Explain uses of line impedance stabilization network

In order to conduct realistic emission and susceptibility testing for radio frequencies, a line impedance stabilization network LISN-B is required by certain electromagnetic compatibility (EMC) and electromagnetic interference (EMI) test standards. LISN is also known as artificial networks (AN) or artificial mains networks.

What Is conducted emissions testing?
A radio frequency (RF) equipment or system may be inspected for flaws by means of emissions testing. This kind of testing is useful for ensuring the safety of various sorts of electrical infrastructure, including electronics and power lines. If operators don’t do conducted emissions testing, EMI interference and other network problems might arise. LISNs acquire data during performed emissions testing that is vital for the technicians caring for the vehicles to know.
For the purpose of measuring conducted emissions, LISN with an RF output are placed into the power lines of the test subject device. Line impedance stabilization networks are often used in test setups to provide a specified source (supply) impedance by inserting them into the EUT supply lines.

What Is A LISN?
When a line impedance stabilization network is used to eliminate RF interference, it maintains the integrity of EMC measurements and prevents the noise from being sent back to the utility company’s wiring. Liquid-Impedance Switched Networks (LISNs) use impedance and enable users to conform to several electromagnetic compatibility (EMC) test standards.
These standards include MIL-STDs, IEC/EN, FCC, CISPR, ISO, and RTCA DO-160. LISNs link up to a power supply and the EUT to do analysis. The testing may be quickly and easily conducted at that time using RF impedance.

Figure: Line impedance stabilization network

Different Types of LISN
In addition to Low Pass Filter and Crosstalk Canceling Device (CCD), other LISN options include Inline LISN and Long Inline LISN. It’s the means by which the lightning impulse and surge are captured that makes these variants different.
Like a power line filter, these gadgets only let low frequencies through while blocking off higher ones. In addition, remember that no LISN-B can prevent ground current routes from occurring as a result of either direct or close indirect lightning strikes.

Inline LISN
“Inline Listening and Speaking Network” abbreviates “Inline LISN.” A method wherein two persons may engage in two-way conversation at the same time is meant here. The telephone conversation, in which two or more individuals may converse at simultaneously, is a frequent example of inline LISN.
As another example, consider a video conference in which many people can see and hear each other at the same time. Despite being present since the early 1990s, inline LISN has not seen widespread adoption owing to its hefty price tag and complicated installation process. But this new technology might be a game-changer since it’s inexpensive and easy for businesses to deploy without a major overhaul of their networks.

Long Inline LISN
The purpose of a statement like “a lengthy inline line impedance stabilization network” is to provoke the reader into contemplation of the subject matter. I’m exhausted is one such expression. The reader will get a sense of how you feel from this line, but they will learn nothing more about you.
This method has a wide range of applications, including grabbing the reader’s attention and pique their curiosity at the beginning of an article or tale. Because it’s crucial for others to know your sentiments when you don’t have anything else to say, it may also be uttered out loud when you’re at a loss for words.

How does it work?
Using this simple setup, conducted emissions from the EUT may be measured. However, the source impedance will impact the amplitude of the conducted emissions. Because it functions as a voltage divider in the system as a whole, the source impedance has a major impact on the observed amplitude of the conducted emissions. Taking the impedance of the supply source out of the calculation ensures consistent findings for the same EUT across laboratories.
A line impedance stabilization network may be wired into any source of electricity and will provide the same voltage and current to the EUT terminals as the source of electricity. Contrarily, the LISN’s source impedance is defined by EMC regulations, allowing for consistent conducted emission measurements in any lab.

How is it implemented?
The LISN has an impedance very near to 50 Ohm for the vast majority of the outlined frequency range. As the frequency drops below 5 MHz, the impedance approaches 0 Ohm, and it is effectively zero at DC.
The impedance of the 5 H inductor becomes quite large at frequencies exceeding 5 MHz. Resistance from the 50 Ohm load is the primary factor in the LISN impedance at the EUT port. The 50 Ohm impedance is the input impedance of the attached spectrum analyzer or measurement receiver when the LISN is used for conducted noise measurement. This also explains why a LISN’s RF output must be terminated with a 50-Ohm load when it is being utilized in an unmeasured environment.
It can be seen from the specific impedance curve that the total LISN impedance reduces below 5 MHz when the impedance of the inductor plus the 1F capacitor begins loading the 50 Ohm resistor.
Regardless of whether the source terminals are open, shorted, or of what impedance they are connected to, the LISN-B impedance must conform to the standard. Within its stated frequency range, the LISN’s impedance is unaffected by the impedance of the associated power supply.

Main functions of an LISN
Stable line impedance
A LISN’s primary purpose is to provide a known impedance at the power input of the EUT, allowing for reliable measurements of EUT noise at the LISN’s measurement port. It’s crucial to know this because the impedance of the power supply and the EUT together act as a voltage divider. If you look at the supply wire behind it, you can see that the impedance changes depending on the shape of the supply wiring.
Another factor in determining the appropriate LISN for testing is the expected inductance of the power line at the site where the EUT will be installed. In automotive measuring standards, for instance, a 5 H inductor is used to simulate a shorter usual wire length, whereas a 50 H inductor is more often used to make a connection in a building.

Isolation of the power source noise
A line impedance stabilization network also shields the system from high-frequency power-source noise, which is crucial. As a low-pass filter, a LISN-B blocks high-frequency RF interference from entering the EUT while allowing low-frequency power to pass through.

Safe connection of the measuring equipment
In order to collect data for an EMC test, a spectrum analyzer or EMI receiver is often employed. Overloading the input port of such a device might cause irreparable harm. The output impedance of the measurement port on a LISN is typically 50. Because of the LISN measurement port’s stable impedance, built-in low-pass filter function, and DC rejection capabilities, coupling the high-frequency noise signal to the input of the measuring equipment is simple.

LISN applications
When setting up a variety of tests, LISN are employed not only to detect conducted emissions, but also to maintain a stable supply line impedance. These are only a few instances among many others.

Conducted emission measurements, voltage method
EMC pre-compliance testing often involves LISN measurements of conducted emissions from power lines.
Equipment that operates off of direct current (DC) is measured at its DC supply lines, whereas that which operates off of alternating current (AC) is measured at its AC supply lines. For devices that need an external power source, such laptops, 3D printers, and mobile phones with chargers, among others, the standard mandates testing solely at the AC-mains side, skipping the supply cord in between. The same will not be true of any disruptive behavior on these lines. Interference from other supply lines will induce interference from the cable itself, which might result in the product failing its radiated noise testing.
Therefore, it is recommended to verify conducted emissions on the connecting cable by inserting a pair of 5H LISN between the power source and the connected device. Conduct an initial measurement with the EUT terminals connected to the power supply terminal, then switch the LISN-B to examine the emissions on the supply input terminals of the related device, since both the power supply and the device may generate conducted emissions. To prevent a nasty surprise at the test house, make sure the emissions are well under the limitations set for the mains-powered side, which are not applicable here.

Conducted emission measurements, current method
By introducing LISN into the supply lines, a certain impedance level is achieved. It is not possible to detect conducted noise at the LISN RF outputs due to their 50 Ohm termination.

Interesting Facts About LISN
While a LISN is not always necessary, it is strongly suggested anytime an electronic equipment with a high radiofrequency radiation output is utilized near humans. Standing waves on the transmission line are caused by the proximity of many measurements equipment with widely varying impedances. Interference from a standing wave may slow down data transmission.

Safety precautions when using
When connecting an AMN/LISN to a standard wall outlet, two extra measures of safety are required. The first is related to the protection of the unit’s operators, since there is a capacitance of around 12 F between the live and earth terminals. This permits a current of around 0.9A to flow in the supply earth wire when 240V 50Hz is applied. If this current were to pass through a human body, it might easily be fatal. Unfortunately, the earth current is still too high for safety, even with the 1.1F version from live to earth.
If the AMN/LISN is not correctly bonded to the supply earth and it is unplugged, the case (and any RF connections attached, for example) will become live. SOLID BONDS TO THE SUPPLY EARTH AND GROUND PLATFORM ARE REQUIRED FOR THE AMN/LISN CASE. The equipment should ideally be permanently installed in the testing facility. Installation of portable AMN/LISNs for on-site work requires extra caution.
Because of this earth current, AMN/LISNs can’t be utilized on mains circuits that have earth leakage or residual current contact breakers installed for safety purposes. To facilitate this and for purposes of optimal safety, an isolating transformer should be installed in the LISN’s mains supply. This won’t have any impact on the network’s performance but may restrict the amount of energy that can be sent to the EUT.
The second measure is for the protection of the measurement equipment. A rich source of transients that may sometimes approach 1kV comes from the supply mains. Although the LISN-B hardware may mitigate some of these spikes, it cannot completely eliminate them.
However, substantial transients may be generated by supply switching operations inside the EUT itself, since current is interrupted via the LISN chokes and then delivered straight to the measuring instrument without any attenuation.
That’s why, if you’re going to be using a spectrum analyzer, you need to make sure there’s a transient limiter included into the AMN/output LISN’s chain (two of the five commercial units investigated included a switchable limiter in the output signal path). The signal will be reduced by 10 dB and the measurement uncertainty will increase by a tiny amount, but these effects are usually tolerable and much less costly than repairs. Keep in mind that some limiters also use a low-pass filter to cut down on the audible frequencies.
Since the front end of a measuring receiver is narrowband and is thus shielded, the installation of a limiter may be optional.

Choosing The Right LISN
A variety of high-quality AC and DC line impedance stabilization networks from LISUN are available at reasonable prices.
Thinking about the test frequency, operating voltage, and kind of current required by your test standard will help you quickly identify the best line impedance stabilization network. To find out more about our LISNs and other conducted emissions test products, contact a representative at LISUN now. In addition, we can advise you on the best LISN to use for your tests.

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