An active‐passive cascaded bandpass filter with an 88kHz–1.1GHz bandwidth on substrate‐integrated suspended line platform

This letter proposes an active‐passive cascaded bandpass filter based on the substrate‐integrated suspended line platform for the first time. By cascading an active‐RC highpass circuit with a passive‐LC lowpass filter circuit, the proposed filter achieves a GHz bandwidth, an extremely low cut‐off frequency, and a controllable in‐band gain. It has the advantages of compact size, DC‐offset cancellation, and self‐packaging. As proof of concept, a sixth‐order bandpass filter was designed and fabricated. The measured 3‐dB bandwidth is from 88 kHz to 1.1 GHz with a 5.8 dB in‐band gain, and the circuit area is only 0.021 × 0.022 λg, where λg is the guided wavelength at 0.55 GHz.