A Compact Coupling Structure for Diplexers and Filtering Power Dividers

This paper presents a compact and novel coupling structure for diplexers and power dividers based exclusively on coupled resonators. It consists of two cross-coupled structures joined together by two common resonators with a cluster of only four resonators. For a diplexer, it represents one of the most compact topologies that produces two 2nd-order channel filters with two fully controllable transmission zeros. This can be used to increase the rejection and isolations between channels without increasing the number of resonators. The same topology can also be used to realise a 3rd-order filtering power divider (FPD), with its embedded cascade trisection (CT) structure generating an asymmetric transmission zero. The coupling matrices of several diplexers and power dividers have been synthesized. Two microstrip diplexers with different positions of the transmission zeros have been demonstrated to verify the device concept. A 1.8 GHz FPD with a fractional bandwidth of 5% has also been prototyped, showing an improved out-of-band rejection from 15 dB to 25 dB below 1.71 GHz. The isolation performance of the divider has been investigated and improved from 7 dB to 18 dB across the band by adding only one resistor.