Why use directional coupler

Their ability to sample either the forward or reverse direction of signal propagation allows a wide range of applications in test, measurement, monitoring, feedback and control. This note should help system designers understand the function, architecture and performance of the coupler, to select a suitable type for their particular application.

Find the right directional, bi-directional or dual directional coupler for your application from the hundreds in Mini-Circuits catalog. You must be logged in to post a comment. Return to MiniCircuits. Urvashi Sengal Applications Engineer, Mini-Circuits Directional couplers are an important type of signal processing device. Definitions Ideally, a coupler would be lossless , matched and reciprocal. Figure 2: Ohm Directional Coupler Advantages: Performance can be optimized for the forward path High directivity and isolation The directivity of a coupler is strongly affected by the impedance match provided by the termination at the isolated port.

Furnishing that termination internally ensures high performance. Disadvantages: Coupling is only available on the forward path No coupled line The coupled port power rating is less than the input port because the power applied to the coupled port is almost entirely dissipated in the internal termination. Advantages: Symmetric design Input and output ports are interchangeable There are two transmission lines.

Coupled line works the same as the mainline It has forward and reverse coupling. Disadvantages: Design is critical to maintaining good performance in both directions.

The directivity of the coupler depends on how well the isolated port is terminated. Advantages: Performance can be optimized for both forward and reverse paths Higher directivity and isolation can be achieved Provides forward and reverse coupling Directivity of one path is not affected by mismatch present on the other path Can also be used to simultaneously monitor both the forward and reverse power of a system.

Disadvantages: Usually involves two back-to-back directional couplers Larger size compared to directional and bi-directional couplers No coupled line is present not accessible at both ends Higher insertion loss than the single directional and bi-directional coupler.

There are different types of directional couplers like single, dual directional, coaxial, waveguide and even combination types. By: Justin Stoltzfus Contributor, Reviewer. By: Satish Balakrishnan. Dictionary Dictionary Term of the Day.

Natural Language Processing. Techopedia Terms. Connect with us. Sign up. Term of the Day. The output and reverse ports are terminated in 50 ohms. It is always a good practice to terminate unused ports in 50 ohms assuming a ohm system. Referring again to Figure 1, the signal generator could be connected to the output port and the RF voltmeter to the reverse port and the results would be the same.

Again, unused ports are terminated in 50 ohms. Directivity is one of the most important characteristics or specifications of directional couplers. Directivity can best be defined by an example.

The setup in Figure 2 is the same as Figure 1, except the RF voltmeter is now connected to the reverse port and the forward port is terminated in 50 ohms.

The RF voltmeter now indicates much less than dBm. The signal level at the reverse port will be equal to the input signal level minus the coupling factor minus the directivity. If the coupling factor is 20 dB and directivity is 30 dB, the signal level at the reverse port will be equal to the input signal level 0 dBm minus 50 dB, or dBm. This assumes a perfect ohm match at the output port.

If the output port were left open unterminated , the RF voltmeter would indicate dBm because the signal power is completely reflected back into the output port and coupled with a 20dB loss to the reverse port. It is easy to see why directivity is such an important characteristic if the directional coupler is to be used to compare forward and reflected power, as it is in a directional wattmeter. To better understand how coupler directivity affects the accuracy of a wattmeter, refer to Figure 3.

The graph is based on a directional coupler having a directivity of 25 dB with a forward power of W at the input and a reflected power of 10 W at the output. A portion of the signal power from the reverse signal is mixed with the forward signal power and vice versa. Normally, the forward power reading is not significantly affected by the contaminant because the forward power is usually much higher than the reflected power. However, the reflected power reading can be seriously affected by the contaminant from the forward signal.

This is especially true at relatively low reflected power levels. To determine the effect of these contaminants, the power levels are converted to voltage levels in 50 ohms and then analyzed using phasors. Figure 4 shows the various voltages and the resultants. The voltage at A represents the voltage in the W forward signal. The voltage at E represents the voltage in the 10 W reflected signal. Most waveguide couplers couple in the forward direction as they rely on multiple coupling holes; a signal incident on port 1 will couple to port 3 port 4 is isolated in our clockwise notation.

Microstrip or stripline couplers are backward wave couplers because they rely on coupled lines: for a signal incident on port 1, port 4 is the coupled port port 3 is isolated in our clockwise notation.

The coupled port on a microstrip or stripline directional coupler is closest to the input port because it is a backward wave coupler. On a waveguide broadwall directional coupler, the coupled port is closest to the output port because it is a forward wave coupler. Forward couplers are in-phase couplers. Backward couplers couple in quadrature the coupled port phase is 90 degrees more negative than the direct port.

We attempt to explain why the phases are in quadrature on our coupled-line coupler page. The Narda coupler below is made in stripline you have to cut it apart to know that, but just trust us , which means it is a backward wave coupler. The input port is on the right, and the port facing up is the coupled port the opposite port is terminated with that weird cone-shaped thingy which voids the warrantee if you remove it.

Luckily Narda usually prints an arrow on the coupler to show how to use it, but the arrow is on the side that is hidden in the photo. On the waveguide coupler below, the input is on the left, while the coupled port is on the right, pointing toward your left ear. There is a termination built into the guide opposite the coupled port, although you can't see it.

Looking at the generic directional coupler symbol below, if port 1 is the incident port, port 2 is the through port because it is connected with a straight line. Port 3 is the coupled port, and port 4 is the isolated port.

For a signal incident on port 2, port 1 is the through port, port 4 is the coupled port and port 3 is the isolated port. Just follow the lines! The symbol below is for a forward coupler. Note: this paragraph was corrected in November thanks to Jim!

If you ever have any schematic questions, there is an IEEE standard that you can probably find with a google search:. Let's first look at some definitions using S-parameters. Let port 1 be the input port, port 2 be the "through" port, and let's assume we are talking about a forward wave coupler port 3 is the coupled port and port 4 is the isolated port in clockwise notation, thanks to Tuomo for pointing out an inconsistency, January ! Ideally, power into port 1 will only appear at ports 2 and 3, with no power at port 4, but in real couplers some power leaks to port 4.

For an incident signal at port 1 of power P1 and output powers P2, P3 and P4 at ports 2, 3 and 4 , then:. The equations are now in agreement with Pozar's Microwave Engineering , edition, page Pick up a copy from our book page! Note that these numbers are supposed to be positive in dB. Quite often, microwave engineers present these quantities as negative numbers, it is not a great faux pas, just look at the magnitude.