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Reprinted from 272501530 at 23:15 on November 11, 2010 Reading (loading. ..) Comments (0) Category: Personal Diary
1.
the type of cable in many environmental applications due to the cable, so the appearance is very different point of view. But regardless of the type of cable, their nature is still the same, they are transmitted as a signal of a conductor. Different types of cables, the quality of the signal transmission performance is a big difference. Except for special applications, the current audio transmission cables generally have these types of single-wire, twisted pair, coax and fiber.
1.1.
single conductor cable single conductor is one of the most basic types (such as wire), which consists of one or a group surrounded by plastic wire coating composition, this cable is generally used to transmit low frequency signals, such as power, audio, computer ID code.
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1.2.
twisted pair is a generic term, the number of wires and strands The type and no restrictions, but the structure of the cable are only two types: shielded twisted-pair network (STP: Shielded Twisted Pair) and non-shielded twisted-pair network (UTP: Uunshielded Twisted Pair).
twisted pair common in the telecommunications, Internet, widely used in professional audio, this cable from two or more independent, mutually insulated wire cable composed of continuous strands, each strand of which is called the group of two cables, transfer impedance is generally 100_, the diameter of single wire specifications 20 AWG (American wire standard: 0.91mm) to 24 AWG (0.61mm) between. Is a relatively inexpensive twisted pair cables, each set of wires with the same anti-jamming capability, can effectively inhibit the external electromagnetic interference (EMI), also effectively shield the transmission signal to external electromagnetic interference.
UTP cable is the most commonly used in telecommunications transmission and computer network environment, according to the type of twisted divided into five categories, over five and six cables, generally up to 100 Mbps (100 million digits per second) transmission rate. STP cable in the wire group, adding a layer of woven outer metal mesh or foil, help to improve signal suppression outside the impact of radio waves. STP cable's metal shell of each connector must maintain good contact with the shielding.
1.3. Coaxial
coaxial cable (Coaxial) is a compound consisting of two conductors, as shown below: coaxial cable center conductor for the transmission of signals from two metal screen effects: first,
Nepal and India monks - Qzone log, public ground as a signal to current loop signals, and second, the shield as a signal net, suppression of electromagnetic noise on the signal interference. Between center conductor and shielding half of polypropylene foam insulation between the cable insulation determines the transmission characteristics, and the effective protection of the middle of the wire.
; coaxial cable is widely used in audio and video, or radio frequency transmission, transfer impedance is generally 75_, has become the video standard impedance (early will also use 50_ impedance for video transmission). Quality standard coaxial cable generally more expensive than twisted pair, coaxial cable and reliable because of the physical characteristics, can provide excellent audio and video performance. Signal frequency, resolution and the effective transmission distance cable systems in the audio and video played a decisive role.
1.4. Fiber
fiber optic cables (Optic Cable) signal is the best choice for long distance transmission, optical transmission is replaced by a photoelectric conversion based electronic transmission of technical means. Optical fiber transmission principle is simple: Analog signal pass optical transmitter, the signal buffer circuit and driver circuit, the input current signal into a voltage signal to drive LED or laser. Thus, the input signal is converted to optical signals by precise optical alignment and guidance, coupled into the fiber. Optical signal by optical fiber transmission, optical signal at the receiver by a photo diode wavelength converted to match the original electronic source, the low-noise linear amplifier and then output.
optical signal transmission to avoid many of the shortcomings of traditional cable transmission, cable transmission can not be compared with many advantages;
excellent anti-electromagnetic interference: long cable, the cable itself is a huge antenna, there will pick up the electromagnetic signal around the space, especially in the more prominent display system, such interference signals shown in the image shows the particles can not eliminate the noise, the core of a glass fiber optic cable, and optical signal transmission is not susceptible to external electromagnetic interference.
very small volume: most of the fiber and the body as thick hair.
low attenuation: as optical transmission is to rely on completion of the glass tube, there is no signal because the cable resistance, capacitance caused significant attenuation, fiber greatly improves the transmission bandwidth capacity and distance. High security optical fiber: Fiber content of the transmitted signal is not easy to be tapped.
Although fiber seems to be the ultimate method of signal transmission, but there are some negative points ;
higher prices: fiber optic cable, transmitters, receivers are expensive
; higher labor: in the process of fiber cloth tube wiring, you need a large number of human resources and special tools. Although the optical signal in optical fiber transmission loss is very low,
new balance mens, but the transmitter and the receiver side of the electro-optical, photoelectric conversion on the signal attenuation is very powerful. Therefore, the insertion loss to ensure that no transmission, it must be added in the transmission of high-gain multi-stage amplifier circuit can also guarantee stability.
2.
Cable structure Most of the video cable is connected between two adjacent devices, such a short distance, does not seem much to consider. But in a perfect video systems, video equipment and even these are found in every corner of the building, the transmission cable completely became crucial factors. Transmission cable to those factors most directly affect the final result of video signal? We have a single coaxial cable transmission as an example.
2.1.
structure type wire cable center conductor means for transmitting electronic conductor, the most commonly used material is copper. Copper is the most cost-effective means of transmission, one of the other materials, including aluminum, silver and gold.
Coaxial cable according to the different diameter, shape differences in size. Conductor structure is divided into two types: single cable and multiple cable combinations. The structure of a single cable easy to shape, but the protection of the skin and the insulating layer of relatively hard, it seems there is no flexibility. Multi-wire cable composed of many small strands forming the cable, increasing the flexibility of the cable. Generally believed that the transmission quality of a single cable is superior to multiple combinations of patterns.
2.2. Wire diameter measurement
principle, the more coarse the same conditions as the more cable for longer distance transmission, because the same length, thick than thin cables with lower DC resistance cables.
2.3.
dielectric dielectric insulator that is, the cable, the coaxial cable served as a double role: first, between the conductor and the dielectric shielding between the formation of a protective conductor of the key organizations; a more important role is that it identifies the physical characteristics of the cable. Such as cable impedance and capacitance. Impedance 75_ cable dielectric is usually half of polyethylene foam.
2.4. Shielding
network in the cable shield and dielectric to conductor surrounded by tight, played two roles: one as a signal of public ground for the current loop signals, and second, the shield as a signal net,
new balance sneakers, suppression of electromagnetic noise on the signal interference. The structure of metal shielding braid and foil the most common. Metal foil with braid lower than the DC resistance, but the electromagnetic interference shielding rate of only 90%, tin foil for electromagnetic interference shielding rate of up to 100%. So many professional cables will take the metal braid and foil double shielding structure can more effectively improve the signal to noise ratio.
2.5. Protective layer
cable protective layer is a full protection of all components within the layer of skin, is very susceptible to temperature, chemicals, liquids and sunlight effects. Cable protective layer must be adapted to any of the conditions of the installation environment. These standards are NEC (National Electric Code) management and has UL (Underwriters Laboratories) certification:
retardant certification: raw materials commonly used in cable protection layer is a good and inexpensive insulated PVC (polyvinyl chloride), flame retardant cable is not only a special certification requirements for fire protection layer, and it must use a special insulating material to prevent the cable exposed to smoke generated by the event. Certified through the flame-retardant cables used in the outdoor and indoor applications can not PIPELINE.
Halogen Certification: most protective layer in the PVC cable halogens are widely used in the manufacturing process (Halogen) as a mixture of halogen flame-retardant and insulation with high chemical properties, but at a high temperature will produce toxic fumes (mostly gas). Halogen-free cable in high temperature protective layer will not produce smoke and toxic gases, which is European security standards (No: IEC33203, IEC61034 and IEC754-1).
3. cable performance and specifications
3.1. length of the
signal attenuation is proportional to the length of the cable, the longer the cable, the greater the attenuation, which is the cable laws of physics. Cable length in feet or meters to the general annotation, audio and video signal through long cables can cause signal attenuation, the final effect is reflected in lower signal to noise ratio,
new new balance shoes, reduce the brightness, image blur and poor synchronization of these apparent differences in cable quality has become the basis for comparison (unit length of cable to transmit the same signal with different attenuation.)
3.2.
frequency capacitance cable and wire materials determines the transmission signal frequency range, at the right distance,
new balance running shoes, if the picture appears blurred, most of the cable does not meet the requirements of high-frequency transmission, resulting in loss of high frequency signals (image for details).
3.3.
cable interference is also a huge antenna, will absorb the space existence of electromagnetic waves. If the cable is bad there is no shielding or shielding, electromagnetic interference of any kind will be directly on the useful signal, lower signal to noise ratio
3.4. Temperature br>
As with all electronic circuits, the physical characteristics of the cable will be affected by environmental temperature, the cable physical parameters in different temperature ranges have different performance. In engineering applications, the cable typically in the walls, ceilings and equipment rack cover design, as the ventilation conditions of these places will not be ideal, easy to produce a higher temperature. Therefore, the choice of cable allows the use temperature range should be applied to these environments.
3.5. Slope
slope is to describe the pair have different lengths of signal transmission time difference, depending on the twisted pair of strands twisted type of process and, when the errors have a greater delay, the need for cable slope compensation.
3.6.
impedance cable impedance is to describe one of the important parameters specifications It processes the signals correctly established the baseline. This signal is maintained throughout the process of power conversion system.
imagination flow of water through a large-diameter pipes, has been maintained as long as the pipe diameter, flow structures and processes will not change. When the water is introduced into a small opening through the pipes, the situation changed: the existence of the bottleneck, the flow structure is disrupted by all of the flow can not be a bottleneck, causing part of the currents flow the opposite direction, and finally into the mainstream of water flow again.
cable transfer impedance mismatch can also cause similar to the above phenomenon: the electronic signals to be re-imported to the original signal of known as reflection, transmission interference wave and the reflected wave amplitude to form the results of the voltage standing wave, with the VSWR (Voltage Standing Wave Ratio) said.
video system, the impedance matching is required in system design issues seriously. Early in the BNC coaxial cable connector type exist 50_ and 75_ two specifications, and now the video impedance unity 75_, 50_ impedance of the cables and connectors in the RF signal in the current application only. Short impedance mismatch will affect the image of the high-frequency detail, causing the screen a ;
the attenuation of the signal cable,
new balance outlet, also known as insertion loss is expressed in decibels (dB), the cable will provide a formal loss of form, description of unit length of cable attenuation of different frequencies value. Attenuation of a cable such as that to mean this cable length 30 meters, the transmission bandwidth of the signal will produce 100MHz-2.2dB insertion loss. Cable insertion loss is cumulative, and the signals of different frequencies are not the same attenuation. The same signal bandwidth, double the length of the cable, insertion loss is also doubled,
new balance shoes, such as the cable length at 60 meters, the transmission bandwidth of the signal will produce 100MHz-4.4dB insertion loss.
Although the design of the cable
insertion loss is necessary, but often the absence of pre-calculation, the cable insertion loss, high-frequency loss of the system are common. In this case, the front-end electronic compensation may be the only solution. So most of the interface and line driver are included to adjust the compensation circuit, usually have level (or gain) and peak adjustment.
By 30 meters Extron BNC5-Mini cable transmission 150MHz bandwidth signal, the signal is not After equalization and amplify any, in the 150MHz band appeared close to-12dB insertion loss, and 0 ~ 150MHz frequency band transmission curve is very steep;; the use of electronic equipment (such as interface, equalizer, amplifier) to signal the appropriate level and peak adjustment, not only the cable-3dB insertion loss decreased to less than 0 ~ 150MHz frequency band but also the transmission curve of the response has been very smooth.
4.1.
level control level control is inserted to compensate for cable attenuation of the signal amplitude, due to insertion of the cable is easy to cause 5% of the signal amplitude or more of the decay, such as an analog signal 0.7V, 5% of the cable insertion loss of up to 35mV of signal attenuation. Usually level control is set to compensate for the attenuation of low frequency, and to restore the signal amplitude in the normal level (eg 0.7V)
4.2. Peak Control
peak control is inserted to compensate for cable attenuation of high frequency signal, high frequency signal attenuation mainly affects the image details. Peak control can overcome high-frequency signals generated in the cable transmission attenuation curve, but will not transition to enhance high frequency information signals,
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听说爱情回来过(很感触的歌词) - Qzone日志, level control and peak control can be used simultaneously. Test equipment and display equipment in some of the data that rely on experience to calculate the cable transmission distance is the real reason cause system failures, although most designers are already aware of the cable is the weakest link in any system design, but still be arranged Finally, an element of consideration.
4.3.
DC resistance of a resistor species is defined as DC resistance of general use cable _ / km indicated by the cable material, diameter, size and temperature of the decision. DC resistance of the signal voltage (amplitude) and long-distance transmission cables or high-resistance cables will make the image look bleak. For example, a 0.7Vp-p video signal via the 60-meter-long cable (cable DC resistance 41_) transmission for display, showing the true side by the signal level available for only about 0.63Vp-p (signal level loss about 10%). Good general composite video cable DC resistance per km should 35_ less.
4.4.
Capacitance
capacitance is defined as The units with pF / m (slightly Law / per meter).
existing coaxial cable capacitance will affect the sharpness and detail of the video signal, which is Because there may be some black video signal - a white shift, the capacitance of the cable will affect the signal change in the rise and fall time, which is affecting the video signal in terms of sharpness and detail changes. Good general composite video cable capacitance per meter distribution should be 60pF or less.
Linear Mode
