[equalization] EQ from PCISIG_tx equalization

本文是记录从PCISIG上学习EQ的相关文章，包含原文、译文及自己的理解。侵删。

什么是均衡?Tx均衡与Rx均衡有何不同?什么是可训练均衡?

提示：取自“What is equalization? How is Tx equalization different from Rx equalization? What is trainable equalization?”

• 均衡是通过变换来扭曲数据信号。
• 均衡可以应用在Tx、Rx，或者两者都应用。
• 均衡的一种简单形式：PCIe Gen1/2中的去加重（数据信号需要翻转时，以full swing；不需要翻转时，以reduced swing）
• 均衡的目的：
  • 减小高频损耗产生的影响
  • 补偿channel中由于阻抗不连续(如过孔或连接器)反射而产生的波纹。
• 均衡的实现：各类算法。
  • 两种常见算法：线性反馈(LE)和决策反馈(DFE)
  • 线性均衡可以在Tx或Rx处实现，而DFE在Rx处实现。
• 可训练均衡：tap coefficients可以调整
• 有关tap coefficients，当前理解是各种算法中使用的滤波器的系数。

什么是均衡?Tx均衡与Rx均衡有何不同?什么是可训练均衡?
均衡是一种使用变换来扭曲数据信号的方法，其表现为信道响应的近似逆。
它可以应用于Tx、Rx、或Tx和RX。

一种简单的均衡形式是在PCIe 1.x和PCIe 2.X中规定的Tx去加重，其中数据在每次极性转换后以full swing的模式发送，之后以reduce swing的模式发送相同极性的所有bits的数据。
去加重减少了Rx看到的低频能量。

由于通道在高频处表现出更大的损耗，均衡的作用是减少这些影响。

均衡也可用于补偿通道中由于阻抗不连续(如过孔或连接器)反射而产生的波纹。

均衡可以使用各种类型的算法来实现;最常见的两种是线性反馈(LE)和决策反馈(DFE)。

线性均衡可以在Tx或Rx处实现，而DFE在Rx处实现。

可训练均衡是指调整tap coefficients的能力。
Tx、channel和Rx的每个组合都有一组独特的系数，产生最佳的信噪比。
训练序列包括对tap coefficients的调整，同时应用质量度量来最小化误差。
在PCIe规范的下一个修订中，均衡类型的选择在很大程度上取决于可以在最低成本点上获得的互连通道优化。

PCI-SIG的目的是以最低的成本、为最常见的拓扑结构提供通道和硅增强的最佳组合。
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原文链接：https://pcisig.com/what-equalization-how-tx-equalization-different-rx-equalization-what-trainable-equalization
分段以方便浏览：
《What is equalization? How is Tx equalization different from Rx equalization? What is trainable equalization?》
Equalization is a method of distorting the data signal with a transform representing an approximate inverse of the channel response.
It may be applied either at the Tx, the Rx, or both.
A simple form of equalization is Tx de-emphasis as specified in PCIe 1.x and PCIe 2.x, where data is sent at full swing after each polarity transition and is sent at reduced swing for all bits of the same polarity thereafter.
De-emphasis reduces the low frequency energy seen by the Rx.
Since channels exhibit greater loss at high frequencies, the effect of equalization is to reduce these effects. 
Equalization may also be used to compensate for ripples in the channel that occur due to reflections from impedance discontinuities such as vias or connectors. 
Equalization may be implemented using various types of algorithms; the two most common are linear (LE) and decision feedback (DFE). 
Linear equalization may be implemented at the Tx or the Rx, while DFE is implemented at the Rx. 
Trainable equalization refers to the ability to adjust the tap coefficients. 
Each combination of Tx, channel, and Rx will have a unique set of coefficients yielding an optimum signal-to-noise ratio. 
The training sequence consists of adjustments to the tap coefficients while applying a quality metric to minimize the error. 
The choice for the type of equalization to require in the next revision of the PCIe specifications depends largely on the interconnect channel optimizations that can be derived at the lowest cost point. 
It is the intent of PCI-SIG to deliver the most optimum combination of channel and silicon enhancements at the lowest cost for the most common topologies.
原文：
What is equalization? How is Tx equalization different from Rx equalization? What is trainable equalization?
Equalization is a method of distorting the data signal with a transform representing an approximate inverse of the channel response. It may be applied either at the Tx, the Rx, or both. A simple form of equalization is Tx de-emphasis as specified in PCIe 1.x and PCIe 2.x, where data is sent at full swing after each polarity transition and is sent at reduced swing for all bits of the same polarity thereafter. De-emphasis reduces the low frequency energy seen by the Rx. Since channels exhibit greater loss at high frequencies, the effect of equalization is to reduce these effects. Equalization may also be used to compensate for ripples in the channel that occur due to reflections from impedance discontinuities such as vias or connectors. Equalization may be implemented using various types of algorithms; the two most common are linear (LE) and decision feedback (DFE). Linear equalization may be implemented at the Tx or the Rx, while DFE is implemented at the Rx. Trainable equalization refers to the ability to adjust the tap coefficients. Each combination of Tx, channel, and Rx will have a unique set of coefficients yielding an optimum signal-to-noise ratio. The training sequence consists of adjustments to the tap coefficients while applying a quality metric to minimize the error. The choice for the type of equalization to require in the next revision of the PCIe specifications depends largely on the interconnect channel optimizations that can be derived at the lowest cost point. It is the intent of PCI-SIG to deliver the most optimum combination of channel and silicon enhancements at the lowest cost for the most common topologies.
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