A Line Impedance Stabilization Network (LISN) is one of the most important pieces of equipment used in EMC testing. It provides a standardized impedance between the power source
A Line Impedance Stabilization Network (LISN) is one of the most important pieces of equipment used in EMC testing. It provides a standardized impedance between the power source and the Device Under Test (DUT), allowing engineers to accurately measure conducted electromagnetic emissions while isolating unwanted noise from the external power supply.
Whether testing industrial equipment, consumer electronics, communication products, medical devices, or automotive components, a LISN is essential for conducted emission measurements required by EMC standards such as CISPR, FCC, and MIL-STD.
Understanding how a LISN works helps engineers obtain repeatable EMC test results and identify conducted noise before formal compliance testing.
A LISN, or Line Impedance Stabilization Network, is a passive network installed between the AC or DC power source and the DUT during conducted emission testing.
It performs three primary functions:
* Provides a standardized impedance for EMC measurements
* Separates conducted noise generated by the DUT from power source interference
* Couples RF noise to the EMI receiver for analysis
Without a LISN, conducted emission measurements can vary significantly because the impedance of the external power supply changes from one test environment to another.
Every power supply contains a certain level of electrical noise.
If the DUT is connected directly to the mains supply, external interference may affect the measurement results.
A LISN creates a stable and repeatable electrical environment by isolating incoming power-line noise while presenting a defined impedance to the DUT.
This allows different laboratories to obtain comparable conducted emission results when testing according to international EMC standards.
Products such as the GE-EMC LISN Series are specifically designed to meet these measurement requirements for EMC compliance testing.
During EMC conducted emission testing, the DUT receives normal operating power through the LISN.
At the same time, high-frequency noise generated by the DUT is directed through the LISN’s measurement port to an EMI receiver or spectrum analyzer.
The measurement process typically follows these steps:
1. The DUT is powered through the LISN.
2. The LISN filters external RF noise coming from the power source.
3. Conducted RF noise generated by the DUT is separated from the power line.
4. The RF signal is transferred through the RF output port.
5. An EMI receiver measures the disturbance level across the specified frequency range.
Because every test uses the same standardized impedance, measurement repeatability is greatly improved.
Although different models are available for various applications, most LISNs contain the following components:
* Precision inductors
* High-frequency capacitors
* Measurement resistor network
* RF output connector
* Ground connection
* Power input and output terminals
These components work together to block unwanted RF energy from the external power source while allowing conducted emissions from the DUT to be accurately measured.
Conducted emission testing evaluates electromagnetic noise that travels through power cables instead of radiating into free space.
The measurement setup typically includes:
* Device Under Test (DUT)
* LISN
* EMI receiver
* Ground reference plane
* Test software
The LISN connects directly between the DUT and the power source.
The EMI receiver measures RF noise through the LISN output port and compares the results with CISPR or FCC emission limits.
Without a properly calibrated LISN, conducted emission measurements cannot meet the repeatability required for EMC compliance testing.
Most commercial EMC conducted emission tests evaluate frequencies from:
150 kHz to 30 MHz
This frequency range covers the majority of conducted RF disturbances generated by:
* Switching power supplies
* DC/DC converters
* Inverters
* Motor controllers
* Digital electronics
* Communication equipment
These disturbances can propagate through power lines and interfere with nearby electronic systems if not properly controlled.
LISNs are widely used across many industries.
Typical applications include:
* Industrial automation equipment
* Consumer electronics
* Medical devices
* Communication systems
* Renewable energy equipment
* Electric vehicle electronics
* Battery chargers
* Laboratory instruments
For products requiring international EMC certification, conducted emission testing using a LISN is often mandatory.
During EMC testing, conducted emission failures are frequently related to product design rather than measurement equipment.
Typical causes include:
Poor EMI Filter Design
Insufficient filtering allows switching noise to enter the power line.
Selecting appropriate filter components during product development can significantly reduce conducted emissions.
PCB Layout Problems
Large current loops and poor grounding increase conducted RF noise.
Optimizing PCB layout is often more effective than adding additional filtering components after testing.
Switching Power Supply Noise
High-frequency switching circuits generate harmonics that commonly exceed conducted emission limits.
Reducing switching noise requires careful design of power stages and return current paths.
Cable and Grounding Issues
Improper cable routing or grounding may increase common-mode currents, resulting in higher conducted emission levels.
A LISN is only one part of a complete EMC measurement system.
Depending on the applicable EMC standard, additional equipment may include:
* EMI receiver
* Spectrum analyzer
* RF signal generator
* RF power amplifier
* Biconical antenna
* Log-periodic antenna
* Horn antenna
* Semi-anechoic chamber
Manufacturers establishing internal EMC laboratories often integrate these systems into complete EMC laboratory solutions for both conducted and radiated EMC testing.
Several factors should be considered when selecting a LISN:
* AC or DC power application
* Maximum operating current
* Voltage rating
* Frequency range
* Applicable EMC standard
* Connector type
* Calibration requirements
Using a LISN that matches the intended test standard helps ensure accurate conducted emission measurements and simplifies future compliance testing.
For companies performing EMC pre-compliance testing, selecting a high-quality, calibrated LISN is one of the most effective ways to improve measurement consistency and reduce certification risk.
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