A Design of an Equal Hybrid Coupler with Low Amplitude Imbalance and Tunable Phase-Difference

Couplers with tunable phase differences have very important applications in wireless communication systems. However, the amplitude of the phase-tuned directional coupler is related to the phase difference, which leads to the imbalance of the amplitude when adjusting the phase-difference. To solve this problem, a new method of the hybrid coupler with low amplitude imbalance and the continuously tunable phase difference is proposed. Through the even-odd mode analysis of the port response of a phase-tuned directional coupler, the two amplitude imbalance signals can be superposed and compensated by another tunable coupler. The tunable phase difference and balanced output amplitude can be achieved simultaneously. Based on this principle, the designed coupler is composed of two phase-tuned directional couplers placed symmetrically in the center, cascading a section of microstrip transmission line with an electrical length of 90°. The phase-difference can be tuned by adjusting the electrical length of the horizontal transmission line of phase-tuned directional couplers. Theoretically, the proposed method reduces imbalances of the amplitude by 2.03 dB compared with the phase-tuned directional coupler. At the center frequency, the output phase difference is continuously tunable in the range of –40~47°, and the output amplitude imbalance is less than 0.6 dB.