A Compact Microwave Low-pass Filter

Low-pass filter is one of the important components in a communication system, which can suppress harmonics and noise signal and has wide application in the communication system. As technology continues to evolve, to fulfill even more stringent requirements, the design of the filter band-band must exhibit good properties: low-band signal loss and reflection small in pass-band, large band attenuation, wide stop-band, the edge of the steep decay, and compact structure.Traditional open-stub low-pass filters and step impedance low-pass filters use the micro-strip line to realize lumped capacitance based on Richard transformation and Kuroda identity relationship. The size of these filters are large, stop-band is very narrow. In order to get a better stop band characteristics and edge of the steep decay, it needs to increase the number of short circuit or open circuit stub, but this will further increase the circuit size, and introduce more of the pass-band insertion loss. So how to design a compact, low loss, wide stop-band low-pass filter becomes the focus of the research of a low pass filter.In this paper, there are three areas to research compact low-pass filters:First, in the traditional open-stub low-pass filter, sizes of the stubs can be shortened by loading lumped capacitance. Second, the use of SIR structure can design compact, wide-stop-band low-pass filter. Third, by using cross-coupled comb structure, it can design quasi-low pass filter which has the low insertion loss, the steep attenuation edge, deep stop-band suppression and other requirements.