Research And Design Of Tri-band Bandpass Microstrip Filter

With the high development of technology, our lives are effected and improved at an unprecedented speed by modern wireless and mobile communication system. Meanwhile, more urgent and higher requires are put forward for us to satisfy. As the crucial component of wireless communication system, filters that have multiple bands working at the frontend of the radio frequency are the main methods to achieve the function of multi-band in a communication system. High performance filters with compact size, low insertion loss and high orthogonal coefficient are the hotspot to be studied and designed along with the proposal and application of the new wireless communication protocols, such as WLAN, Wi MAX, LTE and WCDMA.In this paper, planar tri-band band-pass filters for WLAN and Wi MAX are studied, and two kinds of tri-band band-pass microstrip filters are designed using well performed resonators.Firstly, the ultramodern researches about tri-band band-pass filters are reviewed, including the methods to realize multi-band band-pass filters.Secondly, the structure and principle of stepped impedance resonator(SIR) with a quarter wavelength are introduced in this paper, based on which the first pass-band is created. Then, the structure and resonance principle of the open stub loaded resonator(OSLR) are presented, based on which a tri-band band-pass filter is formed by the combination of OSLRs and SIRs. The second pass-band and the third pass-band can be obtained by the odd mode resonant response and the even mode response of the OSLR. To make sure that the filter has a small size, the basic SLRs are twisted to ?-shaped and coupled together with two SIRs, so that we can get a tri-band band-pass filter fed by a zero-degree feed structure. To improve the performance of the filter, two open stubs are loaded by the side of the input and output feed lines, through which five transmission zeroes whose attenuation level exceed 33 d B are generated, locating at 2.02 GHz, 2.85 GHz, 3.01 GHz, 4GHz and 5.9GHz. With a size of 0.28 0.08?g g? ?, the filter's center frequencies are 2.4GHz, 3.5GHz and 5.2GHz. The insert losses are 0.44 d B, 0.64 d B and 0.34 d B, respectively. The 3d B fractional bandwidths are 11.7%, 9.7% and 9.1%, respectively.Finally, the structure and resonance principle of SIR with a half wavelength are described and the method to design this resonator is researched in the paper. Two C-shaped SIRs are placed symmetrically to construct the first and third pass-band. Then the SIR with a length of a quarter wavelength is applied to generate the second pass-band. By using a zero-degree feed structure, we can acquire transmission zeroes and better return loss. With a size of 0.214 0.112?g g? ?.The filter's center frequencies are 2.4GHz, 3.5GHz and 5.2GHz, respectively. The return losses are 2.19 d B, 0.21 d B, 1.92 d B, respectively. The 3d B fractional bandwidths are 11.3%, 8%, 3.5%, respectively.