Design of a Compact Quad-Channel Diplexer

This paper presents a compact quad-channel diplexer by using two asymmetrical coupling shorted stub loaded stepped-impedance (SSLSIR) dual-band bandpass filters (DB-BPFs) to replace two single-band BPFs in a traditional BPF-based diplexer. Part of its impedance matching circuit is implemented by using a three-element lowpass T-network to acquire the desired phase shift. Detailed design procedures are given to guide the diplexer design. The fabricated quad-channel diplexer occupies a compact circuit area of 0.168 angstrom(g) x 0.136 angstrom(g). High band-toband isolation and wide stopband performance are achieved. Good agreement is shown between the simulated and measured results.