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 achieves wideband flat group-delay and magnitude responses using a tapped-feeding structure and short-stub-loaded unequal-length coupled lines. A closed-form formulation enables accurate synthesis of the desired responses, while the tapped feeding provides additional design freedom by modifying the impedance-transformation ratio. Modified structures are proposed to mitigate the degradation caused by the non-ideal behavior of the tapped feeding. However, this configuration is structurally complex and requires extensive full-wave optimization, so an ideal-circuit-based approximated equivalent circuit is developed to enable efficient optimization and practical implementation. Four filters centered at 3.5 GHz with target group delays of 2.35, 2.40, 2.45, and 2.50 ns were fabricated on a multilayer RO4003C/RO4450F stackup. Measurements show over 500-MHz bandwidth, a maximum group-delay 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.