External Group Delay Equalizer Based on Dual-Mode Substrate Integrated Waveguide Cavity

This letter presents an external group delay (GD) equalizer with low signal attenuation, based on a dual-mode substrate integrated waveguide (SIW) cavity. The design begins with a single SIW cavity inserted as a series-stub at the output of a bandpass filter (BPF) to realize a negative GD circuit (NGDC), compensating for the GD peak near the edge of BPF transmission response. To enhance compactness and performance, a dual-mode SIW cavity is introduced, which is designed to generate dual-band negative GD (NGD). Furthermore, a matched resistive load is incorporated to extend the NGD bandwidth and effectively shape a complementary central positive GD (PGD) region. To verify the design concept, the proposed equalizer is integrated into a third-order SIW BPF centered at 6.51 GHz with a fractional bandwidth of 1.6%. Measured results demonstrate an 84% reduction in GD ripple and a flatness ratio of 223%, indicating effective GD flattening well beyond the nominal passband.