Why directional coupler

These effective permittivities are different, so determination of the optimum length of the coupler is not straightforward. The only choice is to use the average of the permittivities:. Finally, the widths of the feed lines must be determined. EM analysis is often used to refine this synthesized coupler. There are two main uncertainties in the design.

One is the uncertainty in the length of the coupler due to the different even- and odd-mode effective permittivities. The other uncertainty is that the coupled-line equations come from low-frequency analysis—EM analysis will capture the frequency-dependent effects.

However, only minor iteration would normally be required. Lange [8] introduced a coupler, now known as the Lange coupler, in In this design, true quadrature coupling over an octave bandwidth is realized as a consequence of the interdigital coupling section, which also compensates for the differences of the even- and odd-mode phase velocities over the wide frequency range. Note the use of the center bond wires—this was the key invention.

The bonding wires should look, electrically, as close as possible to a short-. In semiconductor technologies, these bond wires are replaced by air bridges, and in structures with two or more metal layers the wirebonds are replaced by vias to another metal layer and a short connection on the second metal layer.

Note that the input-to-direct-output link meanders through the structure and this DC connection identifies the through connection.

The physical length of the coupler is approximately one-quarter wavelength long at the center frequency of the coupling band. As with many distributed components, this element was invented using intuition and empirical iterations.

Since then, analytic design formulas have been developed to enable synthesis of the electrical parameters of the coupler see [5]. The synthesis is based on even- and odd-mode impedances analogous to those developed in Section 5.

Synthesis leads to a design that is close to ideal, and subsequent modeling in an EM simulator can be used to obtain an optimized design accounting for frequency-dependent and parasitic effects. Solution The electrical design of a directional coupler comes down to determining the evenand odd-mode characteristic impedances required.

Footnotes [1] This is shown in a detailed derivation provided in Section In an ideal directional coupler, no signal should appear at the isolated port, however practically, a small amount of power called back power is obtained at Port 4. The directional coupler as the name suggests operates in a single direction i. There are some other types of coupler configurations like Bi-Directional Couplers and Dual-Directional Couplers whose ports can be interchanged. Click here to find RF Couplers from the leading manufacturers.

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Go Ask a Question. Submit Cancel. What is a Directional Coupler? Can you answer this question? Editorial Team - everything RF Aug 14, Upvote Downvote Flag it Comments Tags Coupler.

More Calculators. What is Coupling Factor or Coupling Ratio? What is Directivity in a Coupler? What is a Lange Coupler? What is a Branchline Coupler? What is a Bi-Directional Coupler? What is a Dual Directional Coupler? What is a Notch Filter? What is the difference between a monopole and dipole antenna? The functional diagram in Fig. The top diagram is a 4-port coupler, which includes both coupled forward and isolated reverse, or reflected ports. The lower diagram is a 3-port structure, which eliminates the isolated port.

This is used in applications that only need a single forward coupled output. The 3-port coupler can be connected in the reverse direction, where the port that was formerly coupled becomes the isolated port:. Figure 1: Basic directional coupler configurations. This type of coupler has three accessible ports, as shown in Fig. The basic function of a directional coupler is to sample the isolated reverse signal. A typical application is measurement of reflected power or indirectly, VSWR. Although it can be connected in reverse, this type of coupler is not reciprocal.

Since one of the coupled ports is internally terminated, only one coupled signal is available. In the forward direction as shown , the coupled port samples the reverse wave, but if connected in the reverse direction RF Input on the right , the coupled port would be a sample of the forward wave, reduced by the coupling factor. With this connection, the device may be used as a sampler for signal measurement, or to deliver a portion of the output signal to feedback circuitry. This coupler type has four ports, all accessible for the customer to use.

It has a symmetric design, allowing forward and reverse signals to be sampled simultaneously. This third type of coupler is a combination of two 3-port couplers with their main lines cascaded, and their internally terminated ports facing each other at the interface between the couplers. This configuration provides bi-directional coupler action, but with independent use of the coupled ports. The primary advantage is that a mismatched load applied to either port will not affect the other.

When connected as shown in Fig. This allows measurement of reflected power, representing the degree of mismatch of the load.

When placed at the transmitter output, this configuration can monitor the VSWR of the antenna system, both for measurement and monitoring.

Figure 8: Schematic of a 3-port directional coupler in a simple reflectometer setup.