High selectivity Microwave Bandpass Filter with a wide Upper Stopband

Muhammad Riaz, Bal S Virdee


This paper describes a highly selective microwave bandpass filters based on microstrip integrated technology. The filter initially proposed consists of electromagnetically coupled C-shaped open-ring resonators whose feedlines are loaded with inductive spiral shaped open-circuited stubs that are strategically located along the feedlines in order to introduce transmission zeros in the filter’s stopband response. It is shown that the overall response and selectivity of this filter can be significantly enhanced by interdigitally coupling the resonators with the feedline. The resulting filter exhibits highly sharp roll-off skirts with a passband insertion-loss of 1.4 and return-loss of greater than 15 dB. The out-of-band rejection level of the filter is better than 20 dB between 3.7-9.4 GHz. Simulation results show that the center frequency of this type of filter can be adjusted from 3.3 to 3.8 GHz by carefully modifying the structure’s dimensions. The proposed coupling scheme allows to control the filter’s 3 dB fractional bandwidth, and results show the fractional bandwidth of the filter can be adjusted between 5.52 to 17.7% with negligible effect on the stopband characteristics. The proposed filter is suitable for applications in high interference environments and cognitive radio systems.


Microstrip technology; microwave filters; bandpass filters; high selectivity filters; wide out-of-band rejection

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DOI: http://dx.doi.org/10.22385/jctecs.v29i0.307