An Enabling Multi-Operation Branch-Line Coupler

In this paper, an enabling multi-operation branch-line coupler is proposed initially. The proposed design is configured by ±90° and 180° transmission lines, and the main advantage is that the proposed design can be applied in three different system connection schemes as shown in <xref rid="fig1" ref-type="fig">Fig. 1</xref>. Corresponding to the three schemes, the coupler can be with three operations (e.g. the operations of balanced input with single-ended outputs, single-ended input with single-ended outputs, and single-ended input with balanced and single-ended outputs). This can increase the flexibility and freedom of the coupler for systems with different connection types. Furthermore, the common-mode suppression can be achieved under the operation. The standard and mixed-mode S-parameters are utilized to deduce the circuit parameters. Two prototypes using 3/<inline-formula> <tex-math notation="LaTeX">$4~\lambda _{\mathrm {g}}$ </tex-math></inline-formula> (<inline-formula> <tex-math notation="LaTeX">$\lambda _{\mathrm {g}}$ </tex-math></inline-formula> is the guided wavelength at the center frequency) transmission line (design I) and coupled line with two shorts (design II) to realize the −90° transmission line were designed, fabricated, and measured to verify the theoretical prediction. The sizes of the two prototypes are <inline-formula> <tex-math notation="LaTeX">$0.98\,\,\lambda _{\mathrm {g}} \times 0.2 \lambda _{\mathrm {g}}$ </tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$0.65 \lambda _{\mathrm {g}} \times 0.2 \lambda _{\mathrm {g}}$ </tex-math></inline-formula>, respectively. Design I (design II) exhibits the 15-dB operating bandwidths for the three operations of 8% (9%), 7.3% (9%), and 6.7% (7.8%), respectively.

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