Research On Dual-stub Loaded Dual-band Microstrip Filter

With the rapid development of communication technology,communication systems are developing to the direction of multi-band communication,and communication equipments also need to meet the requirements of miniaturization.As an important part of RF front end of communication systems,the performance of filters will directly affect the performance of entire communication systems.Utilization of dual-band bandpass filters in modern mobile communication systems helps to reduce the size of equipments and improves performance of systems.Flexible way to achieve specified dual passbands,better out-of-band suppression character-istics,smaller passband insertion loss are the development trend of dual-band passband filter.The dual-stub loaded resonator proposed in this thesis can improve design flexibility,strengthen its capability of frequency control,and achieve specified dual band with better out-of-band suppression characteristics.This provides us a new choice for the design of dual-band passband filter.In this thesis,design principle of filter is introduced in detail first,then dual-stub loaded microstrip resonator is proposed,its resonant equations are deduced and resonant characteristics of the resonator are analyzed.It turns out that the resonator can realize the resonant frequency ratio between 1.4 and 2 flexibly,and can adjust the value of f₂f₀ without changingf₁ f₀.So f₂ can be moved away from two passbands,so a better out-of-band suppression characteristics can be achieved.In addition,since the dual-stub loaded resonator can adjust resonant frequencies flexibly,it is possible to obtain two kinds of resonators that their first two resonant frequencies are the same,but the third resonant frequencies are different.Dual-band bandpass filters using this two kinds of resonators can suppress parasitic passbands caused by the second harmonic frequencies,and this helps to obtain better out-of-band suppress-ion charactersitics.Then three examples of filter design are given.The first example is a second order dual-band microstrip bandpass filter,which uses two resonators with the same resonant frequency characteristics.The second example is a fourth order dual-band microstrip bandpass filter,which uses four resonators with the same resonant frequency characteristics.The second harmonic resonant frequency of the resonators has been shifted to3.4 f₀ for fear that the close distance between second harmonic resonant frequencies and the two passbands will deteriorate out-of-band suppression characteristics.The parasitic passband generated by the second harmonics is located at the position of 3 GHz?3.4 f₀?as expected and its insertion loss is higher than-30 dB.So dual passbands are achieved as well as better out-of-band suppression characteristics.The third example is a fourth order dual-band bandpass filter,which uses two kinds of resonators with different resonant characteristics.The first two resonant frequencies of this two kinds of resonators are the same,but the third resonant frequencies are different,so noise signal cann't pass through the postion where the second harmonic resonant frequencies of the two kinds of resonators stands simultaneously.Then parasitic passbands generated by the second harmonics are suppressed.Better out-of-band suppression characteristics can be achieved.