Frequency shifting of microstrip ring filter using open stubs

Dual-mode microstrip ring filter whose center frequency is shifted to a lower frequency using four additional open stubs is presented. Nevertheless, the shifted dual-mode ring filter has to face an in-band matching problem in its passband response. A possible solution to the problem is proposed by modifying the line impedance of the ring filter. In this paper, a frequency shifting of about 33.33 % of its 3 GHz initial value is implemented for a microstrip ring filter on FR4 substrate. All the filter layouts are presented along with the comparison between the simulation and measured results.