Introduction to Linear Impedance Stability Network (LISN)_Century Wisdom (Suzhou) Testing Technology Co., Ltd

English

Home page

Technical column

CASES

Introduction to Linear Impedance Stability Network (LISN)

Release time:

2023-02-22 15:10

Source:

　　Introduction to Linear Impedance Stability Network (LISN)

　　1 Introduction

Conducted interference occurs in the form of common mode (asymmetric) and differential mode (symmetric) of voltage or current. Common mode interference refers to the loop interference formed by the phase line; differential mode interference refers to the loop interference formed between the phase lines.

LISN,Line Impedance the abbreviation of Stabilization Network, that is, linear impedance stabilization network, also known as AMN, artificial power network (Artificial Mains Network). It is the main measuring instrument in the conducted interference test. At present, the conducted interference test of the power supply port of electronic and electrical equipment is mainly measured by the linear impedance stabilization network.

Fig. 1 Single Line LISN

Fig. 2 4-line LISN

　　2 Role of LISN

The purpose of the conducted emission test (CE) is to measure the noise current present on the product's power line.

In different test sites, the environmental noise on the power supply network is also different. External environmental electromagnetic noise enters the tested sample through the AC power line, which affects the accuracy of the measurement results. Although current probes can also perform conducted emission measurements, the impedance of different power systems varies greatly. This change in load impedance affects the magnitude of the noise current conducted from the power supply line. In order to ensure consistency between test sites, the impedance of each test site from the product power cord must be stable.

Fig. 3 Schematic Diagram of Conducted Interference Test Arrangement

Then came LISN. It is used between the sample under test (EUT) and the power input to achieve the following functions and requirements:

1) Isolation. LISN is placed between the power grid and the sample to filter out the electromagnetic noise from the power grid and provide pure power to the sample to ensure the accuracy of the measurement results. It also isolates the disturbance signal from the sample under test to prevent excessive electromagnetic interference from flowing to the grid power supply and affecting other equipment on the same line.

2) Power supply. Provide power supply to the test sample, and can not affect the normal power supply of the grid power supply to the test sample.

3) impedance stability. Provide a stable line impedance within the measurement frequency range specified by the standard. A stable line impedance was artificially developed to provide uniform test conditions for the measurement of the conducted emission voltage. A network impedance of 50 Ω is generally provided within the measurement frequency range.

4) Signal pickup. The high-pass filter of the LISN is used to couple the interference signal generated by the EUT to the EMI receiver and prevent the grid supply voltage from being applied to the EMI receiver.

　　3 Classification of LISN

There are two basic types of LISN: V-type for coupling asymmetric voltages and Δ-type for coupling symmetric and asymmetric voltages, respectively. In CISPR16 standards, the use principles and types of LISN in different frequency ranges are specified, as shown in Figure 4.

Figure 4 Classification of LISN

The impedance of LISN is the impedance measured at the EUT terminal relative to the reference ground when the disturbance output terminal is terminated with a 50Ω load.

Now the commonly used LISN to do a brief introduction:

1) 50Ω/50µH 5Ω V-type LISN, suitable for 9kHz-150kHz frequency range.

Figure 5

2) 50Ω/50µH V type LISN, suitable for 150kHz-30MHz frequency range.

Figure 6

3) 50Ω/5µH 1Ω V-type LISN, suitable for 150kHz-108MHz frequency range.

Figure 7

4)150 Ω V-type LISN, suitable for 150kHz-30MHz frequency range.

Figure 8

5)150 Ω Δ type LISN, suitable for 150kHz-30MHz frequency range.

Figure 9

　　4 Structure and Principle of LISN

At present, the testing standards based on EMC professional laboratories in China mostly use 50Ω/50µH V LISN. The following figure shows a line impedance stabilization network that meets the requirements of CISPR 25 and ISO 7637-2. A LISN needs to be connected in series on the positive and negative power lines respectively.

Figure 10

1µF and 5µH in the circuit diagram are used to filter the interference from the grid power supply and provide "pure" power supply for the tested sample. A power supply system will be connected to a number of electronic and electrical equipment, these devices in the work of their own noise will be introduced into the power supply network. If this electromagnetic noise is not filtered (isolated), it will be measured by the EMI receiver, which will affect the accuracy and repeatability of the test results.

Figure 11

The 0.1µF capacitor in the circuit diagram is used to isolate the through cross. The DC component in the circuit will be transmitted to the EMI receiver to cause damage, and the capacitor can absorb the peak voltage in the disturbance signal. The 0.1µF capacitor has a lower impedance within the test frequency range and does not affect the test results, while it has a larger impedance for the 50Hz power supply frequency.

Figure 12

1kΩ is used to discharge a capacitor of 0.1µF. After a test is completed, the RF port of the EMI receiver is disconnected, and the capacitor needs to be grounded to provide a charge release path. Otherwise, in the high-voltage EMC test, there will always be high voltage on the 0.1µF capacitor, which cannot be completely discharged in a short time and needs to be discharged through the 1kΩ resistor to the ground.

Figure 13

The LISN RF port 50Ω Termination (termination load) is to maintain the symmetry of the two wires. During the EMC test, there are interference currents on the L line () and the N line (-), and the line interference that has not been measured still exists. A 50 ohm resistor needs to be terminated through the RF port of LISN to provide a path for it, otherwise it will affect the test results on the measurement line.

Figure 14

　　5 Precautions for use

The following figure is an example of EUT layout of bench-top equipment for conducted interference test in GB/T 9254.1:

Figure 15 Table top EUT test layout example

It can be seen from the figure that the standard has many detailed requirements for the test arrangement, such as height, distance, length, etc. The reason is that the power line test limit required by GB/T 9254.1 standard is 60dBV, and the conversion from dB to voltage is 1000V, that is, 1mV. The corresponding current is 20A, and the current at grade A is very small. The layout details of grounding resistance, distance, height, etc. are likely to affect the magnitude of this current. Therefore, the standard refines the requirements of EMC Test Setup. Its purpose is to ensure that the test results are credible and repeatable, and to reduce the influencing factors as much as possible, resulting in inaccurate test results, or changing the site, resulting in unrepeatable test results. Therefore, it is very important to carry out the test arrangement according to the standard, otherwise the test data may be invalid.

The common mode current/voltage of the conducted interference test is passed through the housing of the LISN to the metal ground reference plane (GRP), and then back to the equipment under test (EUT). Therefore, the connection between the LISN and the ground reference plane has a great influence on the measurement results. The FCC standard requires that the grounding impedance of the LISN be no higher than 2.5 mΩ.

Whether it is a single-wire LISN or a LISN that can test multiple wires, the shell and the ground reference plane should be connected with a short and thick conductor, preferably an L-shaped copper plate, so as to ensure a sufficiently low ground impedance.

In addition, it should be noted that the GND of LISN must be connected safely, which is not the same thing as the connection with the metal ground reference plane. The former is for safety, and the latter is for measurement accuracy.

Century Wisdom (Suzhou) Testing Technology Co., Ltd

www.300.cn nanjing SEO

Business License

全部

全部
产品管理
新闻资讯
介绍内容
企业网点
常见问题
企业视频
企业图册