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The Problem of Ghost Interference in Cable TV Frontend System

Leading Ghost (Left Ghost)
Reason analysis: The front ghost on the left side of the main signal contour in the cable analog television system is caused byAdjacent frequency modulatorThe space signal received in the system arrives at the user receiver ahead of the main signal transmitted by the cable transmission system, which is often caused by the system user being close to the Launch pad, the air signal being strong, and directly entering the user receiver

Clearing method: The method of clearing is to use frequency conversion during channel setting, that is, to use Digital Video SUSPO at the front end of the computer roomAdjacent frequency modulator(also known as channel converter), which switches the frequency of the spatial signal to another frequency and amplifies the transmission to avoid being on the same frequency as the strong spatial signal. In addition, the adjacent frequency modulator level can be appropriately increased without affecting the distortion index to reduce ghosting interference, but generally it should not exceed 75 decibel microvolts (75Db).
2. Lag Ghost (Right Ghost)
Cause analysis: This interference can occur in both wireless television reception and cable television systems. In wireless television users, when receiving the spatial main signal, the television also receives multiple multipath reflections generated by mountains or tall buildings, which have a significant phase lag compared to direct waves, resulting in right ghosting. In the CATV system, if the open circuit antenna is not properly aligned with the Launch pad or reflector, between the receiving antenna and the feeder, between the cable and the distributor, branch, cable, user box, or the cable is deformed due to the small bending radius, extrusion, or mechanical damage during construction, the impedance characteristics will also change greatly, resulting in impedance mismatch and reflection waves. The reflected wave travels back and forth along the cable and gradually decays, resulting in right ghosting of the image reflected on the TV screen.

Clearing method:
A. Carefully adjust the antenna position and direction. If the problem cannot be solved, strong reflection antennas or phase shifted antennas should be used or appropriate antenna array combinations should be selected to eliminate ghosting.
B. Select a 75 Ω coaxial cable, connector, and connector to ensure impedance matching.
C. Make sure to ensure good contact and impedance matching in all connection links.
D. Check the coaxial cable section by section for deformation and impedance changes, and replace it if there are any changes.