How to choose the test system cable

Coaxial cable is one of the components that are often overlooked in the system unless they fail. When in its best condition, the microwave cable assembly does not add any effect: no loss, no phase distortion. When in the worst case, the cable loss can become so severe that it must be compensated with a wideband amplifier circuit. Coaxial cable and cable assemblies used for test and measurement applications must be "invisible" to the system so that their electrical characteristics are not measured as part of the device under test (DUT).

Although some engineers who build RF/microwave test systems prefer to “build their own” cable assemblies by cutting flexible or semi-rigid cables to length and then terminating the cables in selected coaxial connectors, Most engineers are still willing to order coaxial cable assemblies. There are many suppliers of such components, even for high-performance precision cables required for testing applications, and most suppliers can provide standard-length and custom-length cable assemblies.

Although automatic test equipment (ATE) systems typically use semi-rigid cables, and in these systems the DUT can be automatically placed in a suitable test fixture, many measurement systems based on microwave vector network analyzers (VNAs) need to pass through flexible cable assemblies. Connect the DUT to the test system port. Companies such as Flexco Microwave have provided sophisticated cable assemblies developed specifically for use with VNAs. For example, the company's NTC195 test cable can be used in the frequency range from DC to 26.5 GHz, while the NTC182 test cable can be extended from DC to 40 GHz. Both cables have an outer diameter of 0.5 inches and a minimum bend radius of 1.5 inches. Both 50Ω cables have a minimum shielding effectiveness (SE) of 90dB. Depending on the frequency requirements, these cable assemblies can be terminated with different connectors, including K, 3.5mm, SMA, TNC, and N-type connectors.