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 f(c)) with high suppression level (25 dB) is successfully obtained. Furthermore, the LPF has a small size of 0.067 lambda(g) x 0.134 lambda(g) (lambda(g) 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.