Design And Implementation Of Reverse Optical Transmitter Used In CATV System

Because the draw up of DOCSIS3.1the new standard of CATV access network,all the equipment of HFC network must be updated. The reverse path frequency isbroadening to204MHz. This article aims at researching and developing new reverseoptical transmitter which works in HFC dual network line.Compare with the original optical transmitter, the bandwidth of the new reverseoptical transmitter is broadening from85MHz to204MHz. The wider of thebandwidth will bring about several new difficult point and problem: first thebandwidth become wider, the related circuit will have new request. Secondly, in theprocess of signal transmission and processing, due to the more wide bandwidth, thehigh frequency point will have more loss then the low frequency point. How to keepthe signal of every frequency is another problem need to be solved. In existingproduct the Pilot signal is fixed frequency point, it will not adopt the new demand ofreverse transfer link. At last, due to you tree structure of HFC transfer link. The widerof the working bandwidth will bring more gather noise. How to monitoring the signalof reverse path of HFC system is another new problem.This paper uses for the reference of the design thinking of original reverseoptical transmitter. This paper analyses the possible problems of new reverse opticaltransmitter and did following researches：First do the principle analysis for the circuit according to the project of opticaltransmitter, and require parameters. Design the suitable amplifier circuit, attenuationcircuit, filtering circuit and so on. Secondly combine the character and differencevalue of high frequency, and then put forward to use Trim circuit to compensate thedifference value of different frequency point. Aim at the problem of fixed pilotfrequency, put forward to use variable clock generator to produce Pilot signal. In thismethod the frequency of Pilot signal can be adjustable. At last put forward to addfrequency spectrum detect circuit in reverse optical transmitter. This circuit willdetect the reverse signal from5MHz to204MHz at1MHz step. The data of the signalfrequency spectrum will send to the head end.This article at last accomplishes the development of the reverse optical transmitter of204MHz working bandwidth. Trim circuit will make the of5MHzsignal have1.7dB attenuation then the204MHz signal, this will ensure the gain of allthe bandwidth is equal. The design low pass filter has less than1dB attenuation inpass band and more than26dB attenuation out of pass band. The test result of systemindex (NPR) is29.5dB; satisfy the15dB of operation requirement of the system. Putthe transmitter into the system and make sure the CM to get online.