Technology. F. Tabatabai, Member, IEEE, H.S. Al-Raweshidy, Senior Member, IEEE. filter, normally do not possess high quality factor at the

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1 Proceedings of Asia-Pacific Microwave Conference 2007 C and Ka-Band Wide Bandpass Filter Using LTCC Technology F. Tabatabai, Member, IEEE, H.S. Al-Raweshidy, Senior Member, IEEE Brunel University, Department of Electronic, Room 31 IA, Howell Building, Uxbridge Middlesex UB8 3PH Abstract - This paper presents two types of wideband filters that have been developed at 5 and 28-GHz frequency bands, respectively. These filters have been fabricated with low temperature co-fired ceramics (LTCC) multilayer technologies. Each filter has approximately 20% and 12% 3dB bandwidth, insertion loss less than 1.5 db, and has an attenuation over 30 db at fo +15% respectively. Design procedure is described and design examples are given to demonstrate the features of the proposed coupling structure. A wide-band LTCC filter is simulated and successfully built. The advantages of multilayer LTCC technology such as high integration and vertical stacking capabilities were employed to design edge- coupled embedded microstrip wide BPF. The difficulties in controlling the precise distance between two adjacent resonators in LTCC edge-coupled BPF were overcome by locating the resonators on different layers. Index Terms - Filter, LTCC, multilayer substrate, wideband. I. INTRODUCTION The low-temperature co-fired ceramic (LTCC) is a multilayer substrate, which is manufactured by deposition layer by layer, of dielectric and metallic conductor patterns. Typically, the thickness of each dielectric layer is less than inches, and the relative permittivity of the dielectric is 10. The LTCC manufacturing method allows planar microwave circuits such as stripline based circuits to be integrated easily into the substrate. In recent years, low-temperature co-fired ceramic (LTCC) technologies are widely attracting microwave and millimeterwave engineers' attentions for their superior advantages over other substrate technologies. Threedimensional (3-D) integration capabilities can make sizereduction and low-cost design, and at the same time the small value of dielectric loss tangent reveals its excellent high frequency characteristics [1]. In this paper, we firstly present an edge-coupled C and Ka band pass filter by using 3-D multilayered LTCC technology. The shrinkage after co firing and restricted resolution of the LTCC process makes it difficult to control the precise distance between two adjacent resonators close enough for coupling on the same planar substrate. Locating microstrip resonators on different layers in the multilayered structure can clear those process limitations by vertically separating the adjacent resonators with an intermediate LTCC tape. Further more; by overlapping two adjacent ends of resonators we can control the filter characteristics such as bandwidth, sharpness, and minimum insertion loss in pass band [2], [3] /07/$20.00 w2007 IEEE. The tremendous growth in wireless communications demands the 5-GHz unlicensed national information infrastructure band due to its high data rate, 54 Mbps. Though highly integrated modules are desired, it is difficult or uneconomical to integrate a high quality analogue filter on a chip because the inductors, the key elements for a filter, normally do not possess high quality factor at the frequency of 5 GHz. Also this paper presents an embedded edge-coupled BPF using multilayered LTCC technology for Ka band application. Here, an edge-coupled BPF is considered. This filter also offers low insertion loss and spurious free performance. The procedure for design and fabrication as well as the issues and solutions will be described in detail. II. FILTER DESIGN This paper presents an embedded edge-coupled BPF using multilayered LTCC technology for C and Ka-band application. Edge-coupled filters normally consist of an array of striplines approximately quarter-wave long at the midband frequency of the first passband. In this design, the first and last lines of the array are open-circuited, while the rest of the lines are short-circuited at one end and open-circuited at the other end in an alternating arrangement. This configuration results in a compact filter since all lines in the array serve as resonators for the filter. [5] For a microstrip bandpass filter implementation, the coupling between resonators decreases with increasing substrate height. Generally, microstrip or stripline bandpass coupled line filters; with bandwidths less than about 15% can be easily fabricated. However, where wider bandwidth filters are desired, very tightly coupled lines are generally needed. This can be achieved by reducing the substrate height used, where the track separation required realizing the coupled line filter becomes smaller. However, this small track separation can present a problem in terms of manufacturability. Also, quite often the characteristic impedances of the stub resonator are in reality difficult to realize as well. Thus the trade-offs between substrate height, minimum coupling gap, realizable characteristic impedances and overall loss must be addressed to gain the required filter performance. The LTCC MCM process method on the other hand, does not face these problems. Using this technique, filters and other passive components can be fabricated with a line width and a line spacing as small as 15pm, which is unobtainable with a conventional

2 thick-film process. Coupled line filters can be designed to produce either a maximally flat or equi-ripple response. In this study, a 7th-order Chebyshev bandpass filter has been designed with a center frequency of 5.3GHz with a 3dB bandwidth of 0.8GHz, and 28GHz with a 3dB bandwidth of 6GHz using coupled half-wave resonators to give a maximally flat and sharp end response. The bandpass filter is designed by following the design procedure based on the even- and odd-mode impedances of the coupled lines, and is further optimized using MomentumTM (by HP ADS). The filter is then fabricated on a Alumina substrate (; 9.8) of height 635um using Hibridas metal pastes. Seven resonators were used to guarantee sufficient out-of band rejection. In order to make the relatively high values of the capacitance, the first and the seventh resonator were placed one layer above the transmission lines. The first and the seventh resonators on the first layer are not straight arrayed with the other resonators and transmission lines on the second layer in order to optimize the capacitance values for a trade-off between an insertion/return loss and narrow bandwidth characteristic. A two-layer LTCC substrate was built on bottom ground plane. Resonators were located on two layers above the ground plane. Each length of the resonators was designed to be a half of the wavelength at 28 GHz and 5.25GHz. The seventh order Chebyshev filter is based on edgecoupled resonators on top of an alumina substrate with capacitive coupled input and output lines. This capacitive coupling is realized using broadside coupled microstrip lines [4], where input and output lines are on the top side of the substrate. The BPF in this work was designed to cover the (25 to 31 GHz) and (5.1 to 5.6 GHz). The specifications include a 6- GHz passband with the midband frequency at 28 GHz and a 0.6GHz passband with midband at 5.4GHz, an insertion loss of 1.5 db or lower, a return loss of 15 db or greater, a 0.1-dB ripple in the passband, and an attenuation of 30 db at fo +15%. In addition, the BPF should be as small as possible and easily embedded in a multilayer LTCC substrate. The 50 ohms impedance input and output ports are connected to the top surface of the LTCC substrate. Input and output lines of the filter could be directly connected to interconnect lines on top of the LTCC substrate. This solution, however, leads to stringent tolerance requirements for placing the filter on the carrier substrate, as the required capacitive coupling factors strongly depends on the overlapping length of the two microstrip lines. The coupling length for the interconnect lines was chosen to a quarter wavelength at the center frequency of the filter. The shape and size of the ground plane section on bottom of the LTCC substrate, together with the complete circuit, was optimized to provide a good ground connection and to avoid resonances within the desired frequency range. As a result, the filter substrate simply can be glued on top of the LTCC substrate. No air gap exists, as the complete filter substrate area is covered with the glue. With this, the Ka band BPF filter requires even- and odd mode characteristic impedances (Zoe, Zoo) of 89.3 and 37.5, respectively, for the first coupled line section, which translates to a line width of 321um and line gap of 14um on a 25 thou (635um) Alumina substrate. The next coupled line section requires Zoe and Zoo of 62.2 and 41.9, respectively, yielding a line width of 598um and line gap of 328um. The third coupled line section requires Zoe and Zoo of 57.8 and 44.0, which translates to a line width of 674um and line gap of 47lum. The fourth coupled line section requires Zoe and Zoo of 57.3 and 44.3, which translates to a line width of 682um and line gap of 539um. The last three coupled line sections are symmetrical to the first three, thus they have the same dimensions as stated earlier. All the quarter-wave coupled lines have a roughly length of 800um at 28GHz. Fig. 1 shows the S-parameter simulated results for Ka band BPF. - 1IX fre~ GHz Fig.I SI Iand S21 at 30GHz Another design scale down to 5.3GHz to have actual result, the filter requires even- and odd-mode characteristic impedances (Zoe, Zoo) of 92.7 and 37.6, respectively, for the first coupled line section, which translates to a line width of 355um and line gap of 12.6um on a 25 thou (635um) Alumina substrate. The next coupled line section requires Zoe and Zoo of 64.1 and 41.lspectively, yielding a line width of 574um and line gap of 490um. The third coupled line section requires Zoe and Zoo of 59.0 and 43.4, which translates to a line width of 602um and line gap of 725um. The fourth coupled line section requires Zoe and Zoo of 58.4 and 43.7, which translates to a line width of 604um and line gap of 766um. The last three coupled line sections are symmetrical to the first three, thus they have the same dimensions as stated earlier. All the quarter-wave coupled lines have a roughly length of 5400um at 5GHz. Following figure shows the dimensions of the filter. Fig.2 shows the S-parameter simulated results for C band BPF. I

3 Fig.2 SI I and S21 at 5GHz III. RESULTS OF THE FILTER. Il-.T-T-11-.J The responses of the embedded filters were measured through a network analyzer. The s-parameters of the Ka band BPF with center frequency of 28GHz are measured, the insertion loss and the return loss are plotted in Fig. 3, and also Fig.4 shows the impedance characteristics. Simulated results are shown in Fig. 1. The minimum insertion loss of the BPF simulation is 1.5 db at 28 GHz, and the fractional bandwidth is 20%. The measured insertion loss is about 0.8 db higher than that of simulated value and the center frequency is also shifted from 28 GHz to 28.3 GHz. Also the measured s-parameters for Fig.2 have plotted in Fig. 5. Fig. 6 shows the impedance characteristic at 5.3 GHz. As can be seen, the filter embedded in LTCC exhibits a midband frequency at 5.3 GHz, a bandwidth of 600 MHz, an insertion loss of 1.5 db, and a return loss of better than 10 db. Compared to the simulation, the measured insertion loss is about 0.5 db higher than that of simulated value and the center frequency is also shifted from 5.4 GHz to 5.3 GHz the midband frequency moved 0.1 GHz down. We suppose that the shift of the center frequency is mainly due to the shrinkage and variation of dielectric constant after cofiring. In the LTCC tooling design, the shrinkage in the manufacturer's datasheet resulting from blanket LTCC tapes was utilized instead of that obtained from a shrinkage test substrate with an actual design. B -2Q - m X I X W~s,1 C. I I Fig. 3 Measured S-Parameters at 30GHz., x,,~~~~ 1 re J,, D ''III 'I ' -,,,!6i' An ii III I5i Ii AN 455 S 'ncy 60D, 'r K", II

4 Fig.5 Measured S-Parameters at 5GHz Fig.6 Impedance characteristics at 5GHz V. CONCLUSION This paper presents the design, simulation, fabrication, and measurement of wide band edge-coupled BPF embedded in LTCC for 5GHz and 28GHz band. The BPF with edge-coupled microstrip resonators has been implemented with a multilayer LTCC technology in order to achieve such a wide bandwidth characteristic. The performance of these filters have been confirmed to have excellent properties for practical use, i.e., insertion losses are less than 2.5 db, and attenuations on both sides of the passband are over 30 db. Design and performance of a filter on an alumina substrate have been demonstrated. The filter simply is placed on top and glued to the LTCC substrate using nonconductive epoxy. A special electromagnetic field coupling is used to provide interconnect of signal lines and filter ground. REFFERENCES [1] C. Q. Scrantom, "Where we are and where we're going-ii," in IEEE MTT-S IMS Dig., 1999, pp [2] B.G. Choi, M. G. Stubbs, C. S. Park, "A Ka-Band Narrow Bandpass Filter Using LTCC Technology," in IEEE Microwave and Wireless Components Letter., vol. 13, no. 9, September [3] C.-K. C. Tzuang, Y.-C. Chiang, and S. Su, "Design of a quasiplanar broadside end-coupled bandpass filter," in IEEE MTT-S IMS Dig., 1990, pp [4] G. StrauB and W. Menzel, "Millimeter-wave monolithic integrated circuit interconnects using electromagnetic field coupling," IEEE Trans. Comp., Packag., Manufact. Technol. Part B, vol. 19, pp , May [5] G. L. Matthaei, "Interdigital band-pass filters," IRE Trans. Microw. Theory Tech., vol. MTT-10, pp , Figure.7 has shown the 7 order Chebyshev band pass filter,qt 5C,1H7 hqnnl Fig.7 Two layers 7 order Chebyshev band pass filter at 5.3GHz

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