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Questions to Ask and Answer When Looking for LISN, AN, and ISN

LISN- Line Impedance Stabilization Networks, AN- Artificial Networks, and ISN- Impedance Stabilization Networks all refer to the same device. The difference between the terms is different standards choose to use the different terms. Each one is commonly used in electromagnetic compatibility (EMC) testing of electronic devices to measure the conducted emissions, which are the signals that are transmitted through power and signal lines. The purpose of a LISN is to provide a standardized impedance for the device under test (DUT) to ensure that the emissions measured are representative of those that the DUT would emit when used in the real world. Overall, LISNs are important for ensuring that electronic devices comply with regulatory standards and operate reliably in their intended application environment by filtering out unwanted noise and providing a stable impedance for testing. The LISN will have two main components the stabilization network and a capacitive isolated RF 50 Ω tap to hook up a receiver for measurements.

If you’re performing conducted emissions testing in most cases, a LISN is called out in the test standard(s) that cover your product. Each industry and environment has developed these standards and refined how these LISNs must be built and calibrated to ensure compliance. We will review what you need to know and how to select a LISN, AN, or ISN for your testing.

What test standard are you testing to?

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A product, manufacturer, or international test standard will specify the required LISN.

Some major test standards are:

  • CISPR 32 IT Equipment, which calls out CISPR 16 for LISN performance. This is 50µH LISN primarily for testing AC lines or long DC lines. In many cases, LISNs are for single-phase and 3-phase AC power networks. This standard also requires an ISN for measuring telecommunication lines.
  • CISPR 25 is used in the automotive industry and uses a 5µH LISN. There are now two LV and HV/EV testing requirements requiring slightly different LISNs. However, some LISNs incorporate switches to allow the use of the same LISN for both cases.
  • MIL-STD-461 calls out a 50µH LISN for testing or DC and AC lines. This LISN has, over time, with CISPR standard revisions become different than the one used for CISPR 32/16. The same LISN should not be used for MIL and CISPR testing, even though both are 50uH.
  • DO-160 for commercial aircraft requires a 5µH LISN for testing AC/DC lines. The main difference of this LISN to the automotive CISPR 25 LISN is the frequency range coverage. A CISPR 25 LISN may not meet the requirements up to 400MHz as specified in DO-160. In aircraft, AC power may have frequencies up to 400Hz or even 800Hz, affecting how the LISN is designed.

 

What Frequency range does the testing cover?

The selected test standard will dictate this. Some common requirements:

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What Lines do you need to test?

The test standard dictates the selection of lines to test. In most cases, Single-phase AC, Three-Phase AC, and/or DC power lines are tested. I/O lines that are very long are also tested, such as the case with telecom lines. Each individual line is considered a path. No path is required for Potential Earth (PE) sometimes referred to as ground. Neutral must be measured and requires a path. For single-phase and DC, two paths are required, and for three-phase, up to four paths are required when a neutral is used. LISNs can be offered as one path or can contain multiple networks and, therefore, have multiple paths.  It is acceptable to use a 4-path LISN when only testing a DUT with two lines or three lines. Each path in the LISN is isolated and would not affect the results. A multiple-path LISN for CISPR 32/CISPR 16 testing is common for AC.

 

What are the maximum voltage and current rating of lines of the DUT?

Once you know the lines of your DUT, you need to test and know your DUT’s voltage and current ratings. These should be the maximum levels if you have a range of DUTs. It is OK to use a higher current-rated LISN for lower current applications.  Manufacturers will have different sizes to fit the different needs. Higher voltage and current ratings do increase size and cost. Some DUTs may also have an inrush current. This is a short burst when the product is turned on or when a certain mode is activated. This is normally a second or less, and most LISNs will have a short-time current limit higher than the steady-state current. This is a concern to keep in mind with high current applications and DUTs.

 

What are the connection points on the LISN?

Knowing how your DUT will connect to the LISN/s and how it will connect to the mains is important. In the case of single Path LISNs, the connection can be a simple, easy-to-use post, and wing nut or a more customized safety interconnect. The power rating of the LISN will dictate the size of the posts. Multi-path LISNS for CISPR 32/CISPR 16 testing may have standardized AC outlet connections depending on the part of the world and voltage and power ratings. A 400V, 63 amp 3-phase plug may need to be adapted for a single-phase application to extend the use of the LISN in the lab. These adaptors might be hard to find and may need to be custom-made in-house. When connecting your DUT to the LISNs, removing the power to the LISN is always important.

 

Does the LISN have internal attenuation or impulse protection?

All LISNs should be used with a pulse limiter to protect the receiver from high voltage spikes. Due to its inherent design, the LISN can hold high voltages in the capacitors and, when making the physical connection with the coax, can produce a transient that can damage the receiver. A transient limiter is highly recommended. This can be an external device or, in some cases, built into the LISN.  This limiter normally has an attenuation value of 10 or 20 dB. This needs to be compensated for in your measurements. Care must be taken that a transient limiter is used, not just an attenuator. A 20dB attenuator reduces voltage by a factor of 10. A 3000V pulse will still be 300V at the receiver and may damage it. A pulse limiter will cut the pulse down, and the internal attenuator will further protect the receiver.

 

Does the LISN require power for fans?

Some LISNs require external power for blowers to reduce overheating. Others are air-cooled. When purchasing, it is good to know what is involved for installation and to verify the correct voltage you have available.

 

Do you want automatic line switching?

Multi-path LISNS can also be offered with automatic switching. A control and power line is normally connected with the receiver to take control and switch from line to line without user intervention. This is a nice feature for 3-phase testing in particular. But it requires the receiver to have this capability and connection. As each line is switched, the unused lines are terminated with 50Ω.

 

What physical size is the LISN?

The size of the LISN is important. Different manufacturers may use different techniques to construct the LISN. Some may be larger for the same relative electrical parameters. A smaller size is desirable as it will take up less space in the setup. If you need to place four very large LISNs in the setup, this may make your setup difficult to place your DUT and require more room or a bigger test table.

 

Do you need Calibration adaptors?

Calibration labs use calibration adaptors to test and ensure the LISN meets specifications. They are also used to verify your test setup. Setup verification is a requirement in many quality requirements. It is a good idea to always have the calibration adaptors on hand so you may connect to LISN feed-throughs with an RF connector.

 

Do you require a pre-compliant solution or a fully compliant solution?

In the past years, pre-compliant EMC products have become available. These pre-compliant devices have brought testing costs down dramatically. But how can the pre-compliant products be priced so well in relation to a fully compliant product? The pre-compliant LISN is “built to meet” the standard. It is not tested or tuned or calibrated after it is built.  The components used are not sourced nor have quality inspections. All of this enables these devices to be sold at a better price. And if you are not final testing and using the devices for comparison measurements, these products fit the need when budgets are tight. They can never be used for final measurements. The accuracy can be many dB off from a compliant solution.

It is much better to purchase a full compliance LISN even for pre-testing. You will be assured that the product meets all quality and standard test calibrations.

For the above requirements, you should be able to narrow down and select the proper LISN, AN, or ISN. It is also important to ask and communicate your needs with the manufacturer/reseller. Make sure they understand you and guide you correctly.  In this industry, it is very important to have trust that the purchase you are making is correct and that you will be supported with any questions and concerns you have in the future.

Some important safety factors to remember when using LISNs: When working with AC or DC, higher voltage and power is present, be aware and safe. Follow safety protocols and be properly trained. Read manuals and safety information. The LISN must be tied and bonded to the ground plane. LISNs, the way they are built, have high leakage currents and cannot be on a Ground Fault Protecting Circuit. In some cases, it may require to be on an isolation transformer. It is best to use LISNs in a shielded room, but they can also be used out in the open. Being out in the open, ambient emissions have the potential to affect measurements. Using additional filtering before the LISN and isolation transformer may help with measurements.

 

LISN Examples

Single Path LISNs and 70Amp vs. 800Amp size comparison
Multi-Path LISNS
ISN for Ethernet
Examples of calibration adaptors

 

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