RF couplers are circuits that sample RF transmissions by combining (coupling) signals asymmetrically. There three basic types of devices: unidirectional, bidirectional, and dual directional. Unidirectional RF couplers are four-port devices with a main input and output line, a coupled line with a coupled output, and an internal termination on the fourth port. The power passing from the input to the output is combined with the coupled output, but the coupled output is isolated from the main output. Any reflected power from the main line output is coupled to the termination. Bidirectional RF couplers are similar to unidirectional couplers, but do not provide termination on the fourth port. Main line power is coupled to the forward output of secondary line and reflected power is coupled to the reflected output. For isolation to be achieved, coupled outputs must be obtained through well-matched terminations at each port. Dual directional RF couplers are four-port devices that consist of two unidirectional couplers. They can be connected back-to-back in series, with the main line output of the forward coupler connected to the output of the second coupler; or integrated into one device with a single main line and two secondary lines. Integration in a single device provides several advantages, including a shorter or more compact unit. In addition, because there is only one main line, insertion loss is reduced and high isolation is more easily attained.
Selecting RF Couplers
Selecting RF couplers requires an analysis of performance specifications. Continuous wave (CW) average power measures one-way transmissions through the main line under matched load conditions. Coupling is the ratio of the power measured at the output port relative to that measured at the forward coupled port. Directivity is the ratio of the power measured at the reverse coupled port relative to that measured at the forward coupled port when the output port is ideally terminated. Insertion loss is the total RF power transmission loss resulting from the insertion of a device in a transmission line. It is defined as the ratio of signal power at the output of the inserted device to the signal power at the input of the inserted device. Coupling, directivity, and insertion loss are all measured in decibels (dB). Voltage standing wave ratio (VSWR) is a unitless ratio ranging from 1 to infinity that expresses the amount of reflected energy at the input of the device. A value of 1 indicates that all of the energy passes through. Any other value indicates that a portion of the energy is reflected. Other performance specifications for RF couplers include frequency range, return loss, and reflected power.
There are several mounting styles for RF couplers. Surface mount technology (SMT) adds components to a printed circuit board (PCB) by soldering component leads or terminals to the top surface of the board. Through hole technology (THT) mounts components by inserting component leads through holes in the board and then soldering the leads in place on the opposite side of the board. Flat pack (FPAK) devices have flat leads and are available in a variety of body sizes and pin counts. Connectorized devices attach with coaxial or other types of connectors. Waveguide assemblies consist of a hollow metallic conductor with a rectangular, elliptical, or cross-section. Some conductors contain solid or gaseous dielectric materials.