Types and Applications of Directional Couplers

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Types and Applications of Directional Couplers [Copy link]

A directional coupler is a very useful passive RF device that extracts a small portion of the energy from the main transmission path and directs it to one or more coupled ports. Since it is more beneficial when the coupled ports have high isolation from the main transmission path, the isolation between the ports of a directional coupler is usually high. There are two main types of directional couplers: standard directional couplers with one coupled port and one terminated port; and dual directional couplers with forward and reverse coupled ports. In addition, there are other types of dual directional couplers, which are called forward couplers and reverse couplers according to the type of coupled port that is coupled to the forward or reverse port.

Schematic diagram of common directional coupler

An important point to note is that the amount of coupling provided by a directional coupler has a direct impact on the theoretical minimum insertion loss of the main transmission path. The less coupling a port has, the lower the insertion loss. Typically, the power level rating of the coupled port is lower than the power level rating of the main transmission path, and failure may occur when the difference between the main transmission path power and the coupling strength exceeds the power handling capability of the coupled port. In general, a three-port directional coupler with precision internal matching terminations has higher directivity than a four-port directional coupler with external terminations.
Another factor to consider is the type of termination at the directional coupler's end ports. If the termination resistor is set equal to the inherent impedance of the transmission line (usually 50 ohms), the energy at that termination port can be absorbed with minimal reflection. However, when the termination port is shorted or open, or does not match the characteristic impedance of the transmission line, the energy at that port will be reflected back into the main transmission path. In addition, failure may occur when the power at the termination port exceeds the power limit of the terminator. This becomes particularly bad when the matched termination port fails and becomes a reflective load, which can cause damaging power levels in the main transmission path.
Directional couplers are often used in test and measurement applications. One example is to measure the input power and reflected power of a transmission line by using a dual directional coupler or by performing multiple tests with a directional coupler. This can be used as a measure of the voltage standing wave ratio after the losses of the coupler itself are removed. Other uses include signal sampling, signal injection, and power flow monitoring, where the user must also consider the losses of the directional coupler itself for the best accuracy.
When making precise measurements, the isolation between the ports also needs to be considered based on the quality of the directional coupler. No matter how the coupling is done, there is usually some degree of leakage between the coupler ports. This leakage is usually called isolation and is a measure of how well the coupler is designed to prevent leakage. The directivity of a directional coupler is the ratio of isolation to coupling coefficient and is a common performance specification for couplers. As
with most RF/microwave devices, the exact values of the device parameters are not absolutely consistent at different frequencies. The coupling coefficient, insertion loss, directivity, isolation, etc. mentioned above are usually frequency factors. When making high-sensitivity measurements, the above factors and all manufacturing tolerances must be taken into account. In addition, directional couplers have an operating bandwidth parameter. When designing, trade-offs need to be made between the parameters mentioned above, so the optimal design of the coupler ultimately depends on its application.
Most directional couplers do not allow DC current to pass through because the ports are DC grounded, but only some directional couplers allow DC current to pass through. For directional couplers that allow DC current to pass, it is important to keep the current below the rated value to prevent resistive losses from causing heating or affecting the termination performance. In order to meet the target performance, all ports of the dual directional coupler (or bidirectional coupler) must be grounded. In addition, it is important that the ground quality and the connected load match the port impedance of the directional coupler.
90-degree or 180-degree bridges are also commonly referred to as "couplers." Although the physical design of these devices often looks very similar to directional couplers, their operation is fundamentally different from that of directional couplers. However, since these devices split power between the output and coupled ports (3dB split), they can cause damage if they are mistaken for directional couplers with very low coupling coefficients.