How directional couplers work

What is a directional coupler? What is its function? A directional coupler is a general microwave/millimeter wave component that can be used for signal isolation, separation and mixing, such as power monitoring, source output power amplitude stabilization, and signal source isolation. , frequency sweep test of transmission and reflection, etc. The main technical indicators include directivity, standing wave ratio, coupling degree and insertion loss.

Directional couplers are valuable passive RF devices that extract a small portion of energy from the main transmission path and direct it to one or more coupling ports. The isolation between directional coupler ports is usually high since it is advantageous to have high isolation between the coupling port and the main transmission path. Currently, there are two main types of directional couplers: standard directional couplers with one coupling port and one termination port; and dual directional couplers with forward and reverse coupling ports. In addition, there are other types of dual directional couplers, which are called forward couplers and reverse couplers depending on the kind of coupling port that is coupled to the forward or reverse port.

An important point to note is that the amount of coupling provided by a directional coupler has a direct impact on the theoretical minimum of the main transmission path insertion loss. The smaller the coupling amount of the port, the lower the insertion loss. Usually, the rated power level of the coupling port is lower than the rated power level of the main transmission path. When the difference between the power of the main transmission path and the coupling strength exceeds the power handling capability of the coupling port, a failure may occur. In general, a three-port directional coupler with precision internally matched terminations has higher directivity than a four-port directional coupler with external terminations.

Another factor to consider is the type of termination of the directional coupler termination port. If the termination resistor is set equal to the inherent impedance of the transmission line (typically 50 ohms), the energy at the terminated port can be absorbed with a minimal amount of reflection. However, when the termination port is short-circuited or open-circuited, or does not match the characteristic impedance of the transmission line, the energy at that port will be reflected back to the main transmission path. In addition, when the power of the terminated port exceeds the power limit of the terminator, failure may occur. This situation becomes especially bad when a terminated port that has been matched fails and becomes a reflective load, which can cause damaging power levels within the main transmission path.

Directional couplers are commonly used in test and measurement applications. An example of this would be to measure the input power and reflected power of a transmission line by using dual directional couplers, or by performing multiple tests with directional couplers. This serves as a measure of the voltage standing wave ratio after removing the losses in the coupler itself. Other uses include, for example, signal sampling, signal injection, and power flux monitoring, where for optimal accuracy the user must also consider the losses in the directional coupler itself.

When performing accurate measurements, the isolation between ports also needs to be considered based on the quality of the directional coupler. No matter how coupled, there is usually some degree of leakage between the coupler ports. This amount of leakage is often called isolation and is a measure of how leakproof a coupler design is. The directivity of a directional coupler is the ratio of isolation to coupling coefficient, which is a common performance indicator of the coupler.

As with most RF/microwave devices, the exact values ​​of device parameters are not absolutely consistent across frequencies. The above-mentioned coupling coefficient, insertion loss, directivity, isolation, etc. are usually frequency factors. When making high-sensitivity measurements, each of the above factors, as well as all manufacturing tolerances, must be taken into account. In addition, directional couplers also have operating bandwidth as a parameter. During design, 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 because the port is DC grounded. Only some directional couplers allow DC current to pass. For directional couplers that allow DC current to pass through, it is important to keep the current below the rated value to prevent resistive losses from causing heating or affecting termination performance. To meet target performance, all ports of a dual directional coupler (or bidirectional coupler) must be connected to ground. Additionally, it is important that the ground quality and 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, the way they operate is fundamentally different. However, since such devices split power between the output and coupling ports (3dB split), there is a potential for damage when mistaken for a directional coupler with a very low coupling coefficient. The above is the working principle of directional coupler, I hope it can help you.