Desingn Of Wideband Microstip Balun And Balanced Filter

Balun is the unbalanced to balanced transformer and has already been widely usedin many fields such as balanced mixers, frequency multipliers, push-pull amplifiers andantenna feed networks.The balanced filter integrates balun and filter into one module,which not only reduces the area of the printed circuit board effectively, but also avoidsparasitic effects between the components. Nowadays, the equipments of wirelesscommunication focus on compact, wideband, low power and multifunction, themicrostrip balun and the balanced filter become a hot research topic. This paper analysesthe basic theory of balun and filter in detail, and then three wideband microstrip balunsand two ultra-wideband balanced filters are proposed.Firstly, according to the characteristics of microwave transmission line, three typesof wideband microstrip baluns are designed. One type is slotline to microstrip balun, theother type is slotline to coupled strip line balun. To achieve an out of phase signaldivision over a large frequency range, a T-junction formed by a slot-line and two arms ofa microstrip line is employed. Besides, a multilayer Wilkinson power divider balun isproposed, which gets two signals of equal amplitude and same phase by applying thepower divider, and produces a 180 degree phase shift by using slotline.Secondly, the design theory of filter which uses the low-pass prototype is discussed,and the DGS (Defected Ground Structure) is introduced. The DGS has the characteristicsof single pole, low pass, slow-wave effect and high characteristic impedance. Theequivalent circuit of dumbbell shaped DGS is researched in this paper, and with the helpof electromagnetic simulation software, the effect of changing the size of dumbbellshaped DGS is also discussed. The application of DGS in the design of microstrip filter isstudied.Finally, according to the dumbbell shaped DGS, a new improved DGS is obtained,which reduces the insertion loss of pass-band effectively. Through the application of thenew structure in the signal transmission of the balun, two types of balanced filters aredesigned. They have good performance in passband and good harmonic suppression characteristics in stopband, at the same time, the signals of output ports change fromunbalanced to balanced.