Design Methodologies of LTCC Bandpass Filters, Diplexer, and ...

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720 IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 54, NO. 2, FEBRUARY 2006 Fig. 8. Fabricated 2.4-GHz bandpass filter with transmission zero at the frequency of 1.9 GHz. (a) 3-D structure. (b) Measured and EM simulated results. Two filters mentioned above are fabricated with the substrate of Dupont 951. Their dielectric constant and loss tangent are 7.8 and 0.0045, respectively. The 2.4-GHz LTCC filter is designed on one upper layer with the sheet of 1.57 mil, six middle layers with the sheet of 3.6 mil, and three lower layers with the sheet of 1.57 mil. Its overall size is 110 mil 92 mil 28 mil. After circuit simulation, these values are converted into the LTCC structure. The simulation is carried out with the assistant of the full-wave electromagnetic (EM) simulator Sonnet from Sonnet Software Inc., North Syracuse, NY. In the 3-D structure, the parallel-coupled line is placed on the lower layer to reduce the coupling effect with other capacitors. Fig. 8(a) shows the three-dimensional (3-D) structure of 2.4-GHz LTCC bandpass filter. The on-wafer tester has been chosen to improve the accuracy of measurement. The network analyzer Agilent N5230A PNA_L is used to measure, and the short-open-load-through (SOLT) is adopted to calibrate. Fig. 9. Fabricated 2-GHz bandpass filter with transmission zero at the frequency of 2.5 GHz. (a) 3-D structure. (b) Measured and EM simulated results. As shown in Fig. 8(b), the frequencies of measured and EM simulated transmission zeros are 1.84 and 1.9 GHz, respectively. At the frequency of 2.4 GHz, the measured and EM simulated insertion losses are less than 1.44 and 1.3 dB, respectively; the return losses are greater than 15 and 17 dB, respectively. A 2-GHz bandpass filter is given as another design example. This 2-GHz LTCC filter is designed on five upper layers with the sheet of 3.6 mil, four middle layers with the sheet of 1.57 mil, and two lower layers with the sheet of 3.6 mil. Its overall size is 107 mil 102 mil 32 mil. The 3-D structure of the 2-GHz LTCC bandpass filter is shown in Fig. 9(a). In the 3-D structure, the parallel-coupled line is the same placed on the lower layer to reduce the coupling effect with other capacitors. As shown in Fig. 9(b), the frequencies of the measured and EM simulated transmission zeros are 2.45 and 2.5 GHz, respectively. At the frequency of 2 GHz, the measured and EM simulated insertion losses are less than 1.45 and 1.26 dB, respectively; the return losses are greater than 19 and 39 dB, respectively. The LTCC technology is a kind of thick-film process. In order to realize the physical 3-D circuit, the parasitic effect among ca-
TANG AND YOU: DESIGN METHODOLOGIES OF LTCC BANDPASS FILTERS, DIPLEXER, AND TRIPLEXER WITH TRANSMISSION ZEROS 721 Fig. 11. Diplexer designed with multilayered structure. (a) Photograph. (b) Measured responses. Fig. 10. Diplexer designed with multilayered structure. (a) Equivalent circuit. (b) EM simulated responses. pacitors may cause their model differing from the ideal values provided by the circuit simulator. Figs. 8(b) and 9(b) show the measured and EM simulated responses, which involve the parasitic effect and substrate loss. These effects may cause the measured and EM simulated results such as Figs. 8(b) and 9(b) to not exactly be the same as the ones in Figs. 6(b) and 7(b). In this paper, we design our examples with the substrate of Dupont 951. Even though the thick-film process may provide the linewidth variation within 5% in the plane surface, it can still provide good repeatability. 1 Compared with their EM simulation, the measured frequencies of transmission zeros in the fabricated circuits merely shifted 3.2% and 2% downward, as shown in Fig. 8(b) and 9(b), respectively. Moreover, with a fixed sintering profile, stable dielectric constant and layer thickness 1 DuPont Green Tape Material Systems. [Online]. Available: http://www. mcm.dupont.com/MCM/en_US/Products/greentape/green_tape.html can be obtained in the LTCC process and the resultant fabricated circuits are not sensitive to the temperature variation within the range of 40 C– C. IV. SYNTHESIS OF DIPLEXER AND TRIPLEXER The proposed second-order bandpass filter can be employed to develop the diplexer and triplexer. A. Diplexer The diplexer developed from 2- and 2.4-GHz bandpass filters is taken as the design example. Two filters are connected with the matching line to form a diplexer as shown in Fig. 10(a). There are two transmission zeros, which can increase the isolation between two frequency bands, generated by 2- and 2.4-GHz bandpass filters. Their frequencies are 2.58 and 1.86 GHz, respectively. Two passbands of 2- and 2.4-GHz bandpass filters are within 1.8–2 and 2.4–2.5 GHz, respectively. The EM simulated results of the diplexer are expressed in Fig. 10(b). The diplexer is fabricated with the substrate of Dupont 951. Its dielectric constant and loss tangent are 7.8 and 0.0045, respectively. The LTCC diplexer is designed based on four upper
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