Description
Line Impedance Stabilising Networks (LISNs), also known as Artificial Mains Networks or V-Networks, are passive electronic circuits defined in 3GPP test specifications such as TS 25.113 (for UTRA), TS 34.124, TS 36.124, and TS 38.124 (for terminal conformance testing). They are not a network function but a critical piece of laboratory test equipment used for Electromagnetic Compatibility (EMC) compliance testing of User Equipment (UE) and base station equipment. Their primary function is to present a defined, stable RF impedance (typically 50 Ohms) between the Equipment Under Test (EUT) and the power supply mains across a wide frequency range (e.g., 150 kHz to 30 MHz or higher).
Architecturally, a LISN is placed in series with the AC or DC power line feeding the EUT. It consists of inductors, capacitors, and resistors arranged in a specific network topology (often a 'V' shape, hence V-Network). The inductors provide high impedance to RF noise from the EUT, preventing it from flowing back into the mains supply and the laboratory's power grid, which has a variable and unknown impedance. The capacitors provide a low-impedance path for this RF noise to be directed to a standardized 50-Ohm measurement port, where a spectrum analyzer or EMI receiver is connected.
During testing, the EUT is powered through the LISN and operated in various modes to generate potential emissions. Any RF noise currents generated by the EUT's switching power supplies, digital circuits, or other sources travel along the power cables. The LISN ensures these currents see a consistent 50-Ohm load at the measurement port, regardless of fluctuations in the actual mains impedance. The spectrum analyzer measures the voltage at this port, which directly correlates to the conducted emission current from the EUT. This standardized setup allows for repeatable and comparable measurements between different test laboratories, which is essential for regulatory certification (e.g., FCC, CE marking).
Their role is fundamental in ensuring that telecommunications equipment does not emit excessive electromagnetic interference that could disrupt other electronic devices or radio communications. By using a LISN, test engineers can accurately quantify the conducted emissions on power lines, one of the key pathways for electromagnetic interference. The specifications in 3GPP documents define the exact circuit parameters and performance requirements for LISNs to be used in testing 3G, 4G, and 5G equipment, ensuring global harmonization of test methods for market access.
Purpose & Motivation
LISNs exist to solve a fundamental problem in EMC testing: the unpredictable and variable impedance of real-world AC power mains. Without a stabilizing network, the RF noise currents emitted by a device would encounter different impedances in different test labs or even at different times in the same lab, leading to vastly different voltage measurements at the power port. This would make compliance testing non-repeatable and non-reproducible, undermining the entire regulatory framework for electromagnetic compatibility.
The technology was created to provide a standardized reference impedance for conducted emission measurements, as mandated by international EMC standards like CISPR (International Special Committee on Radio Interference). By inserting a LISN between the equipment and the mains, it isolates the equipment's emissions from the unstable mains impedance and provides a clean, consistent 50-Ohm measurement point. This allows for fair and objective comparison of emission levels against established limits.
Its inclusion in 3GPP specifications (starting from Rel-4 for UTRA testing) was motivated by the need to define precise conformance test conditions for mobile equipment. As devices became more complex and operated at higher frequencies, controlling and measuring their unintended emissions became critical to ensure they do not interfere with the radio networks they operate on or with other services. The LISN is a foundational tool that enables the verification that 3GPP-compliant equipment meets stringent EMC regulations worldwide, facilitating global market access and ensuring network reliability.
Key Features
- Provides a standardized, stable 50-Ohm impedance for conducted emission measurements across a defined frequency range
- Isolates the Equipment Under Test (EUT) from the variable impedance of the mains power supply
- Directs RF noise currents from the EUT to a dedicated 50-Ohm measurement port for connection to a spectrum analyzer
- Defined by precise circuit parameters (inductance, capacitance values) in 3GPP test specifications
- Essential for repeatable and reproducible EMC testing in compliance with standards like CISPR
- Used in testing both AC-powered and DC-powered telecommunications equipment
Evolution Across Releases
Introduced LISN requirements in 3GPP test specifications like TS 25.113 for UTRA base station conformance testing. It defined the initial use of LISNs to provide a stable 50-Ohm impedance on power lines for measuring conducted emissions from 3G Node B equipment, ensuring tests were performed under controlled and repeatable conditions.
Updated LISN specifications in TS 36.124 for LTE User Equipment conformance testing. The requirements were adapted for the new LTE terminal ecosystem, ensuring EMC test methods remained consistent and effective for the next generation of mobile devices, covering the relevant frequency bands for conducted emissions.
Defining Specifications
| Specification | Title |
|---|---|
| TS 25.113 | 3GPP TS 25.113 |
| TS 34.124 | 3GPP TR 34.124 |
| TS 36.124 | 3GPP TR 36.124 |
| TS 38.124 | 3GPP TR 38.124 |