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[屏蔽工序 ] 电缆的屏蔽解决方案

P: 2006-07-26 11:03:48

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随着当今信息传输技术的飞速发展,布线系统的传输速率在稳步加快的同时也带来了某种防干扰的脆弱性,这是因为传输信息的频率越高,信号的敏感性增加,同时它们的能量越来越弱,此时的布线系统就越容易受干扰。
干扰无处不在
    电缆及设备会对其他元件产生干扰或被其他干扰源严重干扰,例如:计算机屏幕, 电动机, 移动电话, 无线电转播设备, 数据传输及动力电缆,等等。
        潜在的窃听者,网络犯罪及黑客不断增加,由于他们对UTP电缆信息传输的拦截会造成巨大的损害及损失。哪怕没有物理连接,拦截UTP电缆上传输的信息也是非常容易的,侵入者只需要有一台无线接收机,信号处理设备及一台便携机即可工作。数据拦截在几厘米至几百米的范围内都有可能发生,并且在大多数情况下完全无法监测。尤其在使用高速数据网络时,拦截大量信息所需要的时间显著低于拦截低速数据传输所需要的时间。
        数据双绞线中的绞合线对在低频下可以靠自身的绞合来抵抗外来干扰及线对之间的串音,但在高频情况下(尤其在频率超过250Mhz以上时),仅靠线对绞合已无法达到抗干扰的目的,只有屏蔽才能够抵抗外界干扰。
        电缆屏蔽层的作用就象一个法拉第护罩,干扰信号会进入到屏蔽层里,但却进入不到导体中。因此,数据传输可以无故障运行。
    由于屏蔽电缆 (STP) 比非屏蔽电缆具有较低的辐射散发,因而防止了网络被拦截。屏蔽网络(屏蔽的电缆及元器件)能够显著减小进入到周围环境中而可能被拦截的电磁能辐射等级。
        百通凭借多年在数据传输及音视频领域的专业应用,开发了一系列独有的屏蔽方式,以提高电缆抵抗电磁干扰/射频干扰的能力,确保了信号的完整性,为数据,音频和视频的顺利传输提供了保证,并避免了故障时间。百通的屏蔽电缆已在各个领域应用多年,所以我们在高度专业化的电子领域内所积累的用于数据传输的经验具有无可比拟的价值。对于特殊的干扰问题,可以要求百通为客户专门设计和制造屏蔽电缆方案。
新型的屏蔽方案
“French Braid”屏蔽
      百通的专利产品“French Braid”是一 种双螺旋形(双层裸铜线屏蔽)屏蔽,而将两个螺旋在一点织连在一起。与传统网状屏蔽相比,该产品提高了柔韧性,这种构造比标准的螺旋屏蔽提高了抗挠寿命,并比螺旋屏蔽或传统的网状屏蔽降低了颤噪或静电噪声。“French Braid”比标准的网状屏蔽容易端接,因为它没有完全织连在一起。它比单螺旋编织屏蔽的DC(直流电)环路阻抗要低,性能更好。“French Braid” 特性在百通新型“Flexnake”蛇型电缆中表现的更为显著。

                                                    “French Braid”

网状屏蔽
     
  网状屏蔽在保持良好的柔韧性及抗挠寿命的同时,提供了超群的结构整体性。这些屏蔽对于降低低频干扰是理想的选择,比起箔层屏蔽来说,降低了直流阻抗。网状屏蔽在音频以及射频范围非常有效。通常,网状屏蔽覆盖率越高,屏蔽效果就越好。

                                       “Braid shield”

 箔层屏蔽
        箔层屏蔽是由聚脂或聚丙烯薄膜上附着一层铝箔形成。这层薄膜给屏蔽提供了机械强度及良好的绝缘性能。箔层屏蔽可100%覆盖电缆。由于它们的体积小,箔层屏蔽通常用于多对数电缆的单线对屏蔽以减少相互的串扰。箔层屏蔽重量轻、体积小,比网状屏蔽造价低,在射频范围内通常更加有效。箔层屏蔽比起网状屏蔽的柔韧性更好,但抗挠寿命较短。与箔层屏蔽一起使用的接地线,使端接更加容易,并将静电释放入大地。
        百通使用了短接工艺来保持金属与金属的接触,以获得高频性能的改善。没有短接就会出现信号泄漏的狭缝。
        为了改进传统的短接方式,百通使用了旨在多对电缆应用和减少串扰的ZFold技术。此技术包括了绝缘折叠及短接折叠。短接折叠提供了金属与金属之间的接触,而绝缘折叠防止了多线对缆单对屏蔽间的相互短路。ZFold的设计增加了高频段箔层屏蔽的有效范围。

                              “Z-Fold” 屏蔽

箔层的设计
          作为屏蔽技术的创新者,百通开发了几种专有的屏蔽设计方案。百通是第一家开发以铝/聚脂箔层作为电缆屏蔽的公司,并被授予“Beldfoil”屏蔽专利。Beldfoil屏蔽首先做到了100%的电缆覆盖,增强抵抗了音频及无线频率的辐射及侵入。
          箔层屏蔽一般是在导体绝缘层上粘合一层附着物。粘合的优点在于进行更加快捷、方便及可靠地端接。此外,粘合还能防止屏蔽层下的水分发生扩散。如果电缆护套破损,粘合的屏蔽还能帮助保护电缆绝缘层免受污染。百通将这些优点都融合在“Duobond® II”屏蔽之中。                                                

                                          “Duobond” 屏蔽

组合屏蔽
       
组合屏蔽包括多层屏蔽。它们能够在整个频谱提供最大的屏蔽效果。箔层/网状屏蔽结合了箔层屏蔽100%的覆盖范围与网状屏蔽的机械强度和低直流阻抗等优点。其他现有的组合屏蔽包括各种箔层/网状/箔层设计、网状/网状或网状/螺旋设计等。

                                 “combination shield”

对于不同的干扰场,应选用不同的屏蔽方式
        干扰场主要有电磁干扰及射频干扰两种。电磁干扰(EMI)主要是低频干扰,马达、荧光灯以及电源线是通常的电磁干扰源。射频干扰(RFI)是指无线频率干扰,主要是高频干扰。无线电、电视转播、雷达及其他无线通讯是通常的射频干扰源。
        对于抵抗电磁干扰,选择编织屏蔽最为有效,因其具有较低的临界电阻;而对于射频干扰,箔层屏蔽最有效,因编织屏蔽依赖于波长的变化,它所产生的缝隙使得高频信号可自由进出导体。而对于高低频混合的干扰场,则要采用具有宽带覆盖功能的箔层加编织网的组合屏蔽方式:编织屏蔽适用于低频范围,而箔层屏蔽适用于高频范围。
帖 子 奖 罚
管理员 奖励 0 金币,1 声望,于 2006-7-26 11:03:48

technical parameter - 技术参数 点赞(0) 投诉

P: 2006-07-26 11:35:14

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Belden® Cable - Reliable Performance in Broadcasting Environments

Innovating Shielding Solutions

From outdoor news gathering to studios and control rooms to post production, the broadcast industry faces critical challenges from EMI/RF interference. Belden® shielded cable ensures signal integrity and provides confidence in audio and video transmissions, preventing downtime and maintaining sound and picture clarity. Because Belden shielded cable has been engineered for a variety of applications over many years, our experience in this highly specialized field of electronics provides unparalleled value for broadcast transmissions. Choose from the widest variety of innovative shielded cable available through your local Belden Authorized Distributor. For special interference problems, call on Belden to design and manufacture a custom-shielded cable solution.

Braid Shields

Braid shields provide superior structural integrity while maintaining good flexibility and flex life. These shields are ideal for minimizing low frequency interference and have lower DC resistance than foil. Braid shields are effective at audio, as well as RF ranges. Generally, the higher the braid coverage, the more effective the shield.

Foil Shields

Foil shields consist of aluminum foil laminated to a polyester or polypropylene film. The film gives the shield mechanical strength and bonus insulation. Foil shields provide 100% cable coverage. Because of their small size, foil shields are commonly used to shield individual pairs of multi-pair cables to reduce crosstalk. They have less weight, bulk, and cost less than braid shields and are generally more effective in RF ranges. Foil shields are more flexible than braid but have a shorter flex life. Drain wires are used with foil shields to make termination easier and to ground electrostatic discharges.

Belden uses a shorting fold technique to maintain metal-to-metal contact for improved high frequency performance. Without the shorting fold, a slot is created through which signals can leak.

Belden improves on the traditional shorting fold by employing the Z-Fold® designed for use in multi-pair applications to reduce crosstalk. The Z-Fold combines an isolation fold and a shorting fold. The shorting fold provides metal-to-metal contact while the isolation fold keeps shields from shorting to one another in multi-pair, individually shielded cables. The Z-Fold design increases the foil shield's range of effectiveness to higher frequencies.

Foil Tape Designs

As an innovator in shielding technology, Belden has developed several shields of exclusive design. Belden was the first to develop the aluminum/polyester foil used as a cable shield and was awarded a patent for the Beldfoil® shield. The Beldfoil shield was the first to offer 100% cable coverage, improving protection against radiated emission and ingress at audio and radio frequencies.

Foil shields may be bonded to a conductor's insulation with a layer of adhesive. The advantages of bonding include faster, easier, and more reliable termination. In addition, bonding prevents moisture from wicking under the shield. Should the cable jacket rupture, the bonded shield helps protect the cable's insulation against contamination. Belden combines these assets in the Duobond® II shield.

Combination Shields

Combination shields consist of more than one layer of shielding. They provide maximum shield efficiency across the frequency spectrum. The combination foil/braid shield combines the advantages of 100% foil coverage, plus the strength and low DC resistance of a braid. Other combination shields available include various foil/braid/foil designs, braid/braid, or foil/spiral.

Special Noise/lnterference Problems in Broadcasting

Triboelectric noise is generated by mechanical motion of a cable, causing movement in the cable's shield. Belden detects and measures triboelectric noise through the use of our own Low Noise Test equipment. The test procedure and equipment have been developed from a combination of three low noise standards--NBS, ISA-S, and MIL-C-17.

Mechanically induced noise is a critical and frequent concern in the use of guitar cords and microphone cables. Generally, test readings should be below 150 millivolts for low impedance microphone cables and below 15 millivolts for guitar cords. The properties of special conductive tapes and insulations are employed at Belden to prevent mechanically induced noise. Consult with Belden Technical Support Department for the proper cable construction needed to combat the noise problems associated with your application.

Aluminum Mylar - 铝箔麦拉Al/Mylar,铝箔麦拉带 点赞(0) 投诉

P: 2006-07-26 13:58:48

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图片要使用单个插入“上传附件”形式才能贴上来。因为所有的图片必须以附件形式存储在服务器上,文字的只需要直接存储到数据库,所有图片是不能直接贴上来的。

head or butt cable - 有头电缆 点赞(0) 投诉

P: 2006-10-31 13:33:12

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zzyygg:
能否用附件形式上传,让我们学习更方便一些

 

Z-fold.doc

 

美國百通.doc

[cuijiude 在 2006-10-31 13:38:32 编辑过]

fabrication - 加工,制造 点赞(0) 投诉

P: 2007-01-18 11:53:54

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好文章,要是在有图片就更好了。

concentric neutral conductor - 同心中线,同心中性线 点赞(0) 投诉

P: 2007-08-07 11:07:58

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谢谢,楼主。长见识了。附件为什么打不开啊

abrasive plate - 摩擦片(越端放线装置的) 点赞(0) 投诉

P: 2007-08-08 10:25:15

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老兄,能不能把带图的资料发一份给我啊?没有图理解起来比较困难.真的非常感谢. ly0468@sina.com

dissymmetry - 不对称 点赞(0) 投诉

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