| With the increasing development of wireless communication,a variety of communication services have emerged in different frequency band,which require the availability of flexible and multi-purpose microwave circuits as well as systems capable of supporting them.In addition,compact and miniaturized devices are becoming more and more important for modern wireless communication.To meet these specifications,multi-band and reconfigurable filters are introduced and developed.As a more challenging design goal,multi-band reconfigurable bandpass filters(BPFs)have been proposed to date,which can make the devices more compact compared to the conventional multi-band filter and widen the frequency tuning range of monoband reconfigurable filter.In this paper,the reconfigurable filters are proposed based on varactors due to frequency agility and low cost.On the other hand,balanced filters show high immunity to environment noise,which may decrease the signal-to-noise ratios in the microwave circuits.Moreover,a whole balanced RF front-end can be constructed to omit the additional baluns in the system and miniaturize the overall system size.Thus,multi-band balanced filters and reconfigurable balanced filters are also essential for modern communication systems.Some circuits based on planar microstrip structure,which focus on novel multi-band reconfigurable BPFs,multi-band balanced BPFs and reconfigurable balanced BPF,are presented in this paper.The main works can be summarized as follow:(?)Three tunable multi-band bandpass filters are reported.Firstly,a tunable dual-band filter using a novel quad-mode resonator is developed,which is very compact.The resonator is modified from the conventional multi-stub loaded resonator,with two types of varactors loaded.Based on the resonance characteristic analysis,it is found that two odd modes of the proposed quad-mode resonator can be controlled independently to form the first passband,and the two even modes can be allocated within the second passband.A dual-band filter with independently tunable passbands is implemented and fabricated.In addition,a tri-band bandpass filter with tunable lower passband is designed based on the novel tri-mode resonator loaded with varactors.Moreover,two different types of stepped-impedance resonators(SIRs)loaded with varactors are proposed which can be used to control the spurious frequency.A tri-band filter with independently controllable passbands is presented using three sets of resonators loaded with varactors.(?)Three kinds of multi-band balanced bandpass filters are displayed.According to the stub-loaded theory,two kinds of novel stud-loaded resonators are investigated.It is found that the differential-mode(DM)resonance frequencies of the proposed resonators can be controlled easily by tuning the loaded stubs.Thus,the proposed resonators are very suitable for designing differential-mode passband with dual-band and tri-band responses.The common-mode(CM)signals may be suppressed by loading extra lumped components or microstip stubs in the middle of the resonators,which cannot affect the differential-mode responses.Moreover,using three sets of uniform impedance resonators and a pair of stepped impedance resontros,a novel quadband balanced bandpass filter with intrinsic common-mode suppression is designed based on the microstip-slotline conversion structure.(?)A compact balanced reconfigurable bandpass filter with frequency and bandwidth tunability is presented based on a novel multi-mode resonator.According to the theoretical analysis,the common-mode resonant frequencies are different from those of the differentialmode response,so the common-mode suppression can be obtained without adding any extra elements.For the DM response,the modal frequencies of odd and even modes can be adjusted simultaneously by varying the varactors.This helps to design the DM response with tunable passband center frequency.Moreover,the even-mode resonant frequency can be tuned independently,leading to controllable passband bandwidth of the DM. |
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