A Study Of Microstrip/slotline Differential Balanced Bandpass Filter

With the rapid development of modern communication system & MMIC, higher requirements for anti-jamming capability of microwave devices have been put forward. A Balanced circuit has shown a great advantage towards the single one as for its high immunity to the environmental noises. With the increasing application of active devices such as PA & LNA, negative devices like antenna and filter are more likely to be applied to the differential structure for a more effective full-balanced RF system. The replacement of traditional balun-filter combination by the differential balanced filter not only satisfies the CM suppression demand, but also accords with the tendency of miniaturization.In this paper, an introduction and research of diffferential banlanced filter has been put forward in details from three aspects: feed methods, the produce of transmission zeros and CM suppression technology. With a comprehensive analysis of all kinds of methods, the mix mocriostrip/slotline structure is determined finally.Fistly, a dual-band differential balanced BPF is designed. This filter consists of the upper microstrip line, the middle dielectic and the bottom slotline resonator. The filter achieves good CM suppression, and the general vias, branches and resistances or capacitors could be avoided. The etching of two nested opening ring resonators on the ground can introduce two independant passbands. A pair of symmetry DGS are introduced to extend the stopband according to the low out-off band suppression problem.Then, a UWB differential balanced BPF is designed. On the basis of the classical wideband microstripe/slotline differential banlanced filter, an improvement has been made by the replacement of uniform slotline with transition structure, and the size is miniaturized.Futhermore,a wideband differential balanced BPF using low-k material is designed. The folded slotline minimizes the dimension of the filter, and the pair of spurline introduces a notch band.The filters above are of great value for application.