Explain what a pin diode switch or transmission line switch does in RF front ends.

• A. It reduces the transmitter output
• B. It modulates the RF signal
• C. It broadens the bandwidth
• D. It switches the RF path for RX/TX with good isolation

Answer: (D)

Routing the RF path with switch devices is what PIN diode switches and transmission line switches do in RF front ends. Their job is to connect the antenna to either the receive chain or the transmit chain and to keep the two paths isolated from each other when the other is active. In transmit mode, PIN diodes are biased to conduct, creating a low-loss path to the antenna. In receive mode, they are biased to block, providing high impedance and preventing the transmit energy from leaking into the receiver, protecting the receiver and preserving sensitivity. Transmission line switches do the same by reconfiguring the physical routing of the RF energy along the lines. The key idea is to switch the RF path with good isolation between RX and TX so the transmitter can drive the antenna without desensing or damaging the receiver.