How does a blocking switch achieve high isolation?

A single series switch element provides 20-30 dB isolation. Adding a shunt element to ground in the off state adds another 20-30 dB. Cascading multiple series-shunt stages achieves 60-90 dB. Each stage uses a quarter-wave transformer for impedance matching. PIN diodes provide the highest isolation; GaAs FETs offer faster switching.

Where are blocking switches used?

Transmit/receive (T/R) switches in radar (protecting the receiver from the transmitter pulse), test and measurement switching matrices, antenna routing in phased arrays, and time-division duplex (TDD) front-ends. Any application where signal leakage in the off state would damage components or corrupt measurements.

Insertion loss vs isolation trade-off?

Adding more switching stages increases isolation but also increases insertion loss. A single series PIN diode: 0.3 dB loss, 25 dB isolation. Series-shunt: 0.5 dB loss, 50 dB isolation. Double series-shunt: 0.8 dB loss, 70+ dB isolation. The application determines the acceptable trade-off.

Passive Components

Blocking Switch

A Blocking Switch is an RF switch designed for maximum isolation in the off state, typically achieving 60-90 dB by cascading series-shunt PIN diode or FET stages. In the off state, series elements open-circuit the signal path while shunt elements short remaining leakage to ground. Used in T/R switches (radar receiver protection), switching matrices (test equipment), and TDD front-ends where signal leakage could damage receivers or corrupt measurements.

Category: Passive Components

Isolation: 60-90 dB

Understanding Blocking Switches

A simple SPST switch uses one series element: in the on state, the element is low impedance (passing the signal); in the off state, high impedance (blocking). Adding a shunt element after the series element creates a voltage divider in the off state, dumping residual leakage to ground.

Quarter-wave transmission line sections between stages provide impedance transformation, optimizing the isolation bandwidth. PIN diodes are preferred for high-power applications (100+ W) due to their power handling. GaAs/GaN FET switches offer nanosecond switching speed but lower power handling.

Isolation by Topology

Series only: 20-30 dB
Series-shunt: 40-50 dB
Series-shunt-series: 55-70 dB
Double series-shunt: 70-90 dB

IL increases: +0.2-0.3 dB per stage

Switch Technology Comparison

Technology	Isolation	IL	Speed	Power
PIN diode	60-90 dB	0.3-1.0 dB	100 ns	100+ W
GaAs FET	40-60 dB	0.5-1.5 dB	5 ns	1-5 W
GaN FET	40-50 dB	0.3-0.8 dB	10 ns	10-50 W
MEMS	50-70 dB	0.1-0.3 dB	10 μs	1-5 W

Common Questions

Frequently Asked Questions

How achieve high isolation?

Series-shunt cascades: series opens path, shunt dumps leakage. Each stage adds 20-30 dB. Quarter-wave sections optimize bandwidth.

Applications?

T/R switches (radar), test switching matrices, TDD front-ends, phased array routing. Anywhere leakage would damage or corrupt.

IL vs isolation?

More stages = more isolation + more loss. Single series: 0.3 dB/25 dB. Double series-shunt: 0.8 dB/70+ dB. Application drives trade-off.

Related Terms

← Blocking Blown Fiber →

RF Switching

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