A RF switch is a basic building block of many RF circuits used in communication, sensing, and test & measurement applications. RF switches are used whenever a signal path needs to be optionally opened/closed, or when specific signal paths need to be selected. An example of this is in a radar RF Front End (RFEE) when the path to the antenna needs to be switched between the transmitter during transmission and the receiver during reception and it is undesirable to have the transmission signal energy directed to the receiver. In this example, the RF switch also provides some isolation between the transmitter and receiver in the radar.
What Does an RF Switch Do?
At its most basic, an RF switch directs or opens/closes a RF signal path one or more RF signal paths. Depending on the type of switch, a single activation event could toggle many signal paths or open/close many signal paths. The number of signal paths controlled by a RF switch is generally described as a “pole” in RF terminology, where the “throw” of a switch is the number of signal paths that can be connected by the switch.
For instance, a single-pole single-throw (SPST) switch connects a single signal path to another single signal path in the close position and the open position disconnects the signal path. A single-pole double-throw (SPDT) switch will connect a single signal path to one of two optional signal paths.
The previous example of the radar antenna switch is a SPDT, where the antenna port can be toggled between the transmitter or receiver signal path. Extending this example would be a single-pole triple-throw (SP3T), where the antenna path could be connected to either the transmitter, receiver, or an RF terminator.
There are also RF switches with multiple poles. For instance, a double-pole single-throw (DPST) switch connects two different signal paths to two other different signal paths in the closed position and disconnects both signal paths in the open position. A more complex switch, such as a double-pole double-throw (DPDT) switch will toggle two different signal paths between two sets of two other signal paths.
What Is Isolation in An RF Switch?
RF switch isolation is the extent at which RF energy at one port is prevented from being coupled into the other ports. For instance, when an SPDT switch is toggled to position 1, the isolation of this switch is how little RF energy from the signal path opened with position 1 is coupled/leaked into the signal path opened by position 2. With more complicated switch types, the isolation between each port combination is important. Isolation is not to be confused with insertion loss, which is the attenuation along a signal path when closed.
Are RF Switches Bidirectional?
When an RF switch path is open, signals can travel in either the forward or reverse direction, hence they are inherently bidirectional.