Directional Couplers Function as Samplers in Telecommunication Circuits

Key Takeaways

• Directional couplers are widely used for power monitoring, source leveling, and isolation applications in RF circuits.

• In telecommunication circuits, the directional coupler is used in multiplexers, demultiplexers, splitters, and optical switches. 

• Key specifications of the directional coupler are directivity, SWR, the coupling factor, transmission loss, bandwidth, and average power.

Directional couplers function in telecommunication circuits to sample incident and reflected electromagnetic power

In RF and microwave circuits, there are various cases where some amount of signal power in one circuit needs to be utilized in another circuit. This sharing of electromagnetic power is enabled by a four-port network called a directional coupler. In telecommunication circuits, directional couplers function in multiplexers, demultiplexers, splitters, and optical switches for power monitoring, source leveling, and isolation applications.

Directional Couplers Function as Samplers

Directional couplers are passive four-port networks that couple a definite amount of power from one transmission line to another and sample the incident and reflected electromagnetic power in RF and microwave circuits. The sampling and coupling of a signal using a directional coupler can be carried out accurately, without causing any disturbance to the transmission line. 

Directional couplers are reciprocal networks formed by a mainline and secondary line.  A fraction of the input signal traveling in the mainline gets coupled to the secondary line (port 3). The amount of power coupled to port 3 depends on the coupling value of the coupler. Physical distance between the mainline and secondary line determines the coupling factor of the directional coupler. The closer the lines, the higher the power coupled to the secondary line. 

Directional Coupler Ports

(a) Single directional coupler (b) Dual directional coupler

According to the number of secondary lines, directional couplers can be classified as single or dual directional couplers. A single directional coupler is the most basic design, consisting of a mainline and a secondary line. A dual directional coupler can be expressed by two secondary lines and a single mainline. Additional classifications of the directional coupler include microstrip, co-axial, and waveguide couplers. 

Ports of the directional coupler are the ends of the mainline and secondary line. Ports of a directional coupler are named as follows:

The ports in a directional coupler are an input port, an output port, a coupled port, and an isolated port

Input port: This is where the incident electromagnetic signal leaves the device.

Output port: Also called the transmitted port. The mainline connects ports 1 and 2. 

Coupled port: Also called a forward coupled port, a  fraction of the incident input signal appears here.

Isolated port: Also called the reverse coupled port. Typically, for impedance matching, a 50-ohm resistor is connected to terminate the isolated port.

Specifications of a Directional Coupler

Some of the key specifications of a directional coupler are:

Directivity: Measures the ability of the coupler to isolate two signals that are traveling in opposite directions. Directivity is expressed in dB.

Standing wave ratio: The standing wave ratio (SWR) of a directional coupler is important to reduce mismatch errors. It is the measure of how well the directional coupler ports are matched to the given characteristic impedance. 

Coupling factor: The coupling factor is the ratio of incident power to the forward power, measured in dB.

Transmission loss: The total loss in the mainline of a directional coupler is given by transmission loss and is the sum of insertion loss and coupling loss. 

Bandwidth: Bandwidth is the frequency range over which performance of the directional coupler falls within a specific limit. 

Average power: Under matched load conditions, average power handling capability is addressed with one-way transmission through the main line of the directional coupler. 

Directional coupler functions are utilized in RF and microwave circuits to isolate, separate, or combine electromagnetic power. Directional couplers function as the main element in reflection and transmission measurements, power monitoring, and leveling applications. By properly designing directional couplers, any required amount of electromagnetic power can be coupled from one transmission line to another.

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