COUPLERS

Couplers in the RF and Microwave industry are essentially special purpose power splitters and fall into two main categories: directional and hybrid. Directional Couplers split RF power but into unequal levels. Directional Couplers generally have a main through path with minimum insertion loss and a coupled path with a significantly reduced power level. Typical coupled levels are 6, 10, 20 or 30 dB meaning the coupled port is 6dB, 10dB, 20dB or 30dB lower in amplitude than the main through path signal. Hybrid Couplers are four port 3dB couplers since they split incoming signals into equal amplitude outputs. Hybrid Couplers come in the form of 90° and 180° devices, each having an assortment of unique applications related to the internal phase shift applied to one or more of the ports.

The directional coupler takes on the basic form shown below:

The term directional comes from the coupling technique used for these devices which has significantly different coupling rates for signal flowing into or out of a given port. Power entering the INPUT port will exit the OUTPUT port at almost the same level and will also be coupled to the COUPLED port but will be reduced in power by the coupling ratio (6, 10, 20 30 dB etc.) Signals flow or propagate in the directions indicated. Conversely, power entering in the opposite direction at the OUTPUT port will be coupled primarily into the internal 50 ohm termination and, to less extend, into the Coupled port. This is in reverse of the direction indicated in the above figure. The difference in power at the coupled port from signals in each direction is the directivity of the coupler. The level of coupling is mainly determined by the distance between the main and coupled lines and is fixed as part of the design process.

Hybrid Couplers are four port 3dB couplers since they split incoming signals into equal amplitude outputs. Hybrid Couplers come in the form of 90° and 180° devices and are commonly depicted as shown below, where the four ports are labeled A through D:

The phase indications of -90º and 180° indicate the phase shift that occurs to signals passing through these paths. Thus a signal applied to port A on the 90º Hybrid will be split evenly between port C and D but the signal appearing at the D output port will be shifted in phase by -90º. If ports C and D are properly terminated, no signal will appear at port B, which is considered the isolated port. Ninety degree hybrid couplers are also called quadrature hybrids since the signals at C and D are in quadrature meaning they are in different quadrants, spaced apart by 90º. The 180° hybrid coupler behaves in a similar manner but can be used in different applications due to the difference in phase shift. The links provided at the beginning of this article provide a wealth of application ideas and more in-depth descriptions of how these devices work.