EMC Part 58. LISN: How to Verify Line Impedance Stabilization Network (LISN) Compliance CISPR 11/32.
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A Line Impedance Stabilization Network (LISN) is a specialized device used in electromagnetic compatibility (EMC) testing to measure the conducted emissions from electrical and electronic equipment. It serves several crucial functions:
Impedance Stabilization: LISNs provide a stable and known impedance to the Device Under Test (DUT). This consistency is necessary because the impedance of power lines can vary, which would otherwise affect the accuracy and repeatability of emission measurements.
Noise Isolation: It isolate the DUT from external noise present on the power lines, ensuring that only the emissions from the DUT are measured.
Noise Coupling: LISNs couple the conducted noise emissions from the DUT to the measuring equipment, typically an EMI receiver or a spectrum analyzer, while blocking the noise from the power source.
It is definitely possible, to measure conducted emissions produced by the EUT, using this simple set up. However, the amplitude of the conducted emissions will be influenced by the impedance of the source. The source impedance is part of a voltage divider in the overall system and has a significant influence on the amplitude of the measured amplitude of the conducted emissions. As it is necessary to get the same results for the same EUT in any lab, anywhere, it is necessary to remove the impedance of the supply source from the equation.
The picture illustrates how a LISN is incorporated into the setup to provide a defined impedance for the conducted emissions traveling on the supply lines of equipment under test
A LISN can be connected to any power supply and provides the same voltage and current capabilities as the power supply at the EUT terminals. The source impedance of the LISN is specified by EMC standards, ensuring repeatable conducted emission measurements in any laboratory.
A typical LISN consists of several key components:
Inductors: These are used to present a high impedance to high-frequency noise, ensuring that the noise is directed towards the measurement equipment instead of back onto the power source.
Capacitors: These are employed to bypass high-frequency noise to ground.
Resistors: They help in defining the impedance and ensuring stability.
5µH LISNs are primarily used for testing equipment installed in vehicles. The 5µH inductance simulates the supply lines with a length of 5 meters, as wiring harnesses in vehicles rarely exceed this length. These LISNs are primarily used for conducted noise measurements in the frequency range of 150kHz to 110MHz. 5µH LISNs designed for testing according to DO-160 and MIL-STD 461 standards cover the frequency range of 10kHz to 400MHz.
On the other hand, 50µH LISNs are primarily used for equipment installed in domestic or industrial environments. The 50µH inductance simulates 50 meters of supply wiring inside a building.
At frequencies above 5MHz, the impedance of the 5µH inductor is high. The LISN impedance at the EUT port is dominated by the 50"Ω" load resistance. In fact, when the LISN is used for conducted noise measurement, the 50"Ω" impedance is actually the input impedance of the connected spectrum analyzer or measurement receiver. This also explains why the RF output of a LISN has to be terminated with 50"Ω" when used without measurement equipment connected. At frequencies below 5MHz, the impedance of the inductor combined with the 1µF capacitor starts loading the 50"Ω" resistor, and the overall LISN impedance decreases according to the specified impedance curve.
Impedance curve of a 5µH LISN from CISPR 25, Annex C, one of the EMC standards that specifies LISN.