An RF switch is a device or module that switches RF signals. They are available in a number of technologies including mechanical, solid-state and micro-electro-mechanical systems (MEMs). Important characteristics of an RF switch include frequency band, high port isolation in OFF states, low insertion loss between conducting ports in the ON state, good return loss (match) to the characteristic impedance the switch is connected to and high isolation to the actuating mechanism. Characteristics like switching speed, power handling ability and the guaranteed number of switching cycles are also important. RF switches are available in a number of configurations including: single pole double throw (SPDT), multiport (SPnT), double pole double throw (DPDT) and bypass switches. Bypass switches add or remove a component from the RF signal path.

Mechanical RF switches utilize contact pressure caused by an actuator moved by electromagnetic induction of a coil energized by a controlling current. It is common to provide mechanical RF switches with a wetting current on the RF signal path – a small DC current that facilitates microscopic oxide layer breakdown between conductor contacts to maintain low insertion loss. They have the advantage of working from DC, conduct high power levels and have high immunity to ESD. Disadvantages include sensitivity to vibration and switch cycling limits due to mechanical wear. They also have relatively slow switching speed and can exhibit long settling times due to contact bounce.

Solid state RF switches use MOSFET, transistor or PIN diode topologies to actively switch the RF path from low to high impedance. They are band-pass, not conducting DC. Solid state switches can support effectively infinite switching cycles and have fast switching speed. PIN diode RF switches exhibit a transition from low to high impedance that can be utilized for shape switching. Shaping the switching waveforms improves the spectral spillage caused by convolving the RF carrier signal by the gate. Rapid switching in RF systems can also cause sudden load changes that can pass back through power amplifier and exciter stages of a transmitter and pull the VCO momentarily off frequency – exacerbating potential noise issues. Shaping the switch-over by using a shape like a raised cosine reduces the interference caused by rapid switching and helps rapidly switching transmitters to stay within regulations required for systems that require time division multiplex (TDM) or cyclic keying.

RF MEMs switches provide the benefits of a mechanical switch whilst not having issues with wetting currents and providing an actuating drive current to the induction coil controlling the switch. They utilize electrostatic forces between capacitor plates to open or close the mechanical contacts in the RF path or micro thermo-mechanical switches. Electrostatic RF MEMs switches have near zero power consumption, extremely high frequency bandwidths, high switch cycles, low insertion loss and can offer excellent isolation between ports. Their main disadvantage is low power handling capability but also have issues with requiring high actuation voltages to reliable operation. Thermo-mechanical switch devices can operate at lower actuation voltages but require high actuation currents.