Fig: LISN (Line impedance stabilization network):

A LISN is a low pass filter placed between AC &DC power source and equipment under test to create a known impedance and to provide a radio frequency noise measurement port. It is used to isolate the unwanted RF signals from the power source. LISN can be used to predict conducted emission for diagnostic and pre compliance testing.

Functions of LISN:

• It isolates power line mains from EUT which leads to clean power delivered with EUT.
• It isolates noise produced by EUT from getting coupled to main power lines.
• The signal generated from EUT is coupled with EMC analyzer using High Pass Filter.

Why we need different LISN: Different LISNs are needed to meet the various EMC test standards such as CISPR, FCC, ISO, EN, etc. Different standards require different test frequencies as they take into account the operating environment for the types of equipment covered by each standard.