Directional Couplers

The directional couplers offered by Mini-Circuits are designed for both commercial and military applications. All models are available in plug-in, flat-pack, surface mount, and connectorized SMA, BNC, Type N, TNC (option) packages. All units are designed and built to meet the environmental requirements of MIL-C-15370 and are manufactured to ensure statistical control of important parameters such as insertion loss and input/output return loss. Especially designed for high power applications, the H series of directional couplers can operate with up to 25W of RF input.

Mini-Circuits directional couplers are reactive devices featuring very low insertion loss. Most models have three ports, and are manufactured with an internal 50Ω termination. In these cases, power coupled from any power incident to the output port (the reflected power) is absorbed and not available to the user. However, all 4-port (bi-directional) models have both the incident and reflected coupled power available. Examples are the ZFDC-20-1H and the BD-suffix models.

The basic function of a directional coupler is to operate on an input so that two output signals are available. However, when the input is applied to the opposite port of an internally terminated coupler, only one output signal is produced.

Directional Coupler Characteristics

1. The output signals are unequal in amplitude. The larger signal is at the main-line output port. The smaller signal is at the coupled port.
2. The main-line insertion loss depends upon the signal level at the coupled port, as determined by design. The relationship is as follows:
3. There is high isolation between the coupled port and the output of the main-line. A schematic representation of the coupler is as follows:
4. The arrows show signal flow – key characteristics of a directional coupler include coupling coefficient, coupling flatness, main-line loss and directivity, defined on the next page. Mini-Circuits’ full line of directional couplers, spanning 5 kHz to 2 GHz, provide excellent performance. They feature:
   1. Flat coupling over a broad bandwidth.
   2. Low main-line loss, as low as 0.1 dB.
   3. Directivity as high as 55 dB.
   4. Wide range of coupling values, from 6 dB to 30 dB.

Directional Coupler Applications

The high performance characteristics of these units enable the following signal processing functions to be accomplished:

• Measure incident and reflected power to determine VSWR.
• Signal sampling.
• Signal injection.
• Signal generator / oscillator leveling.
• Power flow monitoring.

The following diagram shows how a high-directivity coupler is connected to sample reflected power from a load, which is connected to the setup after a fully-reflecting termination has been used to obtain a 0 dB return-loss reference.

The coupler is connected so that its coupled port samples power reflected from the load. It can thus be used to monitor fault conditions causing high VSWR, or to align the load device for low VSWR. A coupler used in this manner is called a reflectometer. A leveling application is illustrated below.

The coupler is connected between a signal generator and a load, such that the coupled port samples power coming from the generator. It can thus monitor generator output independent of load conditions. The coupled-port signal can be used to level the generator output. Since generator output-power variation caused by load mismatch is suppressed by this feedback technique, the generator-coupler combination will have a source VSWR which is much better than that of the generator if it were not leveled.

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