Researches On Novel Balanced Filters

With the rapid development of wireless communication techniques, it’s of great convenience for people’s every single aspect including our study, work and life. However, the limited frequency resource is the bottleneck that this technology has to face up with. As a key device for frequency selection, filters are widely applied in the communication system to regulate the frequency response, which would directly affect the performance of the whole system in most cases. In addition, the processing procedure is often under the interference of environmental noise. Compared with the traditional single-ended circuit, one of the outstanding advantages of the balanced circuit is relatively strong immunity regading to common-mode signal, so that the whole system can avoid the influence of environmental noise. On the other hand, a whole balanced RF front end can be constructed to omit the additional balun in the system. So for the purpose of suppressing environmental noise and reducing extra device, balanced filter is proposed to meet a better noise immunity and more compact system connection, thus the signal-to-noise ratio and the insertion loss of the whole system could be effectively improved at the same time. In this paper, the key principle and major techniques of the balanced filters are researched based on the planar microstrip structure. The main innovations of this paper are including as:(1) Two balanced filters based on three-fourths wavelength stepped-impedance resonators are proposed. In view of the traditional branch-line balanced structure, the proposed three-fourths wavelength stepped-impedance resonators are able to control transmission zeros under common-mode excitation with more design freedom. The final simulated and measured results show that these balanced filters have improved the characteristics of differential-mode passband, out-of-band selectivity and controllability of common-moded zeros.(2) Two balanced filters based on the multi-order coupled resonators are proposed. On the basis of the traditional multi-order coupled resonator structure, electromagnetic hybrid coupling and common-mode harmonic suppression are introduced in these two balanced filters respectively. Simulated results indicate that the former has advantages of compact size, high selectivity, extended upper-band rejection and high common-mode suppression, While the latter features in widen upper rejection.(3) Three balanced filters based on microstrip-slotline conversion structure are offered. Microstrip series-, parallel- and gap-coupling are adopted to feed the slotline resonator on bottom layer, respectively. While maintaining the superiority of intrinsic common-mode rejection, all these structure have achieved sufficient differential-mode transmission characteristics. With several methods on designing the slotline resonator and different loaded stubs, the balanced fitlers keep not only high common-mode suppression, but also enhancement of differential-mode selectivity, upper-stopband extension as well as the amount of tranmission zeros out of band.(4) Two balanced filters based on the principle of dual-path signal interference are presented. By means of realizing the differential-mode transmission and common-mode suppression in the feeding part and improve the diffrential-mode passband performance with mature bandpass filter in the central filtering part, these two structures embody the design thought of the proposed novel balanced filters which is eager for in this paper. Finally, we could expect the following work on application from couplers to balanced filters with the principle of dual-path signal interference.