RF Coupler Basics & Selection
RF & Microwave couplers are passive devices that are used to sample high frequency signals. It takes one signal as the input and provides two outputs – One being the regular output and the other being the coupled output. Based on the required application, the power level of the coupled signal can be varied when designing the device. Couplers have many applications and are used for sampling signals, signal injection, signal generation, used to measure incident/reflected power to determine VSWR and have a number of other applications.
Types of RF Couplers:
- Directional Coupler: This is a four port device – with an input and output port, a port for the coupled signal and a port that is terminated internally. The power that is passing from the input to the output is coupled with a coupled line and passes out through the coupled output port, which is isolated from the main output port. Any reflected power from the main output is coupled to the port that is terminated, resulting in minimal reflections within the device/system.
- Dual Directional Coupler: This is a four port device where two directional couplers are connected in series, in opposing directions or a directional coupler with a single main line and two secondary lines for coupling.
- 90° Hybrid Coupler: 90 degree hybrid couplers split the input signal in to two equal signals with a 90 degree phase differential.
- 180° Hybrid Coupler: 180 Degree Hybrid couplers split the input signal in to two euqal signals with a 180 degree phase differential.
Frequency (MHz): This is the frequency range over which the device can operate while providing a constant level of coupling with minimal loss and reflection.
Coupling (dB): This is the ratio of incident power fed to the main port to the power at the output of the coupled port.
Coupling Variation (dB): The maximum peak-to-peak variation in coupling expected over the specified frequency range.
Insertion Loss (dB): The reduction in output power due to the power transferred to the coupled line.
Directivity (dB): The difference between the power level of the coupled and isolated ports. The directivity is a measure of independence of the coupled and isolated ports.
Average Power (W): The level of power that the device can handle while maintaining its electrical characteristics.
Impedance (Ohms): The impedance of a coupler must match the circuit/system it is connected to; this will minimize any reflection within the circuit/system. Usually the impedance is 50 Ohms or 75 Ohms.