A Simple and Universal Phase Control Method for Designing Directional Couplers With Adjustable Phase Difference Slope and High Linearity

Control of phase plays an indispensable role in applications such as high-efficiency power amplifiers and broadband beamforming networks. Designs of existing radio frequency (RF) components, however, have not comprehensively considered the phase characteristics within a certain operating band, such as phase slope and linearity. In this article, a simple and universal phase control method by using a 90° transmission line with fixed impedance and a 180° transmission line with variable impedance is proposed for the first time. By using this phase control unit in a branch-line directional coupler, the phase difference slope can be adjusted arbitrarily by simply changing the impedance of the 180° transmission line. For demonstration purposes, 0° and −90° directional couplers are designed based on single-section and multisection structures, respectively. Explicit theoretical analyses are given for both directional couplers to illustrate the working principle and parameters design. For verification, three circuits with different phase slopes are designed, fabricated, and measured for each coupler. According to the definition of the phase difference slope by (
$\varphi _{\mathrm {max}}-\varphi _{\mathrm {min}}$
)/(
$f_{\mathrm {max}}-f_{\mathrm {min}}$
), the measured results show that the 0° directional coupler can provide adjustable phase difference slopes from −58.5 to 34 within the operating frequency range from 3.2 to 3.8 GHz. Within the same frequency range, the −90° directional coupler realizes an adjustable phase difference slope from −36.5 to 94.3. In addition to the controllable phase slopes, both couplers achieve high slope linearity and fixed phase difference at the center frequency, which validate the effectiveness of the proposed structure.