How to select an RF Coupler
An RF coupler is a passive device that is used to sample high frequency signals. A microwave coupler takes one signal as the input and provides two outputs – One being the regular output and the other the coupled output. Based on the required application the power level of the coupled signal can be varied when designing the coupler.
Microwave and rf couplers have many applications and are used for sampling signals, signal injection, used in signal generators, used to measure incident/reflected power to determine VSWR and in a number of other applications.
The key specifications to look for in an rf or microwave coupler are:
Type of Couplers: There two main types of couplers:
· Directional Coupler or Unidirectional Coupler: This is a four port device - 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 or Bi 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.
Frequency (MHz): This is the frequency range over which the coupler 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 a coupler can handle while maintaining minimal reflections i.e with a matched load.
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.
Package Type: RF Couplers are 4 port devices available with various connector options – SMA, N Type, BNC etc.
