Bandpass Filter With Wideband Flat Group Delay Based on Tapped Feeding and Short-Stub-Loaded Unequal-Length Coupled Lines

This paper presents a generalized interdigital bandpass filter that simultaneously achieves wideband flat group-delay (GD) and magnitude responses using tapped-feeding and unequal-length coupled lines. Four filters centered at 3.5 GHz with target GDs of 2.35, 2.40, 2.45, and 2.50 ns are designed and fabricated, each providing over 500-MHz (14.3%) bandwidth, a maximum GD variation of 0.03 ns, and insertion loss below 2.5 dB. The fabricated filters occupy a compact 12.5 × 12.5 mm² footprint, corresponding to (λg/4)².