Ultrawideband Balanced Bandpass Filter

A broadside-coupled structure is utilized to design balanced bandpass filter (BPF) with ultrawideband performance. The top and bottom layers of the structure contain tapered microstrip patches. Those patches are coupled via tapered slots in the ground plane, which is located at the middle layer. The employed structure operates as a BPF in the differential-mode, whereas it operates as an all-stop filter in the common-mode. The simulated and measured performance of the filter show a 123% fractional bandwidth centered at 6.55 GHz, a sharp and wide upper stopband that extends beyond 20 GHz, and a sharp lower stopband. The filter suppresses the common-mode signals by more than 27 dB in the simulations and 24 dB in the measurements across the whole abovementioned band. The designed filter reveals a distortionless performance in the time domain with only 0.1 ns peak-to-peak variation in the group delay across the band 3.1 to 10.6 GHz.