做过serdes的请帮看这句话怎么理解，谢谢！
时间：10-02
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以下文字出于jssc05的troy beukema的文章,用于将ffe与dfe进行比较时的描述，这里他提到dfe的一个缺点，但我不理解为什么dfe只能对一小段时间内的isi进行补偿，意思是dfe不能使用太多tap吗?为什么ffe就不受限制呢?
a problem with dfe is that it can only compensate a fixed time span of isi. in contrast, an ffe can compensate isi arising from transmission line loss(i.e., unipolar post cursor impulse response) over a very wide time span since the ffe filter response, unlike the dfe filter response, is convolved with the impulse response of the channel.
therefore, in a very low bandwidth transmission line channel, significant unipolar post cursor isi may fall outside of the time span covered by the dfe and some level of pre-emphasis using ffe becomes required to lower the isi magnitude past the time span covered by the dfe feedback loops.
a problem with dfe is that it can only compensate a fixed time span of isi. in contrast, an ffe can compensate isi arising from transmission line loss(i.e., unipolar post cursor impulse response) over a very wide time span since the ffe filter response, unlike the dfe filter response, is convolved with the impulse response of the channel.
therefore, in a very low bandwidth transmission line channel, significant unipolar post cursor isi may fall outside of the time span covered by the dfe and some level of pre-emphasis using ffe becomes required to lower the isi magnitude past the time span covered by the dfe feedback loops.
对。 DFEtap是有限的，比如最多30左右。 但FFE是analog filter的方法做，可以很低频调整。
是因为IIR和FIR的关系吗?IIR tap太多了不稳定?
太多了需要存储的数据很多，加法器复杂。
take a read here
http://www.edn.com/electronics-b ... -DFE--and-crosstalk
This is also good paper ffe / dfehttp://emlab.uiuc.edu/ece546/ankit.pdf
可是ffe为什么可以使用很多tap，只有ctle才能吧，他的意思是ctle也是ffe的一种吗?
很好的资料，谢谢！
看到论文里对ffe ctle 和deemphasis区分的不是很清楚，是不是tx deemphasis也算一种ffe?rx ctle也算一种ffe?
我感觉文章的意思是，DFE是非线性的，它是看着ISI的大小，用暴力的方法直接减掉post-cursor的ISI再过比较器，有多少tap就减多少cursor. 而FFE放在TX端，它是一个线性的filter，你看它的pulse response是会延展到好多个cursor之后的，channel的pulse response也是延展到好多个cursor之后的，在多个cursor后它们还能抵消。
我说的FFE是他们说的 CTLE，是没与tap的。
谢谢！你说的没错，就是文章里好像对ffe、de-emphasis、ctle好像区别的不够清楚，有时候会混着说
非常感谢！懂了