A 90° Hybrid Coupler is a 4 port devices that is used to equally split an input signal with a 90° phase shift between the ports. To illustrate operation, a signal at the input port (port 1) is divided between the two output ports with half the power flowing to the 0° port and the other half flowing to the 90° port. Any reflection from mismatches at the output port flow to the Isolated port (port 4). This is one of the reasons hybrid couplers are used to split signals in applications where unwanted reflections can damage the circuit. This type of coupler is also know as Quadrature coupler.