A LISN (Line Impedance Stabilization Network, pronounced listen) is a device to create a known impedance on power lines of electrical equipment during electromagnetic interference testing. A LISN is typically designed to allow for measurements of the electromagnetic interference existing on the power line.
It fulfills three main functions:
- It filters the mains voltage and should block higher frequencies than the mains frequency.
- It should provide a characteristic impedance to the device under test (DUT).
- The conducted interference voltage produced by the DUT is transferred to a meter, for example a spectrum analyser or an EMI receiver.
A LISN is basically a very large and effective "Pi-type" electrical noise filter used in the design and measurement of electronic equipment to meet formal specifications such as the "CE Marking". Its large capacitors and inductors (which would never be used in consumer equipment for safety reasons involving potentially high-current short-circuit current paths!) absorb nearly all unwanted noise energy and essentially "isolate" electrically the Device Under Test ("DUT") from any of the power mains. The LISN is supposed to keep unwanted conducted noise from coming in on the power mains and tainting measurement of the DUT, and it is also supposed to keep noise generated by the DUT from escaping back into the power mains. The purpose is to allow accurate measurement of noise generated by the DUT using, usually, a broadband spectrum analyzer.