A hybrid coupler is a directional coupler that is designed to split power equally among two ports. They are most frequently created from two coupled transmission lines that are set near enough together that energy which passes through one is coupled to the other.
Hybrid couplers come in two types: 90 degree hybrids (also known as quadrature hybrids) and 180 degree hybrids. But what is the difference between the two?
90 Degree Hybrid Couplers
90 degree hybrid couplers are four-port devices that are used to either equally split input signals with a 90 degree phase shift among output ports or to mix two signals while upholding high isolation between the ports.
When power is received at the IN port, half of the power travels to the zero degree port while the other half is coupled to the 90 degree port. Any reflections from mismatches that are sent back to the output ports will either travel directly to the ISO port or be canceled at the input. This is the main reason why hybrid couplers are popularly used to split high power signals when undesired reflections may damage the driver device.
90 degree hybrids are also referred as ‘quadrature hybrids’, as a signal that is applied to any input results in two amplitude signals which are quadrant. It doesn’t make any difference which port is the input as the relationship at the outputs stays the same because the devices are both mechanically and electrically symmetrical.
180 Degree Hybrid Couplers
180 degree hybrid couplers (also known as “rat race” couplers) are four-port devices that are used either to equally split an input signal or to add up two combined signals. An advantage of the hybrid ring is to give out equally-split phase-shifted output signals.
Each port is separated by 90 degrees and the center conductor ring is 1.5 wavelengths in circumference. This configuration allows for low VSWR, high output isolation, impeccable phase and amplitude balance, and match output impedances. This device is ideal for mixing high power mixed signals.
When deciding between the use of one hybrid coupler over the other, make sure to keep the following considerations in mind:
- Isolation (dB)
- Frequency Range of Operation
- Insertion Loss (dB) between ports
- Amplitude unbalance (dB)
- Phase unbalance (degree)
Hybrid couplers are also useful for in-building distribution systems to carry several carrier inputs because of the high degree of isolation among the two outputs and two input ports with no undesired interaction between carriers.