Tunable Diplexer for Cognitive Radio RF Front-end Modules

This paper describes the design and measurement of a varactor-tuned planar diplexer for reconfigurable multi-mode/multi-standard transceiver front-end modules. The proposed diplexer of 3 cm × 1.8 cm is composed of two tunable dual-mode bandpass filters with two varactor diodes. The diplexer channels are 8.8% relative bandwidth at 2.48 and 3.2 GHz for 25 V biased voltage, while 7.5% and 4.1% are obtained at 2.075 and 2.606 GHz for 5 V, respectively. The center-frequency tunability for lower and upper channel filters bands are 21.4% and 20.4%, respectively. Measured insertion loss varies from 2.4 to 4.8 dB and 2.8 to 5 dB for the two channels over the tuning frequency range with better than 30-dB isolation. The lower channel filter (with higher capacitance) presents a better 1-dB compression measurement point (P-1dB) over the tuning frequency range.