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Application of CATV optical receivers
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Application of CATV optical receivers

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Application of CATV optical receivers

CATV optical receiver, also known as radio frequency optical terminal unit, CATV optical terminal unit, etc., is an optical terminal unit that receives the CATV optical signal transmitted by the upper optical transmitter and is an optical terminal unit that converts optical fiber to radio frequency.

 

Fundamental principles of transmitting optical signals

Points for Attention in Use

The cause of distortion in optical transmission of cable television


Fundamental principles of transmitting optical signals

Optical transmission is the technology of transmitting optical signals between the sender and the receiver. The working process of the optical transmission TV signal is carried out among the optical transmitter, optical fiber and optical receiver. The optical transmitter in the center room converts the input RF TV signal into an optical signal, which is completed by electric-optical fiber transmission, E/O. The optical signal converted into a receiving device (optical receiver) is received by optical fiber transmission. The optical signal obtained from the optical fiber is converted into an electrical signal by an optical receiver. Therefore, the basis of the optical transmission signal is the whole process of electrical/optical and optical/electrical conversion, also known as an optical link.

 

Points for Attention in Use

The optical fiber adopts APC or PC head with light output. During maintenance, the light output head should be avoided to aim at the human eyes to prevent the light from harming the human eyes. The light source can be pointed at by the camera function of the mobile phone.

The light connector should avoid dust and be cleaned with anhydrous alcohol before connection.

If the voltage fluctuation is large, an AC voltage regulator or UPS should be installed.

When the output terminal is not fully used, the unused terminal should be installed with a 75Ω false load.

The outer shell of the outdoor optical receiver must be locked tightly to prevent rainwater from soaking in and causing a short circuit.

Lightning protection facilities should be provided where optical receivers are installed, and the casing should be well grounded to better protect the transmission system.

 

The cause of distortion in optical transmission of cable television

In an optical fiber transmission system, due to the nonlinear electrical/optical conversion characteristics of a semiconductor laser, the output optical signal is not consistent with the change of the excitation current, which leads to distortion, which is called modulation distortion. The modulation index M value is not allowed to be too large, so it is necessary to choose the optical transmitter with high performance and strong pre-distortion processing technology. The pre-distortion processing technology is to improve the modulation linearity by using artificial design to generate pre-distortion, so as to eliminate and reduce the CSO and CTB in the optical fiber transmission system.

In the optical transmission system, due to the driving RF amplifier and receiving RF amplifier distortion chance is very small, the linear PIN photodiode due to signal level is not too high, the small error can be ignored, and its main reasons come from the semiconductor laser modulation characteristic distortion and optical fiber dispersion.

In laser intensity modulation, the wavelength of light will change, and additional frequency modulation will appear, so that the signal frequency is broadened, chirp effect will appear, mainly manifested as CSO distortion.

The dispersion characteristics of optical fiber will make the group delay of different wavelengths different, resulting in the distortion caused by the inconsistent arrival time of the terminal, which is mainly CSO distortion.

 

These are the application items of CATV optical receivers.


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