Line Impedance Stabilization Networks
What is a LISN?
There are three common types of LISNs, with V-LISNs being the most popular as they're used on power lines. All LISNs regardless of inductance or application have an electrical path through the device for the associated cabling as well as a monitoring port for connection to an analyzer.
While these devices are used in different ways, all share some similar charteristics. These type of devices are designed to have three main functions:
- 1) Provide Stable Impedance (50Ω) on mains
- 2) Block RF Signals on Mains from Entering EUT
- 3) Provides Measurement Port
Characteristics of LISNs
There are a few key criteria that differentiate LISNs and allow for compliance to a particular standard or requirement, these include impedance, phase angle, insertion loss, frequency, power limitations, amongst others. It is best practice when evaluating different LISNs to reference data sheet and manufacturer resources to ensue compliance.
The impedance, phase angle, and insertion loss requirements are provided over a given frequency range at set levels by the associated standard.
The image to the left show the impedance requirements of both CISPR 16-1-2 and ANSI C63-4 with the data for the Com-Power LI-1100C 50 uH LISN.
The setups for LISNs used in conducted emissions will very by standard, however all have common components. The image on the left from the Teseq ISN ST08 data sheet provides a setup example used on screened balanced pairs.
For exact specifications the underlying standard should be referenced, for pre-compliance or troubleshooting requirements will be based on the associated test plan. The most costly part of any compliant measurement setup will be the EMI receiver.
*For some models of LISNs an isolation transformer is required between power and the network*
Types of LISNs
What are the types of LISNs?
Each type of LISN provides different capabilities, with T-ISN (often referred to as just ISNs) being used mainly or data/IO line measurements. V-LISNs designed for power lines can either have an individual line per unit, or lines combined in a single unit with a plug type connection.
The image below, from Schwarzbeck, shows the three main types topology.
- V-LISN: Unsymmetrical (line-to-ground) Power Lines
- T-ISN: Asymmetric (mid point line-to-line) Common Mode Data/IO Lines
- Symmetric & Asymmetric (line-to-line) Power Lines
Inductance in V-LISNs (5uH vs 50uH)
The inductance values used in the design of the LISNs are based upon the anticipated inductance of the power system which the equipment will be connected. For large systems and commercial (includes large aircraft) requirements 50 micro-Henry is often used whereas, for smaller systems (notably automotive) 5 micro-Henry is common.
A generic inductance value of a single straight cable can be calculated using the wire diameter, wire length, and permeability. A digital calculator can be accessed by clicking here. While most cabling does involve some curve or winding, generally the larger length and diameter the higher the inductance. It is best practice to reference the standard as the table below is generic, individual product requirements can vary.
The value of 50 micro-henry was selected as an inductor by MIL-STD as it represents the inductance of 50 meters of power wiring systems common on a ship or cargo aircraft.
The below chart provides a reference for testing requirements and the most common associated inductance requirements. Some standards, as with MIL-STD-461 (standard download), provides a caveats allowing for both to be used in certain sections. It is best practice to reference the standard as the table below is generic, individual product requirements can vary.
What are Artificial Networks (ANs)?
The term artificial network can be commonly substituted for line impedance stabilization network in many applications. Typically ANs are designed for DC applications in vehicles, boats, and aircraft (smaller) where the impedance is used to replicate the impedance of associated power lines.
ANs are commonly used in two types of testing:
- Conducted emissions testing - CISPR 25 & ISO 7637-2
- Bulk Current Injection (BCI) - ISO 11452-4 & OEM Requirements
Impedance Stabilization Networks
Impedance stabilization networks, commonly just called ISNs, are devices that can be used to measure emissions on different data lines as well as used in conducted RF applications. These networks will generally vary by cable types, IE coaxial, RJ11, RJ45, etc.
How to Use LISNs
Why are LISNs Used?
Line impedance stabilization networks are commonly used for two main applications, emissions measurements as well as conducted RF immunity testing. The application and test plan (IE if testing is pre-compliance) will determine the exact methodology how the LISNs will be implemented and associated setup.
Conducted emissions standards specify LISN design requirements, frequency ranges covered, as well as conducted limits in quasi-peak and average measurements commonly in dBμV. They often differentiate testing criteria based upon the type of product or application, this can been seen with FCC Part 15 Subpart B which can be accessed by clicking here.
While some standards cover entire frequency ranges consecutively, other have gaps between set ranges. The associated image shows the test levels for FCC Part 15 provided by the FCC website. The associated test levels are designed for all digital devices, minus those that are considered Class A.
They also regulate how the voltage measurements are made, commonly between each power line and ground at the power terminals with V-LISNs.
Measurements steps using LISNs
When conducting measurements using LISNs it is best practice to run the equipment prior to connecting the analyzer to protect the RF input.
- Leave the RF output of the LISN unconnected
- Connect the EUT to the LISN
- Connect the LISN to the isolation transformer (if used)
- Power on the EUT
- Check the RF output of the LISN using analyzer with a 20 dB attenuator or transient limiter
- Connect the RF cable from LISN output to the spectrum analyzer input
- Carry out the conducted noise scan
- Disconnect the RF cable
- Power off the EUT
LISNs/ANs in BCI Testing
Both the substitution and closed loop method for Bulk Current Injection Testing including ISO 11452-4 and MIL-STD-461 CS114 requirements utilize LISNs/ANs.
LISNs/ANs used in BCI testing don't utilize the RF connection, however do utilize the other characteristics including the impedance to simulate the impedance of the associated power lines. Common BCI LISNs have frequency ranges from 10 kHz - 400 MHz and are 5uH in automotive applications.
Common LISN manufacturers include:
50 uH LISNs
What is a 50uH LISN?
50uH inductance LISNs are the most commonly used and also come in several different configurations. The 50uH inductance is typically used to replicate larger electrical power systems and typical in commercial and military applications.
The image on left from MIL-STD-461G, provides a schematic for the required 50uH LISN.
For many military applications the frequency ranges will extend as low as 10 kHz, making the frequency range an important consideration when evaluating LISNs. The design requirements will vary by standard as well, making it necessary to have multiple LISNs of the same inductance.
Common 50uH LISNs:
50uH LISNs have a few different options for connections including, socket/plug, combined housing, or individually units per line. While each type has advantages, different requirements have configurations that are typically used. The below images illustrates the common housing configurations.
MIL-STD-461G specifies 50uH LISN and under appendix A allows for 5uH LISNs under certain conditions for sections CE101 and CE102. MIL-STD LISNs are typically housed individually by lines, with four units being used to provide monitoring for three phase applications. The associated image helps to illustrate how a two sets of LISNs can be utilized.
What is a 5uH LISN?
5μH LISNs/ANs are typically used in automotive and avionic testing where smaller electrical power systems are common. In the automotive industry a wide variety of new requirements have been put in place for LISNs used in testing electric or hybrid vehicles. These changes are commonly in testing high voltage cabling, requiring new designs for compliant monitoring of emissions levels.
While there are many options available for purchasing 5uH LISNs/AN, the Teseq HV-AN 150s provide unique capabilities with the different jumper configurations for compliance to a variety of standards.
The video includes:
- 1) 5uH LISN/AN Overview and Capabilities
- 2) CISPR 25 Configuration Including Enclosure
- 3) RTCA/DO-160 Setup with 10uF Capacitor
Common 5uH LISNs/ANs:
There are five main standards that require 5uH LISN, the below reference table provides information on each including frequency range and simulated impedance. This table is based upon the most common requirements and doesn't indicate if additional accessories, (IE 10uF capacitor) are required.
CISPR 25 edition 4.0 is the current version was released 10/27/2016 and can be purchased through the IEC webstore. This standard pertains to both vehicles and boats and this newest versions includes specific requirements for electric and hybrid vehicles. The majority of information regarding line impedance stabilization networks is included in section 6, with additional information in annex E and annex I.
Edition 4 placed new requirements for low voltage (LV) testing below 60 volt and high voltage (HV) above the 60 volt threshold. In addition to the different designs, CISPR 25 now also requires a shielded metal enclosure to encompass both Artificial networks (ANs). The below image shows the Teseq HV-AN 150's in the SME HV-AN 150 for compliant emissions testing to CISPR 25 edition 4.
CISPR 25 ed. 4 ANs/LISNs Requirements:
- Low Voltage (LV) & High Voltage (HV) ANs
- Shielded Metal Enclosure (SME)
- Utilization of Several Types of ANs in Setups
5μH LISNs are required in RTCA/DO-160 with an additional 10uF capacitor. These capacitors are typically added externally and are required for compliant measurements. The associate image on the right shows the installation of the Teseq EXT 10uF capacitor.
The Teseq HV-AN 150 LISNs/ANs can be modified through the jumper methodology to meet requirements of many 5uH standards. This is not always the case and compliance should be verified prior to testing.