Abstract:In the cable TV system, some common faults caused ...
In the cable TV system, some common faults caused by cable joints mainly include the following seven points:
(1) Ghosting
When the cable connector fails (such as looseness, deformation, poor contact, water ingress, etc.), the characteristic impedance of the connection will change, resulting in impedance mismatch and ghosting. Ghosting is easy to appear on channels ch1~ch3.
(2) The attenuation of low channel signal level increases
If the coaxial cable has little attenuation to the high-channel signal, and large attenuation to the low-channel signal, this is caused by the increase in the contact resistance between the outer conductor of the cable and the cable joint. In addition to the impedance mismatch causing a certain reflection loss in the low frequency band, the path of the signal in the outer conductor mainly depends on the distributed capacitance. This capacitance is generally small, so the attenuation of the high frequency signal is small, while the attenuation of the low frequency signal is small. Significant increase, the attenuation value is generally between 10 ~ 20dB.
(3) The power supply voltage of the cable fluctuates and drops
When the main line amplifier is powered by cable, due to the looseness, water ingress and corrosion of the cable joint, its contact resistance increases, resulting in an increase in the resistance of the power supply loop, and the voltage drop at this joint increases, which makes the main line amplifier after the supply of this joint increase. The operating voltage is reduced. As a result of the reduced power supply, the loop current increases. When the contact resistance fluctuates, the supply voltage of the mains amplifier also fluctuates.
The connection of trunk cable joints is not good, and sometimes sparking phenomenon occurs, which introduces spark interference, which is manifested in white dot line interference on individual channels in low frequency bands.
(4) Serious signal leakage and interference on individual or some channels
The form of interference is usually expressed as: white sharp point noise; white dot-dash line brushed noise; oblique wriggling white dot line slowly moving left and right or up and down; " noise.
If the cable connector is not in good contact, the electromagnetic shielding of the cable is damaged, and the cable will be equivalent to a pair of transmitting antennas and a pair of receiving antennas at the same time. On the one hand, it radiates the cable TV signal outward, and at the same time, it also receives the interference signal into the cable TV system. The ability to radiate outward is that the high frequency band is greater than the low frequency band; and the channels that are easily interfered are the low frequency bands, especially the ch1 channel.
(5) There is a signal during the day on a certain section of the trunk line, but no signal at night.
This situation generally occurs in areas with a large temperature difference between day and night. Because the core wire of the trunk cable joint is too short, when the temperature is high, the cable core wire can still maintain contact. When the temperature drops, the cable shrinks and the core wire exits the core hole, cutting off the mains power supply, resulting in no signal in an area.
(6) The carrier-to-noise ratio is seriously reduced
(7) The signal of a certain channel is weak, that is, the depth of level fading is very large.
The measures we should take are as follows:
In order to improve the reliability of the system and prevent failures caused by joint problems, the following measures should be taken:
(1) Make cable joints in strict accordance with technical regulations, specifications and standards to ensure correct assembly procedures of various joints, accurate geometric dimensions, moderate reserved length of core wires, no deformation of inner and outer conductors, and uniform contact pressure between outer conductors and joints to achieve contact Reliable and guarantees good cable shielding.
(2) Do a good job of waterproofing the joints, and the self-adhesive butyl rubber insulating tape should be used for the waterproofing of the joints with reliable connection.
(3) Strengthen the protection of the mechanical strength of the cable. There should be enough margin at the connection between the cable and the amplifier, branch, and distributor, and the cable should be tied with the steel strand that hangs it.
(4) Avoid using inferior cable joints. Type 75-9 and 75-12 cables should be phased out using F-type connectors and replaced with waterproof connectors. The outer conductor of the cable should be tinned braided mesh.
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