Compact low‐pass stepped impedance filters with enhanced out of band response

This article presents slow‐wave microstrip transmission lines based on a double‐layer PCB substrate for miniaturized and high‐rejection low‐pass filters design. The slow‐wave effect is obtained by inserting metallic via rows within the lower substrate layer. Realized slow‐wave microstrip transmission lines exhibit a high effective dielectric constant of 7.4 instead of 3 for classical microstrip transmission lines. Based on this concept, miniaturized high‐rejection 7th‐order low‐pass stepped‐impedance filters with a cut‐off frequency of 2.45 GHz were designed, fabricated and measured. Measurement results are in good agreement with simulations. Thanks to the slow‐wave microstrip lines, the size of the realized filters is reduced by 65% as compared to a classical filter prototype, without degrading electrical performances.