Research On Novel Loading Mechanism Of The Variable Units In Microwave Reconfigurable Filtering Circuits

With the rapid development of wireless communication systems,the spectrum resource has become more and more scarce.While using the limited spectrum resources,people hope to design a multifunctional microwave device,which has the advantages of the low cost,the weight of the low load,the high electromagnetic compatibility and simple design,to replace multiple microwave devices.Therefore,reconfigurable technologies have appeared to meet this demand and it has attracted more attention of the domestic and foreign scholars.The filtering circuit has attracted much attention in the design of the reconfigurable filtering circuits due to the characteristics of its frequency selection and the function of eliminating unnecessary interference.This thesis briefly introduces the research background and significance of reconfigurable filtering circuits,and investigates the current research status of reconfigurable filtering circuits at home and abroad.The universal design methods of the filter are given.Based on the filter design,combined with novel loading mechanism of the variable units,a variety of competitive microwave reconfigurable filtering circuits are designed and implemented.The research has the certain guiding significance for reconfigurable filtering circuit.The frequency tuning mechanism that the characteristic parameters of the quarter-wavelength(?/4)resonator and the variable capacitor fused together are overall analyzed by theoretical analysis,formula derivation and computer simulation.The relationship between the characteristic parameters of the resonator and different frequency ratios,and the effects of different loading positions of the variable capacitor for different frequency tuning ranges are studied.Thus,the novel loading mechanism of the variable units can be obtained.It can be found for the first time that the frequency tuning range of third harmonic(f₃)was zero when the variable capacitor loaded at the f₃(lowest harmonic)of the voltage-null point of the?/4 resonator,which means the fixed f₃.The tuning range of the fundamental frequency(f₁)can be widened by optimizing the characteristic parameters of the resonator.The frequency ratio between the fifth harmonic(f₅)and f₃ changes as the characteristic parameters of the resonator change.And the ratio can be smaller so that two close resonant frequencies can be obtained.Based on the above analysis,firstly,a variable capacitor is loaded at the voltage-null point of f₃ on?/4 stepped-impedance resonator(SIR),which is chosen to design and implement a tunable filter with stable wide stopband.The variable capacitor loaded at the voltage-null point of f₃ on SIR is used to tune f₁.Thus,f₃ is not tuned in conjunction with f₁,which can ensure a stable distance between f₁ and f₃,so that the proposed tunable filter can obtain a stable wide stopband.On the premise of ensuring the stability of the upper stopband,the wide frequency tuning range of the tunable filter can be obtained by optimizing the characteristic parameters of the SIR.In addition,in order to make this design more practical,a proper coupling region is selected and another variable capacitor is loaded between the feeding line and the resonator to achieve a constant fractional bandwidth(FBW)in the design process of the tunable filter.The source-to-load(S-L)coupling is introduced to generate a transmission zero(TZ)on each side of the passband to improve the passband selectivity.Secondly,a new loading scheme of the variable units is used to design a reconfigurable filter with dual functions of tunable and switchable simultaneously.The new loading scheme of the variable units is to load the variable capacitor and PIN diode into the same resonator at the same time.Among them,the variable capacitor is loaded at a specific position of the resonator to realize the tunability of the passband,and the PIN diode is loaded on the open end of the resonator to realize a switchable passband.Based on this scheme,the passband can be turned to the ON-or OFF-state in each operation frequency,which is controlled by the PIN diode loaded at the open end of the resonator during the passband frequency tuning range.Similarly,in order to make the design more attractive,the reconfigurable filter also has a constant absolute bandwidth(ABW)and high selectivity.Finally,in order to meet different application requirements,a diplexer with two separately-designable channels(fixed lower channel and tunable higher channel)is designed.The diplexer uses a common element-loaded?/4 uniform impedance resonator(UIR)operating in f₃and f₅,which is used to design the lower channel and higher channel of the diplexer.The stub is loaded at the voltage-null point of f₃ on the?/4UIR and the variable capacitor is loaded at the open end of the stub.The variable capacitor loaded stub can independently tune f₅ but has no effect on f₃ so that two channels can be separately designed.At the same time,the input feed position on the common UIR is properly designed so that a common TZ between f₃ and f₅ exists in both channel filtering responses.In the tunable higher channel design,asynchronous tuning scheme is used to obtain a constant ABW.In addition,by properly choosing coupling schemes of the two channels,more TZs are realized at the desired position.Due to the existing of multiple fixed TZs,both the selectivity and isolation of the diplexer can be improved significantly.