An interposer with a conductive housing is disclosed. Conductive members pass through insulators positioned in openings in the conductive housing. The conductive housing may be grounded, providing a closely spaced ground structure for signal conductors passing through the conductive housing and therefore providing a desirable impedance to signals carried by the conductive members. Such an interposer may be used in a test system to couple high speed signals between instruments that generate or measure test signals and devices under test.