Investigation On The Synthesis Design Of Frequency-dependent Coupling Filter

Filters with compact size and low insertion loss(IL)are finding widespread application in modern microwave/millmeterwave communication systems where high selectivity and wide out-of-band rejection are required for efficient use of an already crowded and limited electromagnetic spectrum.To achieve high performance filter,filter synthesis has been introduced to obtain desired response.On the one hand,filter synthesis can introduce novel topology of filter;on the other hand,it can provide therotical guide.In this dissersation,some issuses of filter synthesis have been discussed,including: in-line topology mixed coupling filter synthesis with high performance;in-line topology filter synthesis with linearly frequency-dependent coupling coefficient;the extraction of frequency-dependent coupling coefficient;filter synthesis with high-order frequency-dependent coupling coefficient and filter synthesis with frequency-dependent source-load(S-L)coupling.1.A compact second-order mixed coupling bandpass filter(BPF)with one controllable transmission zero(TZ)near the passband edge is presented using multilayer circular substrate integrated waveguide(SIW).Two arranged circular SIW resonators can be vertically coupled via the circular apertures etched on the middle metal layer while preserving a compact physical size compared with the conventional horizontally coupled filter made of the single layer.The mixed electric and magnetic coupling can be introduced and adjusted by two etched circular apertures and a flexible TZ can be generated.2.A general synthesis method for second-order mixed coupled microstrip BPFs with separated electric and magnetic couplings is presented.The mixed coupling is modeled in such a way that both magnitudes and phase differences of constituted capacitive and inductive couplings and positions of two ports.The distribution of TZs can be analytically synthesized with different coupling coefficients ratios and different electric length ratios.Three high performance compact filter examples with multiple TZs are designed.Based on designed second-order mixed coupling BPF,a compact balanced BPF with high common mode(CM)suppression is proposed,meanwhile,the high and wide upper stopband for differential mode(DM)responses is achieved.Furthermore,because of mixed coupling and mutual loading effect,two controllable TZs can be generated in the other channel and improve isolation.A compact diplexer with high isolation can be realized by combining two second-order BPFs with different central frequencies without the extra junction matching network.Finally,a compact and high isolation balanced-to-balanced microstrip diplexer with mixed coupling is proposed via the combination of two high selectivity and CM suppression balanced BPFs without extra junction matching network.3.On the basis of a rescaling principle for frequency variables,an analytical method that rotating transformation between the frequency-invariant coupling matrix and the frequency-dependent cioupling matrix is proposed.And a method which is used to extract frequency-dependent coupling coefficient is presented.Then,the frequency-dependent coupling matrix of the second-order filter with high-order frequency-dependent coupling coefficient can be synthesized by an optimization algorithm which is based on the zero-pole goal function.Four frequency-dependent coupling microstrip filters with different number TZs for WLAN application are synthesized.4.The dual-mode filter topology and in-line frequency-dependent coupling filter topology with frequency-dependent source-load(S-L)coupling have been synthesized and designed.Then,a dual-mode microstrip BPF and a box microstrip BPF with compact size and high selectivity are designed.Finally,a compact second-order microstrip balanced BPF with high selectivity and CM suppression performances is presented based on the designed box microstrip BPF with the magnetically coupling resonators.The experiments are fabricated,measured,and the good agreement between the simulated and measured results demonstrate the validity of our designed structures.