensures the minimum distortion to the input pulse when implemented in the UWB system. (a) (b) Figure 3.29 Simulated S parameters of the parallel coupled bandpass filters (a) BPF-1 and (b) BPF-2

29 61 (a) (b) Figure 3.30 Simulated group delay of the parallel coupled bandpass filters (a) BPF-1 and (b) BPF-2 (a) (b) Figure 3.31 Simulated phase of S 21 of the parallel coupled bandpass filters (a) BPF-1 and (b) BPF-2

30 62 Figure 3.32 (a) (b) Snapshot and measurement responses of developed parallel coupled bandpass filters (a) BPF-1 and (b) BPF-2 The response of the Figure 3.31 (a) and (b) show that the phases of S 21 of the designed filters are linear in the passband and suitable for UWB applications. Snapshots and measured results of the parallel coupled BPFs are shown in Figures 3.32 (a) and (b). 3.8 BANDPASS FILTER BASED ON COMPOSITE OF LOWPASS AND HIGHPASS CONFIGURATION The proposed UWB filter based on modified MMR being discussed in this section yields the response exactly (S 11 and S 12 ) as that of Nedil et al (2007), which is a BPF using CBCPW back to back transition structure. CBCPW technology has unwanted bulk modes due to its parallel-plate modes, which is the main drawback of this structure. Moreover, the back to back transition has a complex design and difficult to fabricate. UWB BPF with

31 63 wide passband and small size is developed and compared with existing structures. The features of the developed filter are compact in size with 10.7 mm in length against 16.0 mm in Nedil et al (2007) and 12.5 mm in Prabhu et al (2007), good passband (insertion loss is about -0.5 db at 6.85 GHz in simulation) and -3 db passband from 2.5 GHz to 9.5 GHz Filter Geometry The geometry of the proposed UWB BPF and a snap shot of the fabricated bandpass filter are shown in Figure 3.33 (a) and (b), respectively. This filter consists of three pairs of transversely connected stub-loaded modified MMR in the center section with two identical direct coupled lines on the left and right sections. Out of three transverse stubs, two of them are folded one. During design, the dimensions of folded stubs are properly trimmed to obtain its -3 db higher frequency cut-off near 10 GHz. The middle section of the constituted MMR exhibits an excellent lowpass property and it is linked with external feed lines via direct coupling. The presented UWB filter is the combination of a lowpass and a highpass filter using microstrip technology. The compact and low-loss nature of this filter is realized in microstrip line. The microstrip line filters are proved as versatile and low loss medium for filter design. For the lowpass filter section, very high characteristic impedance is required which is achieved with narrow strip line. For the highpass filter section, strong broadside coupling between wide line section and ground substrate is required to increase the inductance value, which results in small size and a wide passband filter response. The physical parameters of the proposed BPF are optimized to the following values, G1 = 0.25 mm, G2 = 1 mm, G3 = 0.05 mm, G4 = 0.10 mm, G5 = 0.15 mm, L1 = 1 mm, L2 = 9.8 mm, L3 = 3.2 mm, L4 = 3.15 mm,

32 64 L5 = 6 mm, H1 = 1 mm, and W = 3 mm to cover the entire UWB range of indoor mask. The minimum strip width and gap distance used in this structure are 0.10 mm and 0.05 mm, respectively. The 3 mm long lowpass section (left side of the structure) consists of two open stubs and a distance gap. The length of the modified MMR with folded stubs is 3.20 mm and the length of the highpass filter section (right side of the structure) is 3.15 mm, which comprises a uniform impedance resonator with three vertical limbs. To improve the return loss while combining lowpass and highpass filters, a section of transmission line of 4.35 mm length is added between the two filter portions, resulting to overall filter length of 10.7 mm. (a) (b) Figure 3.33 (a) Geometry of the modified MMR based bandpass filter and (b) snapshot of the fabricated filter

33 Results and Discussion The Simulated and measured S parameters of the developed BPF is depicted in Figure 3.34 (a) and (b). From the responses, it is clear that the filter provides ultra-wide passband from 3.1 GHz to 10.6 GHz, very low and flat insertion loss of -0.5 db and the minimum return loss of -35 db. The measured in band insertion loss is about -1.5 db at 6.85 GHz and return loss is about -37 db at 9.4 GHz. The developed BPF is more compact with comparable performances than filters available in the literature. The dimension of the entire filter structure is only around 10.7 (L) 3 (W) mm 2. It is more compact in size with comparable performance than the filters available in the literature. To illustrate the dimension contraction achieved through the suggested filter in this work, it is compared with filter discussed by Hong et al (2005) and understood that the developed filter consumes only 13% of the substrate of that filter which is equal to the size reduction of around 87%. It is also noticed that the filter has size reduction of 83% against the filter presented by Lee et al (2010) and, here, the proposed filter takes 17% of its substrate area only. The Simulated group delay for the filter is shown in Figure 3.35, the variation in the passband is ± 0.02 ns. Figure 3.36 shows the phase of S 21 for the filter that is linear and suitable for UWB applications. (a) (b) Figure 3.34 Simulated and measured S parameters of the modified MMR based bandpass filter

34 66 Figure 3.35 Simulated group delay of the modified MMR based bandpass filter Figure 3.36 Simulated phase of S 21 of the modified MMR based bandpass filter

35 BANDPASS FILTER BASED ON LTCC STRUCTURE The proposed BPF in this section is based on LTCC which uses parallel coupling at center and broad side coupling at the ends of the proposed filter structure. The filter is designed to cover the entire UWB range. The main advantage of the multilayered structure is to shrink the circuit size LTCC Bandpass Filter Geometry Figure 3.37 shows a circuit arrangement for BPF using LTCC for UWB range. It consists of transmission line sections having the length of half wavelength at the corresponding center frequency. Half wavelength line resonators are positioned so that adjacent resonators are parallel to each other along half of their length. This parallel arrangement gives relatively large coupling for the given spacing between the resonators, and thus, this filter structure is particularly convenient for constructing filters having larger bandwidth as compared to other structures. Figure 3.37 Geometry of the LTCC bandpass filter

36 68 Figure 3.38 Orthographic view of the LTCC bandpass filter The gap between the resonators is introducing a capacitive coupling, which can be represented by a series capacitance. The broad side coupling and existence of the substrate results tight coupling, which provides wide bandpass operation. The physical parameters of the proposed BPF are optimized to the following values, l 1 = 5.89 mm; l 2 = 5.86 mm; d = 0.2 mm; g 0 = 0.08 mm; g 1 = 0.1 mm; a 0 = 1.06 mm; a 1 = 1.2 mm; a 2 = 1.02; w 0 = 1.6 mm; b = 1.6 and h = 33.5 mm to cover the entire UWB range between 2 GHz and 9 GHz. Using this configuration, higher coupling is obtained and therefore wider bandwidth is achieved. This structure is used to generate a wide passband and expected to achieve a tight coupling, and lower insertion loss by reducing both strip and slot width. 3D view of an LTCC UWB bandpass filter with parallel and broadside coupling is shown in Figure 3.38, which consists of two layers, resonators and substrate with frames.

37 Results and Discussion The proposed filter is designed to provide a wide passband, low insertion loss and return loss, linear phase over the passband, flat group delay and high FBW. The Simulated S parameters of the proposed UWB BPF using LTCC are shown in Figure It is clear from the response that the proposed filter has a better insertion loss of db and lower return loss of about -49 db. The -10 db FBW computed from the response is about 62.17%. The Simulated group delay for the proposed filter is shown in Figure 3.40, which exhibits a flat group delay response below 0.1 ns over the whole passband. It implies that this proposed UWB filter has a very good linearity of signal transfer and would ensure minimum distortion to the input pulse when it is implemented in the UWB system. The response of the Figure 3.41 shows that the phase of S 21 throughout the -10 db passband between 3.8 GHz and 7.4 GHz of designed filter is acceptably linear. The filter is designed based on LTCC substrate with two upper sheet layers having a thickness of 1.6 mm and 1.93 mm, respectively, with dielectric constant of 7.8 and a loss tangent of Summary of functional parameter values of the proposed and developed bandpass filters are tabulated in Table 3.1. The developed filters are compact than filters available in the literature. A comparative summary of some of the filters with respect to their geometry size and performance is tabulated in Table 3.2.

38 70 Figure 3.39 Simulated S parameters of the LTCC bandpass filter Figure 3.40 Simulated group delay of the LTCC bandpass filter

39 71 Figure 3.41 Simulated phase of S 21 of the LTCC bandpass filter Table 3.1 Consolidated functional parameter values of the proposed and developed bandpass filters in the literature Sl. No. Geometry/ Circuit S 21 (db) S 11 (db) Passband (GHz) FBW (%) GD (ns) Size (mm 2 ) 1. Discoid Hexagonal Ring Wired Ring Parallel Coupled LTCC LP-HP

40 72 Table 3.2 Comparison of developed filters with existing best reported filters Sl. No Authors Yang et al 2006 Shaman et al 2007 Cheng et al 2008 Kuo et al 2008 Liu et al 2009 Cai et al 2010 This work 8. This work Geometry/ Circuit Passband (GHz) FBW (%) GD (ns) Size (mm 2 ) Size Reduction (%) w.r.t Sl. No. 8 U slot Cross coupling LCP SIR LP-HP DGS Square DGS Discoid MMR Hexagonal MMR SUMMARY Development of compact microstrip filters using different shapes of MMRs such as discoid, hexagonal, ring and ellipsoid and various coupling schemes are discussed in this chapter. Though all four presented filters perform well in terms of functional parameters especially insertion loss, return loss, FBW, group delay, passband and phase, hexagonal MMR based bandpass filter is well suited for UWB systems, since it has higher passband than the other filters. In addition, four filters based on dual line coupling, parallel coupling, modified MMR with direct coupling and LTCC using parallel and broadside coupling are discussed in this chapter.

41 73 Wired ring resonator based BPF using dualline coupling structure is smaller than the previously developed filter. But, in terms of passband and other functional aspect it has low profile. Still to compact the filter size, again BPF based on parallel coupled line microstrip structure is developed. Though the filter is compact, the FBW for the filter is low when comparing with other developed filter, for those it is more than 110%. A miniaturized UWB BPF with improved passband is designed, developed and characterized using composite of lowpass and highpass configuration with direct coupling. The overall dimension of the filter is only 10.7 by 3 mm 2 which is more compact than all other filters developed in this thesis and best filters reported in the literature. Then, BPF for UWB applications using parallel and broadside coupling on LTCC is presented. This proposed filter demonstrated wide bandwidth in compact size but provides a low FBW. The filters discussed in this chapter are fabricated with FR4 substrate and characterized by measuring insertion loss, return loss, group delay and FBW. The measured results are appreciably in good agreement with the simulated results and this proves the validity of the proposed filters. The proposed filter structures are simple, small size, easy to fabricate and can be integrated into any UWB systems. Hence, these types of filters are suitable for UWB indoor applications. With the knowledge acquired in developing BPFs, an attempt is made to develop notch filters for suppressing the narrowband services, particularly 5 GHz and 2.4 GHz WLAN, which are presented in the next chapter.