Directional Coupler Do’s and Don’ts
Directional couplers are extremely useful passive RF components capable of extracting a small portion of the energy from the main transmission path, and redirecting it to one, or more, coupled ports. As isolation from the coupled ports to the main transmission path is desirable, directional couplers typically have high isolation among the ports. There are two main types of directional couplers, a standard directional coupler with a single coupled port and a terminated port, and a bi-directional coupler with a forward and reverse coupled port. Variations of bi-directional couplers exist, called forward coupler and reverse coupler, which impact which of the coupled ports is coupled to the forward or reverse port.
It is important to note that the amount of coupling offered by a directional coupler directly impacts the theoretical minimum of the insertion loss of the main transmission path. The less coupled the ports, the lower the insertion loss. It is common for the coupled port to be rated at a lower power level than the main transmission path, and if the main transmission path power minus the couple strength exceeds the coupled ports power handling ability, failures can occur. Generally, a 3-port directional coupler with a precision internally matched termination will allow for higher directivity than a 4-port directional coupler with an external termination.
Another factor to consider is the type of termination at the terminated port of a directional coupler. If the termination is set to the intrinsic impedance of the transmission line, often 50 ohms, then the energy in that terminated port should be absorbed with minimum reflections. However, if the terminated port is a short, open, or a mismatch to the characteristic impedance of the transmission line, then the power at that port will be reflected back to the main transmission path. Moreover, if the terminated port power exceeds the power limits of the terminator, failures can occur. This could be especially bad if a matched terminated port fails becomes a reflective load, leading to damaging power levels on the main transmission path.
Directional couplers are commonly used in test and measurement applications. An example of this, is to use a bi-directional coupler, or perform multiple tests with a directional coupler, and measure the incident and reflected power through a transmission line. This will provide a measure of the VSWR, minus the losses associated with the coupler itself. Signal sampling, injection, and power flow monitoring, are other example applications, where a user must also account for the losses associated with the directional coupler for optimum accuracy.
Depending on the quality of the directional coupler, the isolation between the ports may also need to be considered when conducting accurate measurements. There is often some leakage between the ports, not associated with the intended coupling. This quantity is often referred to as isolation, which is a measure of how well the coupler design prevents leakage. The directivity of a directional coupler is a ratio of the isolation to the coupling factor, which is a common figure of merit for couplers.
As with most RF/microwave components, the exact measure of the parameters of a device are not absolutely consistent over frequency. The specified coupling factor, insertion loss, directivity, isolation, and etc. are usually all factors of frequency. This may need to be considered, along with any manufacturing tolerances, when conducting sensitive measurements. Directional couplers also have a bandwidth in which they operate, and there are design tradeoffs among the mentioned parameters, so the best coupler design ultimately depends upon the application.
There are some directional couplers which enable DC current to pass through. Many do not, as the ports are at DC ground. For those that do pass DC current, it is important to keep the current below the rated limit, as the resistive losses could cause heating, or affect the performance of the termination. Also, grounding all ports of a dual-directional coupler, or bi-directional coupler, is necessary to meet specified performance. The quality of the grounding and connection load is also important match with the port impedance of the directional coupler.
Hybrid couplers, either 90 degree or 180 degree hybrids, are also commonly referred to as “couplers”. These components intrinsically operate differently than directional couplers, though they often look similar in physical design. However, these devices provide a power splitting, 3dB split, between the output and coupled port, and could cause damage if confused for a directional coupler with much lower coupling factor.