Flexible Tri-Band Coupler With Three Independently Tunable Frequencies

A highly flexible tri-band coupler with three independently tunable frequencies is proposed. The proposed coupler is based on the branch-line structure with tunable loads. Theoretical analysis of the basic structure is conducted to show the relationship between the operating frequencies of the coupler and the admittance of the loads. A tunable load can be implemented by installing three varactors on a coupled line to provide positive, near-zero, and negative imaginary parts of admittance in the lower, middle, and upper bands, respectively. The load admittance in different bands can be controlled by the varactors in different positions. Taking advantage of this property, the operating frequencies of the coupler can be tuned independently by changing the biasing voltages of the varactors. For demonstration, a tunable tri-band coupler is designed, fabricated, and measured. The coupler exhibits independent tunability in each band while keeping fixed frequencies in the other two bands. The measured frequency-tunable ranges of the fabricated coupler in the lower, middle, and upper bands are 1.3–1.5, 2.3–3.0, and 3.9–4.3 GHz, respectively. Good return loss and isolation are achieved in all the three bands, with equal power division and 90 <tex-math notation="LaTeX">∘</tex-math> phase difference.