Tunable directional coupler for RF front‐end applications

Abstract

A tunable directional coupler for radio frequency (RF) front‐ends of handheld mobile devices is presented. The coupler is implemented in a 0.13 μm bulk RF‐CMOS technology with all elements monolithically integrated on a single die. A digitally‐tuned termination network at the isolated port of the coupler is realised solely with RF‐switch transistors using the channel resistance of a metal‐oxide‐semiconductor (MOS) device. The fine‐tuned termination network allows the coupler to achieve a directivity of over 25 dB in the whole cellular frequency range from 699 MHz up to 2.7 GHz. The worst‐case insertion loss in the transmitted path is 0.15 dB at 2.7 GHz.