Miniaturised tri‐band lowpass–bandpass filter using lumped‐element structure

In this Letter, a miniaturised tri‐band lowpass–bandpass (LP‐BP) filter is proposed by using lumped‐element structure. The lowpass passband (LP) and the first bandpass passband (1‐BP) are produced by using the transmission zeros to divide a lowpass filtering response into a dual‐band LP‐BP filtering response. The second bandpass passband (2‐BP) is produced by further loading a lumped‐element circuit. As an example, an LP with 3 dB cut‐off frequency of 1 GHz, and two BPs centred at 2.45/5.2 GHz for the WLAN applications are designed. The fabricated tri‐band LP‐BP filter has low passband insertion losses of 0.08/0.69/1.38 dB, and occupies a compact circuit area of 0.055 λ gc × 0.063 λ gc .