Design of a high band-isolation diplexer based on asymmetric stepped-impedance resonators with side-coupling structure

In this paper, a high band-isolation diplexer based on two bandpass filters (BPFs) formed using asymmetric stepped-impedance resonators is presented. The passbands of the diplexer are operated at 1.8GHz for global system for mobile communications (GSM) and 2.4GHz for wireless local area network (WLAN). Side-coupling structure is adopted in BPFs to create two additional transmission zeros, thus enhancing the band isolation and stopband region. The measured results of the proposed diplexer show the passband at 1.8GHz with a low insertion loss of 1.09dB, a 3dB fractional bandwidth (FBW) of 9.4% in the first channel and a passband at 2.4GHz with an insertion loss of 2.6dB, a 3dB FBW of 6.2% in the second channel. The measured results of the fabricated diplexer also show a good agreement with the simulated ones.