An RF measurement reference plane is the electrical boundary at which the corrected quantity is defined. A VNA calibration may establish correction at cable ends, while the project needs S-parameters at connectors, probe tips, package pads or an internal fixture boundary. State both the calibration plane and the reporting plane, then list every adapter, cable, bias tee, probe and fixture section between them.
De-embedding is required when the result must exclude a stable, linear and characterized network between those planes. Use measured or defensible fixture parameters with the correct orientation and port mapping. Retain the fixture file, method, frequency range, connector torque, temperature and verification result. If the project accepts the complete evaluation assembly, reporting the assembly result may be more honest than removing an uncertain fixture.
Do not de-embed a nonlinear, time-varying, poorly repeatable or inadequately characterized path. Reject a corrected result when it creates nonphysical gain, passivity or causality behavior without explanation, becomes extremely sensitive near fixture nulls, or cannot reproduce a check standard. Fixture removal also carries uncertainty; it is not a free accuracy improvement.