What is a Branchline Coupler?

 A Branchline Coupler is a four-port passive directional coupler that splits an input signal in to two parts by amplitude while providing a phase shift of 90˚ between the two output ports. It is also referred to as a Quadrature hybrid or 90˚Hybrid Coupler since the output signals received by the two output ports are in two different quadrants that are separated by a phase of 90˚. This unique characteristic makes them suitable for use in single antenna transmitter/receiver systems or generating In-Phase/Quadrature (I/Q) signals for Quadrature Amplitude Modulation (QAM) radios in broadband wireless communication systems.

Branchline couplers are constructed using four quarter wavelength (λ/4) transmission lines and are connected to each other at a junction point as shown in the above figure. Here λ refers to the wavelength of the electromagnetic signal. However, it is also possible to use multiples of this length i.e. 3λ/4, 5λ/4, and 7λ/4, depending on the application requirements. Using lengths of this order increases the wavelength, which decreases the operating frequency, thereby limiting the bandwidth of the input signal. Furthermore, the overall size of the coupler also increases with an increase in the length of each transmission line as the frequency comes down.

As seen in the diagram above, port 1 is the input port, ports 2 and 3 are output ports, and port 4 is the isolated/terminated port.

When an input signal is injected at port 1, this signal travels through the transmission lines and reaches port 2 with a phase shift of 90˚. This is because the quarter wavelength line shifts the phase of the input signal by 90˚. A part of the input signal is coupled to port 3 and therefore, this signal travels through another vertical transmission line to this port. Thus, the total phase shift at port 3 is 180˚. It can be seen that the phase difference between ports 2 and 3 is 90˚.

Port 4 is referred to as the isolated port and is usually terminated with an impedance of 50 ohms. Any impedance mismatch i.e. when the input and output impedances are not exactly equal, will result in a portion of the output power being reflected back to ports 1 and 4. At port 4, this power will be absorbed since the port is terminated. However, it is very likely that the reflection at port 1 will damage the coupler and when a higher power is transmitted from the input, a larger reflected power might damage the device permanently. Therefore, careful consideration needs to be taken when designing transmission lines with accurate impedance values and soldering the junction points with high precision to avoid mismatch-related issues.

The amount of power coupled to port 3 depends on the coupling value of the coupler being used. For instance, when a 3 dB coupler is used, the power split between ports 2 and 3 will be 50% (roughly). Depending on how much power should be split between the output ports as per the application needs, an appropriate coupler needs to be chosen. Another important parameter is the directivity, which refers to the ratio between the power at the coupled port and the reflected power at this port. It provides a good measure of how well the device isolates the forward and reflected power at the coupled port and is measured in dB (or decibels). In general, if the directivity is high, the isolation will be better and therefore, higher will be the performance.

The amount of power delivered is another equally important parameter to be considered. This depends on the type of transmission line used, materials, and other requirements of the end application.

While branchline couplers can offer high coupling and I/Q signal quality, they have a few limitations. The main limitation being its size. The size of this coupler depends on the size of the transmission lines which is based on the frequency of operation.

A Branch Line Coupler is the simplest type of a Quadrature Coupler or 90° Hybrid Coupler. This device is basically a 3 dB, four port directional coupler with 90° phase difference between its output ports. It can be used as a single antenna Transmitter/Receiver system or an I/Q signal splitter/combiner.

90 Degree Hybrid Coupler Schematic