| With the development of modern wireless and mobile communication systems, and the application of many kinds of new wireless communication protocols( WLAN, Wi MAX, LTE, WCDMA, EVDO ect.), high performance filters with compact size, low insertion losses, and high selectivity are the goals for designing. Due to the increasing kinds of wireless services, the trend that multi-wireless services can be provided by a single wireless terminal also makes multi-band bandpass filter an essential element.In this paper, we study the planar tri-band bandpass filter for WLAN and Wi MAX. We design two kinds of planar tri-band bandpass filters using several well performed resonators discussed before.Firstly, the lately research conditions about tri-band bandpass filter are reviewed, including the realization methods of multi-band bandpass filter.Secondly, this paper introduces the structure and principles of tri-section stepped-impedance resonator(TSSIR), and focuses on the analysis of the design method of this resonator. The resonator, which is different from the conventional half-wavelength stepped-impedance resonator(SIR), has two impedance ratios K1 and K2. So we can locate the first spurious resonant frequency and second spurious resonant frequency at the desired frequencies. According to the theory of TSSIR, the impedance ratios K1 and K2 can be determined by the frequency ratios of the three resonant frequencies. Therefore, we can use TSSIRs to move the three passbands to the desired frequencies with appropriate K1 and K2. Open stub lines are embedded in the electric coupled C-shaped TSSIRs to adjust the passbands precisely. By using the cone-shaped zero-degree feed structure, we can acquire the transmission zeros out off bands and better return loss in passbands. The centre frequencies of the simulation and measurement are 2.4GHz, 3.5GHz, 5.2GHz and 2.42 GHz, 3.57 GHz, 5.39 GHz, respectively. The return losses are 0.57 d B, 0.2d B, 1.84 d B and 0.99 d B, 1.24 d B, 2.46 d B, respectively. The 3d B fractional bandwidths are found to be 6.9%, 8.1%, 5.8% and 5.6%, 7.6%, 5.8%, respectively. The entire size of the tri-band filter is 0.55 0.15 g gl ′l. With an attenuation level of more than 40 d B, three transmission zeros are realized at 2.11 GHz, 2.81 GHz, and 4.31 GHz, which effectively improve the isolations between passbands.Finally, the other tri-band bandpass filter is formed by the combination of open stub-loaded resonator(OSLR) and a quarter wavelength SIR. The SLR consist of a conventional half wavelength resonator and an open stub located at the midpoint of the microstrip line. The second passband can be obtain from the odd mode resonant frequency and the third one can be obtain from the even mode after analyzing the relationship between the odd even mode and the dimension parameter. The E-shaped SLR and a quarter wavelength SIR are coupled together, so that we can get the first passband. The centre frequencies of the measurement are 2.35 GHz, 3.62 GHz and 5.28 GHz. The return losses are 2.8d B, 1d B, 1.9d B. The 3d B fractional bandwidths are found to be 11.7%, 14.1%, 9.45%. With an attenuation level of more than 35 d B, three transmission zeros are realized at 1.93 GHz, 3.16 GHz, and 4.45 GHz. The entire size of the tri-band filter is 0.28λ_g×0.08λ_g. |
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