CATV光链路传输系统C/N指标的分析和计算

目前，国内外CATV光链路传输，均采用AM光纤传输系统，其技术指标与同轴电缆传输系统的技术指标基本一样，也有C／N、复合二次差拍（Composite Second Order Beat．组合CSO）和复合三次差拍（Composite Triple Beat．CTB）。这三大指标在CATV光链路AM光纤传输系统中，与许多因素有关。本文就如何防范CATV光链路传输系统的噪声积累、提高系统C／N指标．进行了分析和计算。     

论有线电视光链路传输系统的载噪比

有线电视CATV光链路传输系统的任何部分都可能产生噪声,并在信号传输的过程中逐步叠加,致使系统载噪比指标下降。CATV光链路AM(AmplitudeModulation:幅度调制,简称调幅)传输系统的技术指标与同轴电缆传输系统的技术指标基本一样,也有C/N(CarriertoNoiseratio:载噪比)、组合CSO(CompositeSecondOrderBeat:复合二次差拍)和CTB(CompositeTripleBeat:复合三次差拍)。但这三大指标在AM光纤CATV传输系统中,与许多因素有关。调幅AM是CATV光链路传输系统噪声的主要来源     

PDI优质GaAs推挽模块S834在CATV前端调制器中的应用

在CATV前端邻频调制器中，一般会采用一只CATV放大模块作为输出级放大。因此，这只放大模块的性能将很大程度上影响着调制器的性能指标。由于调制器是单频道使用，其对放大模块指标要求，除了传统的CTB，CSO，XMOD等以外，还特别强调模块的：二次谐波指标。但由于CATV放大模块多用于HFC网络上，一般情况下CATV放大模块生产厂家不给出二次谐波这一指标，使调制器生产厂家在使用模块时多有不便。     

PDI新技术、新产品专栏(13)

PDI优质GaAs推挽模块S834在CATV前端调制器中的应用:在CATV前端邻频调制器中,一般会采用一只CATV放大模块作为输出级放大。因此,这只放大模块的性能将很大程度上影响着调制器的性能指标。由于调制器是单频道使用,其对放大模块指标要求,除了传统的CTB,CSO,XMOD等以外,还特别强调模块的二次谐波指标。但由于CATV放大模块多用于HFC网络上,一般情况下CATV放大模块生产厂家不给出二次谐波这一指标,使调制器生产厂家在使用模块时多有不便。为在CATV前端调制器更好的使用放大模块,PDI和用户一起对PDI的GaAs34dB推挽放大模块S…     

羽折衷法解决CATV系统中模、数信号的相互影响

目前的CATV系统大都同时传送模拟信号和数字信号,系统中模拟信号所产生的失真信号CSO（复合二次差拍失真）、CTB（复合三次差拍失真）如果落在数字频道上,将会使数字信号产生误码。同样,当数字信号产生失真时也影响着与它相邻的模拟频道的C／N（载噪比）值。现在的CATV系统中,数字频道被安排在系统的高端频率上,而模拟频道则被安排在系统的低端频率上。在模拟频道中,由于数字信号失真使得其带内噪声功率变高,我们将因失真使其他频道噪声增加的那部分噪声功率称为CIN,频道固有的噪声功率称为Ⅳ,那么在存在数字信号与模拟信号相互影响的那些频道上的载噪比应是：C／（N＋UN）。     

色散补偿技术在1550nm模拟CATV长途传输系统中应用的研究

色散因素是制约1550nm模拟CATV系统无电中继传输距离的主要因素,本文提出利用已经商用化的色散补偿器件对长距离普通光纤CATV模拟传输系统中的色散进行补偿,并对比了色散补偿光纤和啁啾光纤光栅进行色散补偿的优缺点,同时文章给出了光纤光栅进行色散补偿的最佳位置。模拟系统采用皮长传输,采用啁啾光纤光栅实现对模拟信号的色散补偿可以充分发挥啁啾光纤光栅的低损耗的特性和对单波长进行补偿的特性,本文在国内首次提出采用啁啾光纤光栅（CFG）对1550nmCATV模拟系统长距离光纤传输进行了散补偿,并引用试验证明采用光纤光栅在保证较好载噪比（C／N）和三阶差拍失真（CTB）指标的情况下,能够大幅度地补偿二阶组合失真（CSO）的恶化。     

放大器非线性失真与最大输出电平的修正

通常，人们在计算放大器非线性失真参数时，往往忽视了一个实际问题，即对计算结果的修正。比如在计算单个放大器组合交调比时，使用公式CM＝CMo＋２（Sob－So）（１）计算多台相同“干放”级联时的干线组合交调比时使用公式CMn＝CMo＋２（Sob－So）－２０lgn（２）（１）和（２）式中：Sob为放大器标称输出电平；So为放大器实际输出电平；CMo为放大器在标称输出电平和标称频道数时的组合交调比；n为干放级联数。又比如在计算单个放大器组合三次差拍比时，使用公式CTB＝CTBo＋２（Sob－So）（３）计算多台干放级联时的干线组合三…     

MMDS微波传输系统的规划与设计(连载Ⅲ)

4MMDS的传输质量要求及其指标分配MMDS联网的CATV系统要求4级以上图像质量。按GYH106－2标准规定,系统要求达到C／N＞43dB,CTB≥54dB,CSO≥54dB。由于MMDS系统设备的功率放大是甲类线性放大,超线性设计,因此H次失真CSO分量很小;另外,采用平衡式混频器、其二次失真也很小;还有,MMDS下变频器的二次失真也在带外,因此在MMDS系统中不考虑复合二次失真CSO分量,仅考虑载噪比C／N和复合三次失真CTB分量。4．1袖标分留根据MMDS传输网及同轴电缆分配网的规模大小不同,其C／N和CTB的分配比例也不同。l）载噪比C／…     

试论CATV系统中的CSO和CTB

在多频道传输的CATV系统中，由于相关设备存在着一定程度的非线性传输特性，必然会产生大量的非线性失真产物，这些产物落在传输频道内，会在图像载波附近聚集成"簇"，对频道形成"综合"差拍干扰。其中，由二次失真产物聚集而成的"综合"互调干扰为CSO，由三次失真产物聚集而成的"综合"差拍干扰为CTB，衡量这两种"综合"差拍干扰对频道影响程度的性能参数就是C／CSO与C／CTB，理论联系实际地对这些失真产物进行了分析、讨论，说明其产生的机理。     

入射偏振态偏离TE模对光纤AM CATV外调制发射机性能影响的研究

从理论上分析了入射到调制器的偏振态偏离ＴＥ模时对外调制光发射机组合二次失真（ＣＳＯ）和组合三次差拍（ＣＴＢ）的影响。得出在ＴＥ，ＴＭ模插入损耗相同时，传输５９个ＰＡＬ－Ｄ频道，在保证ＣＳＯ＝－７０ｄＢｃ时，偏离ＴＥ模的角度应小于６°。单频传输的实验结果和理论分析一致     

Calculating the CSO/CTB spectrum of CATV amplifiers and opticalreceivers

The discrete CSO/CTB intermodulation distortion has a big influence on the CATV net quality. This article describes a new method to calculate the composite second order (CSO) and composite triple beat (CTB) intermodulation distortion spectrum for the full frequency range of a coaxial cable CATV amplifier for any input frequency plan. The calculation takes into account the output level ripple and preemphasis, using frequency dependent intermodulation coefficients. These coefficients have been established through CSO/CTB measurements on multichannel test systems for standard frequency plans at 5-8 intermodulation frequencies or on distortion arrays for the two (three) oscillator measuring test system. The defined intermodulation coefficients are unrelated to the frequency plan and the input/output level ripple. It is also possible to calculate the CSO/CTB spectrum caused by analogue channels within digital QAM channels. The results equally apply to optical CATV receivers. Simulation results have been verified using extensive data from multichannel measurement systems     

光子晶体光纤在有线电视传输系统中的应用

文章介绍了一种利用光子晶体光纤作为宽带色散补偿装置的直接幅度调制残留边带(AM-VSB)有线电视传输系统,测试结果表明,与未加补偿的外调制方式相比,本系统具有更高的载噪比(CNR),更优的复合二阶失真(CSO)和复合三阶差拍性能(CTB).将光子晶体光纤用作色散补偿,不但可以应用到CATV系统中,还可以广泛应用于其它宽带高速长距离光纤通信领域.     

工作点偏移对光纤AMCATV外调制传输性能的影响

对于光纤ＡＭＣＡＴＶ外调制传输，调制器的偏置工作点设置是非常重要的。通过理论分析得出：当工作点偏离最佳偏置点时，主要对系统的组合二次失真ＣＳＯ（ＣｏｍｐｏｓｉｔｅＳｅｃｏｎｄＯｒｄｅｒｄｉｓ－ｔｏｒｔｉｏｎ）产生影响，并导出相应的公式。对于ＰＡＬ／Ｄ制５５０ＭＨｚＣＡＴＶ系统，要保证ＣＳＯ小于－７０ｄＢｃ，工作点的偏移不能大于１０ｍＶ；实验结果和理论分析一致。     

The impact of CSO/CTB distortion on BER characteristics by hybridmultichannel analog/QAM transmission systems

The FEC limit (uncorrectable errors) with CSO/CTB (composite second order/composite triple beats) distortion presence in a channel is not for modulated lasers only, but for coaxial networks with amplifiers too. The system quality reserve depends not only on CNR, but is more limited on CSO/CTB. It is important, for the definition of the system quality reserve, to calculate the CSO/CTB spectrum caused by analogue channels within analogue and digital QAM channels and also to calculate the changing of CNR from the intermodulation interaction between analog and digital channels in a CATV network     

Analytical formulation of distortion and chirp in CATV DFB lasersincluding spatial hole burning

Analytical formulas for distortions permitting the calculation of composite second order (CSO) and composite triple beat (CTB) in distributed feedback (DFB) laser diodes are given, including gain compression and longitudinal spatial hole burning (LSHB). An improved chirp expression including both effects is also proposed. Gain compression is compared to LSHB over the CATV band: LSHB effects on distortion and chirp are found to be dominant in actual CATV lasers. Nonmonotonous behavior (dips) function of frequency and bias current is verified in second- and third-order distortions in agreement with recent results     

Frequency-dependent distortions of composite triple beat inlightwave CATV transmission systems

The frequency-dependent distortion of composite triple beat (CTB) for analog AM multichannel lightwave transmission systems is studied. A modified circuit model which takes into account the small signal behavior of the leakage path is developed, and it is shown that it is adequate to describe the measured frequency-dependent behavior of CTB in a 42-channel load lightwave CATV (cable television) transmission system using a cap-mesa buried-heterostructure (CMBH) distributed-feedback (DFB) laser inside the transmitter module     

A hybrid CATV/256-QAM/OC-48 DWDM system over an 80 km LEAF transport

A hybrid dense-wavelength-division-multiplexing (DWDM) system for CATV, 256-QAM and OC-48 trunking is proposed and demonstrated. It used two wavelengths for AM-VSB CATV, one wavelength for 256-QAM digital passband signals and four wavelengths for OC-48 (2.5 Gb/s) digital baseband signals transport. Good performances of carrier-to-noise ratio (CNR) /spl ges/ 50 dB, composite second order (CSO) /spl ges/ 67 dB and composite triple beat (CTB) /spl ges/ 65 dB were obtained for AM-VSB CATV; and low bit error rate (BER < 10/sup -9/) had been achieved for 256-QAM and OC-48 digital signals over an 80 km large effective area fiber (LEAF) transport.     

Performance comparison between DCF and RDF dispersion compensation in fiber optical CATV systems

An architecture of an externally modulated AM-VSB CATV 77-channel erbium-doped fiber amplifier (EDFA)-repeated system which use the combination of a single-mode fiber (SMF) and reverse dispersion fiber (RDF) as a dispersion compensation device is proposed and demonstrated. Compared to the conventional externally modulated fiber optical CATV systems with or without a dispersion compensation fiber (DCF), excellent performance of composite second order (CSO) /spl ges/ 78 dB accompanied by satisfied carrier-to-noise ratio (CNR) /spl ges/ 50 dB and composite triple beat (CTB) /spl ges/ 65 dB was obtained in the proposed system.     

Numerical simulation of clipping induced distortion in externallymodulated lightwave AM-SCM systems

For the first time to our knowledge the results of numerical simulation, and experimental verification, of clipping induced distortion in externally modulated lightwave AM-SCM CATV systems are presented. Comparison of these results with clipping induced distortion in directly modulated systems show a remarkable outcome: under the same modulation conditions the composite triple beat (CTB) distortion component in externally modulated systems is 6 dB higher than in directly modulated systems