Definition
Isolation is the attenuation between RF paths that should be disconnected. It is normally expressed in dB. Higher isolation means less unwanted signal couples from the active path into the isolated path.
Why it matters
Poor isolation can allow a transmitter to leak into a receiver, a test stimulus to contaminate another channel, or one signal path to interfere with another. High-dynamic-range systems often need isolation well beyond what is obvious from nominal signal levels.
Frequency dependence
Isolation generally changes with frequency because parasitic capacitance, coupling and the physical dimensions of the RF structure become increasingly significant. Always evaluate the minimum isolation across the full required band.
Port-to-port versus path isolation
The relevant measurement depends on switch topology and state. Engineers should identify which ports are active, which are isolated and the exact signal path that represents the worst-case system vulnerability.
Cascaded isolation
Adding switches or attenuating structures can improve system isolation, but dB values should only be combined when the physical paths and leakage mechanisms justify it. Package-to-package coupling and cable routing can limit the isolation achieved in the assembled system.
Practical verification
For demanding systems, measure isolation in the final fixture with production cables, connectors, grounding and neighboring RF hardware. A high-performance switch cannot compensate for poor external shielding or coupling.
Key takeaway
Isolation is a frequency-dependent measure of unwanted coupling into paths that should be disconnected.