High performance LPF with negligible group delay using circular resonator and transfer function analysis

Using circular and rectangular resonators, a new design of a compact microstrip lowpass filter (LPF) with negligible losses is proposed and fabricated. The proposed LPF has a 3 dB cut‐off frequency at 1.375 GHz with an adjusting range of 14.5%; moreover, the suggested configuration leads to a competitive roll‐off rate of 189 dB/ GHz and a negligible group delay of 0.47 ns. Attenuator cells have capability to produce numerous transmission zeros (TZs) at high frequencies; consequently, an ultra‐wide stopband (14 fc) with high suppression level (25 dB) is successfully obtained. Furthermore, the LPF has a small size of 0.067λg × 0.134λg (λg is the guided wavelength at 1.375 GHz). The transfer function is also calculated to adjust the location of the main TZs and the cut‐off frequency. Finally, an acceptable similarity between the simulated results and the tested ones verifies the simulation results.