A device comprises a planar substrate having conductive formations defining a substrate integrated waveguide test resonator; the test resonator comprising a three-dimensional region formed at least partly within the substrate having first and second planar conductive layers extending parallel to the plane of the substrate and one or more conductive sidewall formations perpendicular to the plane of the substrate defining a resonator side wall extending around the three-dimensional region; in which one of the first and second planar conductive layers comprises a test port comprising a conductive test connection electrically isolated from the rest of that planar conductive layer.