ENGINEERING ANSWER
An impedance coupon is useful when it represents the production construction and a defined measurement method. Its geometry, copper, dielectric spacing and processing should reflect the lines whose impedance it is meant to verify.
What to decide before release
Agree target, tolerance, sampling and reporting with the supplier. Decide whether time-domain testing, RF measurements or both are needed. A coupon located on the panel is evidence for a process, not a complete test of every connector and transition.
Checks to put in your design brief
- Match coupon and circuit construction
- Define test method and sampling
- Retain reports by lot
Important distinction
Passing a coupon does not establish the return loss of a finished RF assembly.
Sources & basis
Material facts follow the cited source. Design checks are preliminary guidance, not supplier process instructions.