Switch
Understanding RF Switches
RF switches are found in virtually every communication system. A cellular phone uses switches for antenna selection, band switching, and TDD transmit/receive switching. Test systems use switch matrices to route signals between instruments and DUTs.
Switch Types
- PIN diode: Fast switching (1-100 ns), moderate isolation (20-40 dB), handles high power. Used in TR switches and attenuators.
- FET (GaAs, SOI-CMOS): Very fast (< 10 ns), good linearity, low power consumption. Dominant in cellular front-end modules.
- MEMS: Micro-electromechanical switches. Excellent isolation, very low insertion loss. Slower switching (1-10 us).
- Electromechanical (relay): Best isolation (60-90 dB), lowest insertion loss (< 0.1 dB), handles high power. Slow (ms). Used in test systems.
Key Specifications
- Insertion loss: 0.1-2 dB depending on type and frequency.
- Isolation: 20-90 dB depending on type.
- Switching speed: 1 ns to 10 ms depending on technology.
- IP3: Linearity under multi-signal conditions.
Frequently Asked Questions
What is an RF switch?
An RF switch selects between signal paths, routing RF energy from one port to another. It enables band switching, T/R switching for TDD systems, test system routing, and antenna selection. Types include semiconductor (PIN, FET), MEMS, and electromechanical.
What type of switch is best?
It depends on the application. FET switches dominate mobile devices (fast, small, low power). PIN diode switches handle high power. MEMS switches offer the best combination of isolation and loss. Electromechanical relays provide the best overall performance but are slow and large.
What is the difference between absorptive and reflective switches?
A reflective switch presents a short or open circuit when off, reflecting incident power. An absorptive switch presents a matched load when off, absorbing the signal. Absorptive switches provide better system VSWR and prevent reflections from causing problems.