The Filter Research Based On Multi-band And Anti-electromagnetic Interference

With the rapid development of wireless communication technology,whether in the area of astronomy,military field or the coming era of 5G communication,the trend of wireless communication RF systems are required to be integrated,multi-service,low-cost and small size.On the other hand,a variety of electromagnetic wave signals lead to a shortage of spectrum resources,electromagnetic interference between various types of communication systems is becoming more and more serious.As a pivotal component of the wireless communication front-end of RF,the filter is mainly used to separate the communication frequency band from the spectrum.Based on the above requirements,Designing a filter with small size,multi-service integration and strong anti-electromagnetic interference has become a attracting issue today,especially the design of wide stop-band and high noise suppression filters.The main research contents are as follows:（1）A compact triple-passband bandpass filter（BPF）with multi-mode and a wide stopband based on a symmetrical multi-mode stepped impedance cross-shaped resonator（SI-CSR）is investigated.The SI-CSR is composed of two section of open-ended SIRs,which are crisscross each other.For demonstration,three resonant modes is designed at 2.45,2.95,and 3.56 GHz for the application of Wi-Fi and 5G communication by designing the appropriate electrical length ratio.The wide stopband are suppressed effectively by tuning impedance ratio（K）.Furthermore,the feedline loaded a pair of open stubs,which can produce two independently controllable transmission zeros（TZs）at the upper stopband to further enhance the suppression level of harmonic responses.（2）An improved balanced dual-band bandpass filter（BPF）using stub loaded short-circuited resonator（SL-SCR）is proposed,and the structural characteristics of the resonator are analyzed.The fundamental mode（f₀）and the third harmonic mode（3f₀）of this resonator are adopted as the working bands.The shorting branch is loaded at the voltage zero position of the 3f₀.Resistance and inductance components are loaded on the symmetry plane of the circuit,which significantly improves the suppression of common-mode noise.Based on the above mentioned,a compact dual-passband balanced filter operating at 3.1 GHz and 5.56GHz is designed,with the common-mode rejection greater than 20 dB.（3）A"H"type multi-mode resonator loaded multi-stub SIR is recommended.Three differential-modes and two common-modes are generated this multi-mode resonator.They have a certain degree of frequency shift.Therefore,a tri-band balanced filter is obtained.Loading the open stub on the symmetry plane produces a transmission zero to further suppresses the common mode noise.Three frequency bands operate at 1.5,3.2,and 5.8 GHz,respectively.The suppression of common mode noise more than 20 dB is basically achieved during the three operating bands.