Comparison of SYV video cable and SYWV video cable

Same point

1. The same characteristic impedance - 75 ohms;

2. The outer sheath, the shielding layer structure, the outer diameter of the insulating layer, the number of the selection, the material selection, and the number of shielding layers are basically the same.

Difference between SYV and SYWV

1. The physical properties of the insulating layer are different: syv is 100% polyethylene filled, dielectric constant ε = 2.2-2.4; and sywv is also filled with polyethylene, but filled with 80% of nitrogen bubbles, polyethylene only contains 20%, macro The average dielectric constant ε=1.4 or so; ε=εǎ?jε", where ε" is the loss term, and the air ε" is basically "0". This process was achieved in the 1990s, which effectively reduced the coaxial cable. Dielectric loss;

2. The diameter of the core wire is different: 75-5 is taken as an example. Due to the structure standard of the-5 cable, the outer diameter of the insulation layer (ie, the inner diameter of the shielding layer) is 4.8 mm, which cannot be changed. In order to ensure the characteristic impedance of 75 ohms, the characteristic impedance It is only related to the diameter ratio of the inner and outer conductors and the dielectric constant ε of the insulating layer. The ε large core wire is thin, the ε small core wire is thick, the core wire diameter: syv is 0.78-0.8 mm, and the sywv is 1.0 mm; It can be single or multiple strands; this difference leads to a difference in core resistance. Such as the measured Tiancheng, Aipu syv75-5 cable, 1000 m core wire DC resistance 39Ω, typical sywv75-5 cable, 1000 m core wire DC resistance 19-20Ω;

3. The above two fundamental differences determine the transmission characteristics of the two cables - the transmission attenuation is different, the syv cable is the earliest coaxial cable, which has been transmitted for several hundred years, including the transmission of RF signals; Later, when sywv appeared, the frequency band above the RF was rarely applied to syv. Because the difference in high-frequency attenuation is too great; slowly syv is basically used mainly in surveillance video transmission, and the "elementary" of this RF cable is renamed "video cable". But this does not mean that: syv "video cable" video transmission characteristics better than sywv, the actual opposite, sywv video transmission characteristics are also better than syv cable. The misunderstanding in this respect is very common, and the misunderstanding in southern China is more serious than in the north. It is believed that the transmission of video signals requires "video cable." Measured 1000 m cable video transmission performance, sywv75-5/64 braided cable: 0.5m-5.15db, 6m-19.12db; national standard high quality syv75-5/96 braided cable: 0.5m-6.43db, 6m-21.76db (same series The cable attenuation of the network structure is 3db larger than that of the foamed cable - that is, 1.4 times larger than that of the foamed cable. There is a well-known manufacturer's product, syv75-5/128 braided cable, 6m-25.22db, and the attenuation is 6db or more larger than the foamed cable. - that is, more than 2 times larger);

4. Regarding high-knit cables, generally refers to cables of 96-128 or higher. The obvious characteristics of the high-knit cable are: the DC resistance of the shielding layer is small, and the low-frequency attenuation below 200khz is reduced, which is beneficial for suppressing low-frequency interference. The measured results show that the frequency of 200khz-6mhz is the same as the attenuation of 128 and 64 codes due to the "skin effect". (The high-frequency current is only on the outer surface of the core wire, and the inner surface layer of the shield layer flows). From the perspective of frequency distortion (difference between high and low frequency attenuation), high-speed cables are rather serious. The direct influence of frequency distortion is the normal proportional distortion of various frequency components of the video signal, which directly affects image distortion;

5. Copper clad steel core wire: This is a kind of sywv cable. It is used for radio frequency transmission of cable TV above 46mhz. Due to the “skin effect”, the current flows only in the copper skin outside the steel wire, and the attenuation characteristics and pure copper core Like the wire, the tensile strength is much higher than that of the copper wire; but this cable is not suitable for video transmission, and the low frequency attenuation of 0-200khz is too large;

6. sywv cable video RF transmission characteristics are excellent, and because of the huge cable TV market support, the output is large, the price also has an advantage;

7. There are misunderstandings and misunderstandings about the video and RF lines. The heated debate in the forum is an example. But everyone should respect the practice: use a 1000-meter 75-5 cable to transmit the signal of a color camera, the end is sent to the monitor, the monitor loop is output to the oscilloscope, and the amplitude of the "color synchro" is measured. The original signal is 0.3v. For comparison, the longer the cable, the greater the difference between the two, the easier it is to compare.