| In recent years, with the rapid development of the multi-mode wireless communication systems, more and more users favor the multi-functional handsets, because users can access various services with a single handset or device. Multi-band microwave bandpass filters have important applications to the miniaturization of circuits and systems, because a single multi-band filter has multiple functions instead of several single-band filters. This paper has studied the basic theory and design method of the separate electric and magnetic coupling (SEMC) and the defected coplanar waveguide structure (DCPWS) multi-band bandpass filters.Firstly, the basic theory on microwave filter is researched, and the design principles of multi-band microwave bandpass filters are expatiated. Secondly, the multi-band bandpass filters have been proposed base on the separate electric and magnetic coupling resonator and the defected coplanar waveguide structure resonator, their design methods have been studied. SEMC Stepped impedance resonators are used in filters design. Electric and magnetic coupling are introduced between a pair of SIRs by the microstrip gap and the grounded via, respectively. It is convenient to control the electric and magnetic coupling, due to they are in different coupling paths. It is noted that electric field centers on SIRs ends coupling gap, it can be equated with a coupling capacitor, and the magnetic field centers on grounded via, the two coupling paths are of the electric and magnetic ones, respectively. The simulated results indicate that a pair of transmission zeros with attenuations of -45dB and -40dB can be realized with SEMC resonator adding the source-load coupling. The transmission zeros at the lower and higher stopband are attribute to electric-dominant coupling and magnetic-dominant coupling, respectively. Transmission zeros (TZ) can improve the filter’s frequency selectivity and out band suppression greatly. This paper has deduced the equivalent circuit and the formulas of resonant conditions, electric and magnetic coupling coefficients of SEMC resonator. Based on this resonator, independently controllable dual-band and quad-band bandpass filters have been designed by embedding a similar resonator inside. The filters have been fabricated and measured, the measured results indicate that good frequency selectivity and good out band suppression can be obtained using this SEMC resonators, and it is easy to design multi-band bandpass filter with good frequency selectivity.For DCPWS multi-band filters, L-shape defected structures are used for a DCPWS resonator unit to break up the distribution of electromagnetic field on coplanar waveguide line, so, resonances can be produced in the frequency responses. This paper has deduced the equivalent circuit and the expressions of resonant frequency for the DCPWS resonator. Independently controllable dual-band and tri-band bandpass filters have been designed by using L-shape DCPWS with different sizes. The filters have been fabricated and measured, the measurements are in good agreement with the simulations. It indicates that multi-band bandpass filters with good performance can be obtained using the DCPWS resonators. |
