RF Switch
An RF switch routes a single antenna or shared signal port to one of multiple alternate paths, usually controlled by a CMOS-level digital input. Common parts: Skyworks SKY13355-378LF SPDT, Qorvo QPC1010 SP3T, Peregrine PE42442 50 Ω 60 GHz SOI switch, Analog Devices HMC641A SPDT.
Solid-state RF switches use controlled conducting paths and, in many designs, shunt paths. ON resistance contributes insertion loss; OFF-state capacitance allows leakage that limits isolation. An absorptive switch terminates unused ports, while a reflective switch does not provide the same matched termination.
In plain terms
A railway track switch at a junction: a single train (the antenna signal) is steered down one of several tracks (TX or RX paths), and the switch must do it cleanly without spilling cars onto the wrong line (low isolation between paths).
Why designers use it
- Switch a single cellular antenna between TX and RX paths in TDD bands.
- Select between multiple LO sources, antenna ports, or filter banks under digital control.
- Bypass the low-noise amplifier on strong signals to prevent compression.
- Multiplex one ADC or signal generator across a bank of test points in measurement instruments.
Best for
- TDD radios
- Antenna selection
- RF test gear
Key specifications
- Insertion loss: Loss through the selected path
- Isolation: Leakage into an unselected path
- Termination: Absorptive or reflective behaviour
- Power and timing: Check operating power, switching conditions and settling separately
When not to use it
- When the highest possible IIP3 is required and switching speed doesn't matter — PIN-diode switches still beat FET switches in 1-dB compression at hundreds of MHz.
- DC-coupled paths where the switch's bias scheme would interfere — most RF switches AC-couple through internal blocking caps that limit low-frequency operation.
Common mistakes
- Assuming OFF means perfect isolation at every frequency.
- Choosing a reflective switch where the rest of the circuit requires a matched unused port.
- Using compression or linearity figures as maximum operating power.
- Ignoring switching sequence, termination power and the exact control truth table.
Where you will find it
- A test fixture can switch between RF measurement paths, with each path characterized for loss and isolation.
- A radio can route a signal between filters or antennas, provided the chosen switch supports the frequency, power and required off-state behaviour.
A short history
An RF switch selects a signal path. Reflective designs leave unused ports unterminated, while absorptive designs terminate them to reduce reflections. In a solid-state switch, ON resistance contributes to insertion loss and OFF-state capacitance limits isolation. These trade-offs matter when routing signals between antennas or test-system ports.
Good to know
- Isolation generally changes with frequency.
- The best off-state termination depends on the surrounding RF system.