The directional coupler is a directional power coupling (distribution) element. It is a four-port component, usually composed of two transmission lines called a straight line (main line) and a coupled line (secondary line). A part (or all) of the power of the through line is coupled to the coupled line through a certain coupling mechanism (such as a gap, hole, coupling line segment, etc.) between the through line and the coupled line, and the power is required to be transmitted to only one of the coupled lines. Output port, the other port has no power output. If the propagation direction of the wave in the through line becomes opposite to the original direction, the output port of the power in the coupled line and the port without power output will also change accordingly, that is, the coupling (distribution) of power is directional , So called directional coupler (directional coupler).
As an important part of many microwave circuits, directional couplers are widely used in modern electronic systems. It can be used to provide sampling power for temperature compensation and amplitude control circuits, and can complete power distribution and synthesis in a wide frequency range; in a balanced amplifier, it helps to obtain a good input and output voltage standing wave ratio (VSWR) ; In balanced mixers and microwave equipment (for example, network analyzers), it can be used to sample incident and reflected signals; in mobile communications, use
The 90° bridge coupler can determine the phase error of the π/4 phase shift keying (QPSK) transmitter. The coupler is matched to the characteristic impedance at all four ports, which makes it easy to be embedded in other circuits or subsystems. By adopting different coupling structures, coupling media and coupling mechanisms, directional couplers suitable for different requirements of various microwave systems can be designed.
