Whitepaper
Why So Many Different Types of LISNs?
This application note from Com-Power Corporation explains the purpose of the Line Impedance Stabilization Network (LISN) and why so many variants exist. The LISN performs four core functions during conducted emissions testing: it provides a stable, normalized impedance on the power line so that measurements are consistent and repeatable regardless of the facility's wiring; it prevents external RF noise from the mains from coupling into the measurement; it allows clean power to pass through to the equipment under test; and it safely couples the low-level RF noise from the EUT to the 50-ohm input of the spectrum analyzer or EMI receiver while blocking the higher mains voltage. The document then explains why a single LISN design cannot serve all applications. Differences in test frequency range require different inductor values and filter topologies, with the standard 50 µH design suited to FCC and CISPR testing from 150 kHz to 30 MHz, while a 250 µH two-stage design is needed for testing down to 10 kHz. The 50 µH value was originally chosen to represent the inductance of approximately 50 meters of power distribution wiring on ships or cargo aircraft, while smaller platforms like fighter aircraft call for 5 µH designs. Variations in current handling requirements, operating voltage, AC versus DC operation, and the number of power phases further expand the range of LISN types needed.