| As the rapid changes in technology today,wireless communication technology is developing rapidly.This dissertation studies the design of multi-band filter,which is an important component of the radio frequency(RF)front-end of the wireless communication system.With the development of wireless communication system,the single-band filters could not meet the needs of system.And if using multiple filters working at different bands,the system will largely increase the RF front-end volume.Therefore,there is an urgent need to research multi-band filters that can operate in multi-bands simultaneously.This dissertation mainly studies the resonator characteristics of the stepped impedance resonator(SIR)and the stub load resonator(SLR).And several multi-band filters are designed by using these resonators.First,based on the SIR structure,we calculate the corresponding impedance ratio and electrical length,when the impedance ratio is less than 1.The corresponding physical length can be calculated from the relative permittivity of the known dielectric substrate.The SIR is properly bent and coupled with an interdigital structure by using the zero-degree feed structure.By adding the open-circuit line to the center line of the SIR resonator,we can adjust the center frequency more precisely.Then we design a dual-band band-pass filter with center frequencies of 2.4GHz and 5.2GHz,respectively.The insertion losses of pass-band at 2.4GHz and 5.2GHz are 0.06 dB and 0.13 dB,respectively,and the return loss is 23.84 dB and 42.66 dB,respectively.The 3dB fractional bandwidths are found to be 16.46% and 11.02%,respectively.Three transmission zeros are generated at 1.8GHz,3.8GHz,5.7GHz,whose attenuation level exceed 31.08 dB.Then when the impedance ratio is bigger than 1,the basic SIRs are twisted to “C”-shaped and formed by electrical coupling with a zero-degree feed structure.By adjusting the impedance ratio and electrical length,we design a tri-band band-pass filter and a quad-band band-pass filter,respectively.The center frequencies of the tri-band band-pass filter are working at 2.4GHz,3.5GHz and 4.5GHz.The insertion losses of each pass-band are 0.29 dB,0.16 dB and 0.49 dB,respectively.The 3dB fractional bandwidths are found to be 6.67%,11.71%,5.73% respectively,and the return loss are 22.31 dB,27.16 dB,17.66 dB respectively.Four transmission zeros are generated at 2GHz,2.7GHz,4GHz,and 5.3GHz,whose attenuation level exceed 28 dB.The center frequencies of the quad-band band-pass filter are 1.9GHz,3.45 GHz,4.4GHz and 5.7GHz.The insertion losses of each pass-band are 0.43 dB,0.14 dB,0.21 dB and 0.63 dB respectively.The 3dB fractional bandwidths are found to be 7.21%,15.86%,12.5% and 4.35% respectively.Finally,the characteristics of the stub load resonator(SLR)are analyzed.Based on the characteristics of open SLR(OSLR),we assume that the odd-mode frequency is 3.5GHz and the even-mode frequency is 5.4GHz.To make sure that the filter has a small size,the basic SLRs are twisted to "?"-shaped and formed by electrical coupling with a zero-degree feed structure,so that we can get a dual-band band-pass filter.Then by embedding the quarter-wavelength SIR resonators with the short-circuited end of the resonator,we design a tri-band band-pass filter combined with OSLR structure.The measured center frequencies are 2.44 GHz,3.55 GHz and 5.41 GHz respectively.The insert losses are 0.68 d B,0.55 d B and 0.59 d B.The return losses are 19.13 d B,13.40 d B,18.11 d B respectively.The 3d B fractional bandwidths are 7.92%,9.43% and 9.07% respectively.Four transmission zeroes whose attenuation level exceed 42.52 d B are generated,which are locating at 1.96 GHz,2.77 GHz,4.39 GHz and 6.01 GHz respectively.The isolation between pass-band is good and the suppression in stop-band is higher. |
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