高速电路设计基本概念之——近端串扰和远端串扰

以下内容摘自英文版的信号完整性分析一书（SIGNAL INTEGRITY By Eric Bogatin）：

The noise between two adjacent transmission lines can be measured in the configuration shown in Figure 10-3. A signal is injected into one end of the line, with the far end terminated to eliminate the reflection at the end of the line. The voltage noise is measured on the two ends of the adjacent quiet line. Connecting the ends of the quiet line to the input channels of the fast scope will effectively terminate the quiet line.

两条相邻传输线之间的噪声可以由下图所示的结构来量测，信号从传输线的一端输入，在远端进行端接以消除传输线末端的反射。电压噪声在两条相邻的不受干扰的传输线（quiet line）两端来量测。将quiet line的两端连接到示波器的输入通道，可以有效地端接quiet line。

Figure 10-3. Configuration to measure the cross talk between an active and quiet net, looking on the near end and far end of the quiet line

Figure 10-4 shows the measured voltage noise in a quiet line adjacent to an active signal line that is driven by a fast rising edge. In this case, the two 50-Ohm microstrip transmission lines are about 4 inches long, with a spacing about equal to their line width. The ends of each line are terminated in 50 Ohms, so the reflections are negligible.

下图显示了当快速上升边沿驱动active line的时候，在与之相邻的quiet line内量测到的电压噪声。此处，两条50Ohm的微带线长约4inch，间距约等于线宽，每条线的两端均端接了50Ohm电阻，所以反射可以忽略不计。

Figure 10-4. Measured noise on the quiet line when the active line is driven with a 200-mV, 50-psec rise-time signal. Measured with an Agilent DCA TDR and GigaTest Labs Probe Station.

The measured noise voltage has a very different pattern on each end. To distinguish the two ends, we label the end nearest the source "the near end" and the end farthest from the source "the far end." The ends are also defined in terms of the direction the signal is traveling. The far end is in the "forward" direction to the signal propagation direction. The near end is in the "backward" direction to the signal propagation direction.

测量到的电压噪声在每端的模式不同，为了区分这两端，标记靠近源端的为近端，远离源端的为远端，同样可以用信号传输的方向来定义。远端在信号传输方向的前方，近端在信号传输方向的后方。

When the ends of the lines are terminated so multiple reflections do not play a role, the patterns of noise appearing at the near and far ends have a special shape. The near-end noise rises up quickly to a constant value. It stays up at this level for a time equal to twice the time delay of the coupling length and then drops down. We label the constant, saturated amount of near-end noise the near-end cross talk (or NEXT) coefficient. In the example shown above, the NEXT is about 13 mV. With an incident signal of 200 mV, the NEXT is about 6.5%.

如果传输线两端都有端接并且不存在多次反射，在近端和远端出现的噪声形式都具有特殊的形状。近端串扰快速上升到一个固定值，并且保持一段时间，这段时间相当于耦合长度延迟时间的两倍，然后跌落。把这个固定值称为近端串扰系数。上图中入射信号200mv，NEXT大约13mv，大概是入射信号的6.5%。

The NEXT value is special in that it is defined as the near-end noise when the coupling length is long enough to reach the constant, flat value and in the special case of matched terminations. Any change in the terminations may change the amount of near-end noise. Obviously, the value of the NEXT will depend on the separation of the traces. Unfortunately, the only way of decreasing the NEXT is to move the traces farther apart.

NEXT是怎么定义的呢？1.传输线两端有端接匹配；2.耦合长度足够长，能够使近端噪声达到稳定平滑值，这个值就称为近端串扰。端接发生任何变化，近端的噪声值就会改变。显然，NEXT的值取决于传输线之间的距离。降低NEXT的唯一方法就是让两条线离得足够远。

The far end has a very different signature than the near end. There is no far-end noise until one time of flight after the signal starts. Then it comes out very rapidly and lasts for a short time. The width of the pulse is the rise time of the signal. The peak voltage value is labeled as the far-end cross talk (or FEXT) coefficient. In the example above, the FEXT voltage is about 60 mV. This is with a signal of 200 mV, or a FEXT ratio of nearly 30%. This is a huge amount of noise.

不同于近端串扰，远端串扰会在信号开始一段时间之后产生，出现十分迅速，持续时间很短。脉冲宽度就是信号的上升时间，峰值电压就是远端串扰系数。上图中远端串扰电压值大约60mv，为输入信号的30%，这是一个很大的噪声。

If the terminations are not matched and reflections affect the magnitude of the noise, we still refer to the far-end cross talk, but the magnitude is no longer labeled as FEXT. This coefficient is the special case when the terminations are matched.

如果端接不匹配，反射影响噪声的幅度，这样就不能再称为远端串扰，因为远端串扰的条件是要端接匹配。

有三个因素可以减小远端串扰：减小耦合长度、增加上升时间、以及拉大传输线之间的距离。

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时间:2018.4.7 作者:Tom 工作:HWE 说明:本文主要是学习王剑宇的<高速电路设计实践>,已注明转载.

高速电路设计基本概念之——近端串扰和远端串扰

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