本帖最后由 iC921 于 2010-3-10 18:47 编辑
最近琢磨这个概念，感觉不少人不会太详细了解，现翻译一份外文资料上的陈述，译得不当之外请大家指正！
iC921@2010.03.10
Jensen AN-002 ：ANSWERS TO COMMON QUESTIONS ABOUT AUDIO TRANSFORMERS
by Bill Whitlock
The purpose of a balanced line is to transfer a "signal" from one place to another while rejecting "ground noise", which is not white noise or hiss, but power line related hum and buzz. To accomplish this noise rejection, two signal lines are used and the IMPEDANCE of the two lines to ground must be equal or "balanced". Since anything connected to the line affects its impedance, always consider the "system" consisting of the driver, the line itself, and the receiver. It is a popular belief that the signals must have opposing polarity and equal amplitudes, or symmetry. Signal symmetry has NOTHING to do with noise rejection. The system must reject noise even when there is no signal and, in fact, this is usually how system noise testing is done. Illustrated below is a generalized balanced system schematic, rearranged to show that it takes the familiar form of a Wheatstone bridge.
平衡传输线，是将信号从一个地方传到另一个地方的同时抑制“接地噪声”。这些接地噪声，指的是工频电源类的hum噪声和buzz噪声，而不是抑制白噪声或hiss声。为抑制噪声，要采用两根对地阻抗相等或平衡的信号线。因任何连接都有阻抗存在，因此，总会考虑构成“系统”的驱动器、传输线和接收器。通常地，所传输的信号往往是极性相反，且幅值相等，或者具体对称性。不过，这种对称性对于噪声抑制没有意义，系统应当能在没有信号时也能抑制噪声的发生（侵入），事实上，这通常是系统测试所需要做的。下面是通常意义的平衡系统的具体原理，重绘后的电路，就是大家所熟知的惠斯通电桥。
A Wheatstone bridge has a "generator", four impedance "arms", and a "detector". In our schematic, the "generator" is the ground noise, the "arms" are the common-mode impedances of the driver outputs ZC1 and ZC2 and receiver inputs ZC3 and ZC4, while the "detector" is the receiver itself, which is sensitive only to the voltage between the two lines.
惠斯通电桥有一个信号发生器，四个阻抗桥臂和一个检测器。在上述原理图中，其信号发生器是指接地噪声，它的桥臂是指输出驱动器的输出阻抗ZC1和ZC2，接收器的输入阻抗ZC3和ZC4等四个共模阻抗，它的检测器就是图中的接收器，接收男厕只对两根信号线间的电压（差）比较敏感。
If the ratio of ZC1 to ZC3 is precisely the same as the ratio of ZC2 to ZC4, the bridge is said to be "nulled". In this ideal condition, none of the ground noise will appear at the receiver no matter how large the noise is. In the real world, however, all kinds of mass-produced equipment will be interconnected and exact matching of these impedances at inputs and outputs cannot be reasonably expected. Equipment designers can minimize the effects of these mismatches or "unbalances" through judicious choices of input and output stage circuit topologies and common-mode impedances.
若ZC1和ZC3之比，和ZC2和ZC4之比相等，则电桥输出为零，没有意义。这种理想条件的有利之处是，此时不论接地噪声有多大也不会（对接收器产生）影响。实际中，各种大批量生产的互连设备，不能过度期望其输入输出阻抗都能做到精确匹配。设备设计师能通过审慎地选择输入输出级电路的拓扑和共模阻抗来最小化失配或“非平衡”的效应。
The Wheatstone bridge is most sensitive to small fractional impedance changes in one of its arms when all the arms are the same impedance. It is least sensitive when one pair of arms is very low impedance and the other pair is very high (we change the impedance of pairs because the ratios of the two sides must match in order to null the bridge). Therefore, we can minimize the sensitivity of our balanced system (bridge) to impedance unbalances by making common-mode impedances very low at one end of our line and very high at the other.
惠斯通电桥在四个桥臂阻抗相同时而其中的一条桥臂发生哪怕是极小的变化都非常敏感。而当一对桥臂阻抗非常低而另一对桥臂阻抗非常高时，电桥是最不敏感的（我们成对地改变桥臂阻抗是因为两边的阻抗比的匹配可令电桥为“NULL”）。因此，我们能通过使得一边桥臂共模阻抗非常低而另一边非常高的方法使得平衡系统（平衡电桥）阻抗不平衡时的（接收）敏感性降至最低（最小化）。
To deliver the maximum signal voltage from driver to receiver, the differential impedance must be low at the driver and high at the receiver. Most line drivers with low differential output impedance will also have inherently low common-mode output impedances, so the low impedance arms of the bridge are almost always at the driver. Generally, in professional audio, low impedance means under 100 Ω and high impedance means over 10 kΩ.
由驱动器向接收器提供最大的信号电压，驱动器的差分阻抗应当取低，而接收器中应当取高。有低的差动输出阻抗绝大多数的线路驱动器，同时也固有地有低的共模输出阻抗，这样，低阻抗桥臂几乎无一例外地位于驱动器一侧。专业音频上说的低阻抗通常在100 Ω以下，而高阻抗则在10 kΩ以上。
Active circuits having high differential input impedance (sometimes called "bridging" since several can be bridged across a line with minimal level loss) are widely used, but the importance of the common-mode input impedances of these circuits has long been ignored by most designers. Almost all such "electronically balanced" inputs have common-mode input impedances in the 5 kΩ to 20 kΩ range, which seriously degrades their performance as a balanced line receiver. The common-mode input impedances of an input transformer are inherently about 1000 times that of most "actively balanced" inputs, giving the transformer about 60 dB better ground noise rejection in the real world, where significant source impedance unbalances almost always exist.
有高差分输入阻抗的有源电路（有时之所以被称之为“（电）桥”是因为连接导线有最小有电平损耗）被广泛应用，但重要的是这些电路的共模输入阻抗被大多数设计师所忽略。几乎所有号称“电气平衡”的输入电路都有范围为5 kΩ ～ 20 kΩ的共模输入阻抗，这会严重降低平衡线路接收器的性能。输入传输变压器的共模输入阻抗，固有地有这些占绝大多数“有源平衡”电路（输入阻抗）约1000倍的输入阻抗，它可以获得优于60 dB接地噪声抑制的实际效果，几乎可以忽略源阻抗始终存在的非平衡性（共模阻抗）。 |
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