Compact Microstrip Lowpass Filter With High and Wide Rejection in the Stopband Utilizing Flabelliform Resonators

Abstract

Abstract-In this paper, a microstrip lowpass filter with −3 dB cutoff frequency of 3.8 GHz consisting of two cascaded resonators with flabelliform patches and two symmetric suppressing cells is proposed. To design the filter, the impact of each transmission line on the frequency response is determined by extracting the equations of the insertion loss (S 21 ) and return loss (S 11 ) on the basis of the equivalent LC circuit of the main resonance cell and the cascaded structure. The designed filter is constructed and tested, and a good agreement between the results of simulation and measurement is obtained. In the whole stopband region, a return loss close to zero and an acceptable suppression level of −30 dB from 4.47 to 25.17 GHz are achieved. Furthermore, a flat insertion loss in the passband and a low return loss (−23.02 dB) in this band can prove desired in-band and out-band frequency response.

A Design of Microstrip Lowpass Filter with Wide Rejection Band and Sharp Transition Band Utilizing Semi-Circle Resonators

“…The suppressors suppress spurious frequencies with a suppressing factor of 32 dB, which is better than the other works. Clearly, the proposed filters in [11] and [17] have the closest suppressing factor to this work. As it can be observed, the presented filter, in spite of occupying 262.44 mm 2 at the operating frequency of 2.1 GHz, has brought about a figure of merit equal to 24241.69, which is much more than the other published articles.…”

“…In [16], employing modified hairpin resonator with long straight slots leads to designing a lowpass filter with sharp skirt performance and wide rejection band, but its return loss performance is not good enough. Some other methods to present symmetric LPFs with sharp transition slope are utilizing flabelliform resonators and triangular patches [17] and [18], although these circuits suffer from a narrow stop-band bandwidth and large circuit size, respectively. In this letter, a microstrip lowpass filter (LPF) operating at -3-dB cut-off frequency of 2.1 GHz has been designed.…”