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 lambda(gc) x 0.063 lambda(gc).