Analysis of Conducted Disturbance Measurement of Telecom Port_Century Wisdom (Suzhou) Testing Technology Co., Ltd

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Analysis of Conducted Disturbance Measurement of Telecom Port

Release time:

2022-05-23 00:00

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　　1 Introduction

　　Conducted Emission Test, referred to as CE Test, is also commonly referred to as Conducted Emission Test or Disturbance Voltage Test.

　　According to the nature of the conductor, we usually divide the conduction disturbance into: conduction disturbance at the power supply end and conduction disturbance at the telecommunications end. The power port can generally be measured by using a linear impedance stabilization network LISN (Line Impedance Stabilization Network). As for the telecommunications port, due to the wide variety of cable types, it is necessary to use different measuring instruments and adopt different test (arrangement) methods.

　　2 What are the methods for measuring the conducted disturbance of the telecommunication port

　　There are three commonly used test methods for conducting disturbance measurement of telecommunication ports: voltage method, current method, and voltage & current method. Next, we will introduce the three test methods respectively.

　　2.1 Voltage method

　　Which cables does the R voltage method apply to?

　　The voltage method is usually applicable to unshielded balanced lines (less than or equal to 4 pairs), shielded/coaxial lines.

　　Brief description of the R test method

　　Insert the ISN into the port (cable) under test. The ISN picks up the common-mode disturbance voltage on the cable under test and transmits it to the receiver. The final measured voltage value is compared with the voltage limit to determine whether it meets the requirements of the standard regulations.

　　Calculated as follows:

　　V=U+ F

　　In the formula:

　　V is the final measurement common mode disturbance voltage, the unit is dBuV;

　　U is the port measurement voltage, the unit is dBuV;

　　F is the ISN voltage division coefficient, usually 9.5dB or 34dB;

　　R test arrangement

　　1- 40cm for desktop equipment, no more than 15cm for vertical equipment

　　2- There is no rigid regulation on the distance between AE and GRP

Figure 1 Schematic diagram of test layout for telecom port voltage method

　　How to choose R measuring instrument

　　In order to ensure the ground impedance of 150 ohms, different cable types (line pairs) need to choose different types of ISNs, which we need to distinguish when purchasing and using.

　　For a pair of unshielded balanced lines (such as xDSL), the ISN with the structure in Figure 2 can be used to measure the conducted disturbance voltage of the telecommunications port.

Figure 2 ISN for 1 pair of unshielded balanced lines

　　For 2 pairs of unshielded balanced lines (such as FXS), the ISN with the structure shown in Figure 3 and Figure 4 can be used to measure the conducted disturbance voltage of the telecommunications port. Due to different coupling impedances, Figure 3 and Figure 4 have different voltage division coefficients. The ISN voltage division coefficient of the structure in Figure 3 is 34dB, and the ISN voltage division coefficient of the structure in Figure 4 is 9.5dB.

　　Ca=33nF

　　Ra=576Ω

　　Rb=6Ω

　　Rc=44Ω

　　L1=4x7mH

Figure 3 ISN for 2 pairs of unshielded balanced lines (voltage division factor=34dB)

　　Ca=33nF

　　Ra=400Ω

　　L1=4x7mH

Figure 4 ISN for 2 pairs of unshielded balanced lines (voltage division factor=9.5dB)

　　For 4 pairs of unshielded balanced lines (such as UTP5), the ISN with the structure shown in Figure 5 and Figure 6 can be used to measure the conducted disturbance voltage of the telecommunications port. Due to different coupling impedances, Figure 5 and Figure 6 have different voltage division coefficients. The ISN voltage division coefficient of the structure in Figure 5 is 34dB, and the ISN voltage division coefficient of the structure in Figure 6 is 9.5dB.

　　Ca=33nF

　　Ra=1152Ω

　　Rb=6Ω

　　Rc=44Ω

　　L1=8x7mH

Figure 5 ISN for 4 pairs of unshielded balanced lines (voltage division factor=34dB)

　　Ca=33nF

　　Sun=800Ω

　　L1=8x7mH

Figure 6 ISN for 4 pairs of unshielded balanced lines (voltage division factor=9.5dB)

　　In addition to the ISN with the above structure, the ISN with the high LCL structure in Figure 7 can be used to measure multiple pairs of unshielded balanced line telecom ports. This type of ISN can take into account the measurement of 1 to 4 pairs of balanced lines, and it is also the most mainstream today. ISN.

　　C=82 nF

　　Rd=390Ω

Figure 7 ISN for 1 pair, 2 pairs, 3 pairs, 4 pairs of unshielded balanced lines (voltage division factor = 9.5dB)

　　For shielded cables, there are currently two types of ISNs, the coaxial cable ISN shown in Figure 8, and the multi-core shielded cable ISN shown in Figure 9.

Figure 8 ISN for coaxial cable (voltage division factor = 9.5dB)

　　L1=(n+1)x7mH, n is the number of signal lines

Figure 9 ISN for multi-core shielded cables (voltage division coefficient = 9.5dB)

　　R Measuring instrument ISN technical specification requirements

　　In order to ensure the repeatability of measurement results, the standard has the following requirements for ISN performance:

　　a. The voltage division coefficient of ISN needs to meet the standard requirements, 9.5dB or 34dB

　　b. 150 ohms common mode impedance within the measurement frequency range

　　c. The AE end needs to have sufficient isolation factor

　　2.2 Current method

　　Which cables are suitable for the R current method

　　The current method is usually suitable for shielded cables

　　Brief description of the R test method

　　The shielding layer is connected to the GRP through a 150 ohm resistor, and the length of the connecting wire is not more than 30cm. Use a ferrite close to the AE terminal to provide sufficient common-mode impedance. Use the current probe to measure the current at a distance of 10cm from the connection point. The current probe converts the magnetic field into a corresponding voltage signal and transmits it to the receiver. The measurement software converts the received voltage into a current through the conversion coefficient of the current probe to obtain different frequencies. The amplitude value of the upper disturbance current. The final measured value is compared with the current limit value to judge whether it meets the requirements of the standard regulations.

　　Calculated as follows:

　　I=U+ F

　　In the formula:

　　I is the interference current, the unit is dBuA;

　　U is the port voltage, the unit is dBuV;

　　F is the conversion factor of the current probe, the unit is dB/Ω.

　　R test arrangement

　　1- 40cm for desktop equipment, no more than 15cm for vertical equipment

　　2- There is no rigid regulation on the distance between AE and GRP

Figure 10 Schematic diagram of test layout for telecommunication port current method

　　R Specification Requirements for Current Probes

　　a. There must be sufficient measurement bandwidth, that is, -3dB bandwidth

　　b. Insertion impedance is not greater than 1 ohm

　　2.3 Current and voltage method

　　What cables does the R apply to?

　　The current voltage method is suitable for unshielded balanced lines, unshielded unbalanced lines without suitable ISN

　　R test method

　　Simultaneously use capacitive voltage probes and current probes to measure voltage and current values. The current measurement value is compared with the current limit value, the voltage measurement value minus the current margin (the current margin is less than 6dB) or the voltage measurement value minus 6dB (the current margin is greater than 6dB), the adjusted voltage measurement value and the voltage limit value Compare. The current and voltage values must meet the current and voltage limits at the same time.

　　R test arrangement

　　1- 40cm for desktop equipment, no more than 15cm for vertical equipment

　　2- There is no rigid regulation on the distance between AE and GRP

Figure 11 Schematic diagram of test layout for telecommunication port current and voltage method

　　R Measuring Instrument Specifications Requirements

　　current probe

　　a. There must be sufficient measurement bandwidth, that is, -3dB bandwidth

　　b. Insertion impedance is not greater than 1 ohm

　　capacitive voltage probe

　　a. The capacitance between the tested cable and the cable is less than 10pF

　　b. Typical voltage division coefficient 34dB

　　c. Sufficient measuring caliber

　　d. Good anti-interference ability

　　3 A picture summarizes the measurement method of conducted disturbance of telecommunication port

　　The disturbance voltage (or current) limit is defined on the basis of 150 ohm common mode impedance. Therefore, the uniform standard common mode impedance is extremely important to the reproducibility of test results. Generally speaking, if the ISN is not used, the common mode impedance seen from the EUT to the AE end is uncertain, but not all types of cables have a suitable ISN, so when there is no suitable ISN, it is necessary to The test is carried out using a non-invasive method, namely the current and voltage method.

　　4 A graph summarizing the advantages and disadvantages of various measurement methods for conducted disturbance of telecommunication ports

　　Different types of cables require different test methods, and there may be multiple test methods for the same cable. We can choose the appropriate test method according to the actual cable type.

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