Three Methods for Measuring Noise Figure of RF Devices

The computer network structured cabling system (SCS) is a module system based on star topology structure launched by experts from Bell Labs in the United States after years of research. It is also the preferred system for local area network construction. The system has the advantages of practicality, flexibility, economy, modularity and expandability. It can realize the interconnection of data communication equipment and other information management systems, as well as the connection of these devices with external communication networks. This structure is easy to expand and the expansion method is simple. The change of any node will not affect the work of other nodes. It is easy to maintain and has high reliability. By using SCS, users can accurately troubleshoot network failures and effectively manage and control the entire network system.

SCS is the most basic and important component of the network. It is the link between every server and workstation. As a medium for transmitting high-speed data, SCS has strict requirements on cables. Once a cable fails, it can cause the entire network system to be paralyzed in serious cases. Therefore, after the wiring project is completed, the entire wiring system must be fully tested. Usually, a testing team composed of technicians from the wiring company and the enterprise will conduct a continuity test on all information points. For example, the Category 5 test is conducted on 20% of all information points. After acceptance, the technicians will be responsible for the daily maintenance and management of the network.

Test instruments and test standards

Before testing, we need to select the appropriate test instruments and test standards. Usually, we use internationally recognized testers for testing. For example, when testing copper cables, we use cable testers to perform basic connectivity (conductivity) tests. When testing Category 5 cables, we use Microsoft's Pentascanner Category 5 tester to test Category 5 cables. When testing fiber optic loss, we use Microsoft's fiber optic cable tester. At the same time, we choose the EIA/TIA568ATSB-76 standard as the basis for testing.

Specific testing methods

At present, there are three main types of cables used in network construction, namely optical fiber, unshielded twisted pair, shielded twisted pair quotation, parameters, pictures, group music and coaxial cable. Optical fiber has the characteristics of high transmission rate, good anti-interference performance and long-distance transmission, and is mainly used for trunk lines. Unshielded twisted pair is a widely used communication medium in recent years. It has good transmission performance and can be switched by switches or hubs. It is often used for short-distance transmission. Coaxial cable is a transmission medium widely used in early networks. Due to various disadvantages, it is less used now.

1. Unshielded twisted pair test

From an engineering perspective, structured cabling unshielded twisted pair testing can be divided into two categories: one is continuity testing and the other is certification testing.

In order to ensure that the cable installation meets the performance and quality requirements, the construction workers will test it during the construction process. This method is called continuity test, which can ensure that every completed connection is correct. The continuity test focuses on the connection performance of the structured cabling and does not care about the electrical characteristics of the structured cabling.

Certification testing refers to testing the structured cabling system in accordance with the standards to determine whether the structured cabling meets all the design requirements. Usually, the channel performance of structured cabling depends not only on the construction process of the cabling, but also on the quality of the cables and related connection hardware used. Therefore, certification testing, also known as Category 5 testing and certification, must be done on structured cabling. Through testing, we can confirm whether the installed cables, related connection hardware and their processes can meet the design requirements. This test includes connection performance testing and electrical performance testing.

(1) Link verification test

Cable installation is a work that focuses on installation craftsmanship. Since no one can work completely flawlessly, we must perform link testing to ensure that the cable installation meets performance and quality requirements. Without testing tools, some errors may occur in the connection work. Common connection errors include incorrect cable labels, open circuits, and short circuits.

① Open circuit and short circuit During construction, open circuit or short circuit faults may occur due to lack of tools, wiring skills or wall threading technology.

② Reverse the connection of the same pair of wires by connecting the pin positions at both ends in reverse, for example, one end is 1-2 and the other end is 2-1.

③ Connect one pair of wires to another pair of wires at the other end, for example, one end is 1-2 and the other end is connected to 4-5.

④Series winding The so-called series winding refers to the process of separating the two original pairs of wires and then re-forming a new pair of wires. Since the end-to-end connectivity is not affected when this fault occurs, the cause of the fault cannot be checked with an ordinary multimeter and can only be checked by using a dedicated cable tester. [page]

Certification testing does not improve the channel performance of the integrated wiring system, but only confirms whether the installed cables, related connection hardware and its process can meet the design requirements. Only when the test instruments that meet the specific requirements are used and the test methods are followed, the results are valid.

For example, use Microsoft's Pentascanner 5-category tester to perform 5-category testing. The method is: first connect the two ends of the jumper with the tester, then press AutoTEST to test, then press F1 to display the test results, and finally print the test results.

The certification test includes 9 items and 5 categories of technical indicators, including Length, Next (Near end crosstalk), Attenuation, Acr (Attenuation to crosstalk), Wire Map, Impedance, Capacitance, Loop Resistance and Noise. When all test results are "PASS" (as shown in Figure 1), it means that the cabling system meets the transmission technical requirements of Category 5 Cable.

If some problems occur during the test, we can analyze them from the following aspects and then troubleshoot them one by one.

① The reason for the failure of near-end crosstalk to pass may be a problem with the near-end connection point, or it may be due to the string pair, external interference, short circuit of the far-end connection point, performance problems of the link cable and connection hardware, not the same type of product, and cable termination quality problems, etc.

② The wiring diagram fails because the connectors at both ends are open, short, cross or broken, or there is a jumper error, etc.

③ The reason for attenuation failure may be that the cable is too long or the temperature is too high, or there is a problem with the connection point, or it may be a performance problem of the link cable and connection hardware, or they are not the same type of products, or there may be a problem with the cable termination quality, etc.

④ The reason for the failure of length test may be that the cable is too long, open circuit or short circuit, or the total length of the equipment connection and jumper wire is too long.

⑤ Tester failure. The possible causes of the failure are that the tester does not start (this problem can be solved by replacing the battery or charging it), the tester does not work or cannot perform remote calibration, the tester is set to an incorrect cable type, the tester is set to an incorrect link structure, the tester cannot store automatic test results, and the tester cannot print stored automatic test results.

2. Fiber optic transmission channel test

Although there are many types of optical fibers, the basic testing methods for optical fibers and their transmission systems are generally the same, and the testing instruments used are also basically the same.

The fiber characteristics test must be carried out on the spliced ​​optical fiber or optical fiber transmission system to make it meet the test standards of optical fiber transmission channel. The basic test contents include continuity and attenuation/loss, optical fiber input power and output power, analysis of optical fiber attenuation/loss and determination of optical fiber continuity and the location where optical loss occurs. The actual test also includes contents such as optical cable length and delay. The main fiber test index is attenuation. If the attenuation is within the standard range, it is "PASS", otherwise it is "FALL". If there are some problems in the process of testing the optical fiber, we need to check whether the optical fiber splicing is correct and whether the optical fiber heads correspond one to one.

Wiring is an investment for the future, so you must be forward-thinking when wiring, especially emphasizing the quality of wiring, because the quality of wiring directly affects whether the network can work properly. Wiring testing is an indispensable "threshold" to ensure the normal operation of the network. Only after strict testing can the network system be put into operation.