Selection and technical application of the hottest

Cable selection and technical application

1 Types of cables

cables look very different in appearance because they are used in many different environments. However, regardless of any cable type, they are all used as a kind of conductor for signal transmission. These different types of cables also have different performance in the quality of transmitting different signals. Except for some special applications, at present, the cables used for audio and video transmission are generally a single one, some white pollution can be turned into green environmental protection products, and the metal will inject ions into the surrounding tissue wires, twisted pairs, coaxial lines and optical fibers

1.1. Single wire

single wire is the most basic type of cable (such as wire), which is composed of one or a group of wires surrounded by plastic protective layer. This kind of cable is widely used to transmit low-frequency signals, such as power supply, audio, computer ID code

1.2. Twisted pair

twisted pair is a general term. There are no restrictions on the number of wires and the type of stranding, but there are only two types of cable structure: shielded twisted pair (STP) and unshieldedtwistedpair (UTP)

twisted pair is widely used in telecommunications, interconnection and professional audio. This kind of cable is composed of two or more independent and mutually insulated cables continuously twisted. Two of the cables twisted together are called groups, and the transmission impedance is generally 100 _, The diameter specification of a single wire is between 20awg (American cable standard: 0.91mm) and 24awg (0.61mm). Twisted pair cable is a relatively cheap cable. Each group of wires has the same anti-interference ability, which can effectively suppress external electromagnetic interference (EMI) and effectively shield the external electromagnetic interference caused by the transmitted signal

utp cables are most commonly used in telecommunication transmission and computer network environments. They are divided into five categories, super five categories and six categories according to the type of stranding, and generally can reach a transmission rate of 100Mbps (100 million bits per second). STP cable adds a layer of braided metal or tin foil on the periphery of the conductor group, which is more conducive to improving the signal and suppressing the impact of external radio waves. The metal shell of each connector of STP cable must be in good contact with the shield

1.3. Coaxial cable

coaxial cable is a composite composed of two conductors, as shown in the following figure: the central conductor of coaxial cable is used to transmit signals, and metal shielding plays two roles: one is to provide current circuit for signals as the common ground wire of signals, and the other is to act as the shielding of signals to suppress the interference of electromagnetic noise on signals. The central conductor and shield are between the semi foamed polypropylene insulation layer, which determines the transmission characteristics of the cable and effectively protects the middle conductor

coaxial cable is widely used in the transmission of audio and video or radio frequency, and the transmission impedance is generally 75 °, It has become the standard impedance of video (50 _ impedance characteristic will also be used for video transmission in the early stage). High quality standard coaxial cable is generally more expensive than twisted pair cable, because the reliable physical characteristics of coaxial cable can provide excellent audio and video performance. The frequency and resolution of the signal and the effective transmission distance of the cable play a decisive role in the audio and video system

1.4. Optical fiber

optical cable is the best choice for long-distance signal transmission. Optical fiber transmission is a technical means based on photoelectric conversion to replace electronic transmission. The simple principle of optical fiber transmission is: the analog electrical signal is transmitted to the optical transmitter, and the input voltage signal is converted into current signal through the signal buffer circuit and drive circuit to drive the light-emitting tube or laser. In this way, the input electrical signal is converted into an optical signal, which is coupled into the optical fiber through precise optical alignment and guidance

after the optical signal is transmitted through the optical fiber, the optical signal at the receiving end is converted into the original electronic source by a photodiode with matching wavelength, and then output after being amplified by a low-noise linear amplifier

optical fiber signal transmission avoids many shortcomings of traditional cable transmission and has many advantages that cable transmission cannot compare

excellent anti electromagnetic interference: in long-distance cable transmission, the cable itself is a huge antenna, which will pick up the electromagnetic wave signals existing in the surrounding space, especially in the display system. This kind of interference signal shows particle noise that cannot be eliminated in the image display. The core of the optical fiber cable is glass, and it transmits optical signals, which is not easy to be disturbed by external electromagnetic waves

very small volume: most optical fibers are as thick and thin as human hair

very low attenuation: because optical fiber transmission is completed by glass conduit, there is no significant attenuation of signal caused by cable resistance and capacitive reactance. Optical fiber greatly improves the bandwidth capacity and transmission distance of transmission. High security of optical fiber: the signal content transmitted by optical fiber is not easy to be eavesdropped

although optical fiber seems to be the ultimate method of signal transmission, it also has some disadvantages

higher price: optical cables, transmitters and receivers are expensive

higher labor: in the process of laying and wiring optical cables, many human resources and special tools are required. Although the transmission loss of optical signal in optical fiber is very low, the electro-optical and photoelectric conversion at the transmitter and receiver attenuates the signal severely. Therefore, to ensure insertion loss free transmission, it is necessary to add a high gain multistage amplifier in the transmission and ensure that the circuit can work stably

2. Cable structure

most video cables are only connected between two adjacent devices. Such a short distance does not seem to have many problems to consider. However, in a perfect video system, these video devices are even distributed in every corner of the building, and the transmission cable has completely become a crucial factor. What factors of the transmission cable will most directly affect the final effect of the video signal? We take the single transmission of coaxial cable as an example

2.1. Structure type of wire

wire refers to the conductor used to transmit electrons in the center of the cable, and the most commonly used material is copper. Copper wire is one of the most cost-effective transmission methods. Other materials include aluminum, silver and gold

the size of coaxial cable varies according to the wire diameter. There are two types of wire structures: single cable and multiple cable combinations. The structure of a single cable is easy to form, but the protective sheath and insulating layer are relatively hard, so it is not flexible. Multiple cables are twisted by many cables with small wire diameters, which increases the flexibility of cables. It is generally believed that the transmission quality of a single cable is better than that of a combination of multiple cables

2.2. Measurement of wire diameter

in principle, the thicker the cable under the same conditions, the longer the transmission distance, because the same length, the thicker the cable has lower DC impedance than the thin cable

2.3. Dielectric

dielectric is the insulator of cable, which plays a dual role in coaxial cable: first, dielectric is between conductor and shield, forming an important organization to protect conductor; Another more important role is that it determines the physical characteristics of the cable. Such as the impedance and capacitive reactance of the cable. Impedance is 75_ The dielectric of cable is usually semi foamed polyethylene

2.4. Shielding

shielding tightly surrounds the conductor and dielectric in the cable, playing two roles: one is to provide the current circuit for the signal as the common ground wire of the signal, and the other is to act as the shielding of the signal to suppress the interference of electromagnetic noise on the signal

the shielding structure is metal braid and tin. Most enterprises' financing range is between 5million and 50million yuan. Foil is the most common. Metal braiding has lower DC impedance than tin foil, but the shielding rate of electromagnetic interference is only 90%, and the shielding rate of tin foil to electromagnetic interference is as high as 100%. Therefore, many professional cables will adopt the double shielding structure of metal braiding and tin foil, which can more effectively improve the signal-to-noise ratio of the signal

2.5. Protective layer

protective layer is a layer of skin that comprehensively protects all components inside the cable, and is easily affected by temperature, chemicals, liquids and sunlight. The protective layer of the cable must adapt to the installation environment under any conditions. These standards are managed by NEC (nationalelectriccode) and have UL (American underwriters' laboratory) certification:

flame retardant certification: the common raw material of cable protective layer is well insulated and cheap PVC (polyvinyl chloride). Flame retardant certification requires that cables not only have special fire protective layer, but also use special insulating materials, which can prevent fireworks caused by cable exposure. Cables with flame retardant certification can be used in outdoor space, and indoor applications do not need pipe protection

halogen free certification: halogen will be widely used as a mixture in the manufacturing process of PVC protective layer of most cables. Halogen has extremely high flame retardant and insulating chemical properties, but it will produce toxic smoke (the main component is gas) at high temperature. Halogen free cable protective layer will not produce smoke and toxic gas at high temperature, which is the European safety standard (No.: iec33203, iec61034 and IEC)

3 Cable performance and specification

3.1 Length

the attenuation of the signal is proportional to the length of the cable. The longer the cable, the greater the attenuation. This is the physical law of the cable. The length of the cable is generally marked in feet or meters. Audio and video signals will cause signal attenuation through long-distance cables. The impact on the final effect is reflected in the reduction of signal-to-noise ratio, brightness, image blur and poor synchronization. These obvious differences also become the basis for comparing the cable quality (different attenuation of the cable per unit length for the transmission of the same signal)

3.2. Frequency

the capacitive reactance of the cable and the conductor material determine the range of the transmission signal frequency. If the image is blurred within the appropriate transmission distance, most of the cables do not meet the requirements of high-frequency transmission, resulting in the loss of high-frequency bands of the signal (details of the image)

3.3. Interference

cable is also a huge antenna, which will absorb electromagnetic waves in space. If the cable is not shielded or the shielding effect is poor, any type of electromagnetic interference will directly act on the useful signal, reducing the signal-to-noise ratio of the signal

3.4 Temperature

like all electronic circuits, the physical characteristics of the cable will also be affected by the ambient temperature, and the physical parameters of the cable have different performances in different temperature ranges. In engineering applications, cables are typically covered on walls, ceilings and instrument racks, because the ventilation conditions in these places are not ideal and are prone to higher temperatures. Therefore, the allowable temperature range of cables should be suitable for these environments

3.5. Slope

slope describes the time difference of signal transmission caused by different lengths of twisted pair, which depends on the stranding process and stranding type of twisted pair. When there is a large delay error, it is necessary to compensate the slope of the cable

3.6. Impedance

impedance is one of the important parameters to describe the technical specifications of cables. It establishes a baseline for the correct flow of signals

this signal process maintains the power conversion of the whole system

imagine that the water flows through a large-diameter water pipe. As long as the diameter of the water pipe remains constant, the structure and flow of the water flow will not change. When the water flow was introduced into a small water pipe, the situation changed: due to the existence of a bottleneck, the structure of the water flow was disrupted, and all the water flows could not be at the same time