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标题: 讨论差分线松耦合和紧耦合问题 [打印本页]

作者: Ivan_GONG 时间: 2012-5-25 10:30
标题: 讨论差分线松耦合和紧耦合问题
因为差分线之间存在串扰，所以在外层走线时需要松耦合
因为内层没有远端串扰，所以在内层走线时要紧耦合
大家板砖扔过来啊！

作者: nishiyufrank 时间: 2012-5-25 12:30
和楼主讨论一下，我也目前正遇到差分线的问题，是DDR3的三对差分线，阻抗要求是100欧，线宽为5mile，线距为7mile，请问楼主这个松耦合和紧耦合有具体的量化吗？
恳请赐教

作者: willyeing 时间: 2012-5-25 12:32
lz不会吧，差分线相位180，自身会有窜扰问题吗，好像第一次听说哎！！！

作者: qiangqssong 时间: 2012-5-25 16:04
差分线对应该都走紧耦合比较好，跟内、外层没有多少关系！！

作者: yejialu 时间: 2012-5-30 09:56
本帖最后由 yejialu 于 2012-6-1 14:33 编辑

目前我们做差分线都倾向于紧耦合，就是线间距不超过线宽的2倍为紧耦合，3倍以上为松耦合，没区分走外层还是内层。对100欧的差分对，外层不容易做到100欧。紧耦合的好处有以下几点：1，节省布线空间。2，更好地抑制共模噪声。3，更好地使走线的阻抗与过孔的阻抗匹配（过孔阻抗按60到80欧姆估计）。目前只想到这些，希望对大家有帮助。

作者: Ivan_GONG 时间: 2012-5-30 20:59

nishiyufrank 发表于 2012-5-25 12:30
' p& G, {6 Q/ K/ ^( U% Q- L和楼主讨论一下，我也目前正遇到差分线的问题，是DDR3的三对差分线，阻抗要求是100欧，线宽为5mile，线距为 ...

量化的的话没有确定的值，可以参照5楼所说的3W的法则

作者: Ivan_GONG 时间: 2012-5-30 21:00

yejialu 发表于 2012-5-30 09:56 & R! f4 [6 E4 ^* Y. O& `
目前我们做搽粉线都倾向于紧耦合，就是线间距不超过线宽的2倍为紧耦合，3倍以上为松耦合，没区分走外层还是 ...

请问是否考虑过查分线之间的串扰？

作者: Ivan_GONG 时间: 2012-5-30 21:01

willyeing 发表于 2012-5-25 12:32 ( W# h8 X" b* ?+ W" m6 @8 s; g/ O
lz不会吧，差分线相位180，自身会有窜扰问题吗，好像第一次听说哎！！！

你可以仿真试试，串扰是减不掉的

作者: nishiyufrank 时间: 2012-6-1 10:14

Ivan_GONG 发表于 2012-5-30 20:59 1 ~. t& R+ [, x, D
量化的的话没有确定的值，可以参照5楼所说的3W的法则

谢谢楼主

作者: scott 时间: 2012-6-2 09:53
差分紧耦合是可以抑制外界对差分对的干扰，差分对相位相反，故可将外界叠加的串扰相抵消；
对于差分对内的相互串扰，还真不好说，串扰的话应该会因为相位相反，使得相位差变小，影响的话不太清

楚。

作者: understudy 时间: 2012-6-15 21:54

yejialu 发表于 2012-5-30 09:56 ' D* ^: h* T3 N( m
目前我们做差分线都倾向于紧耦合，就是线间距不超过线宽的2倍为紧耦合，3倍以上为松耦合，没区分走外层还是 ...

紧耦合会使差分线的线宽变窄，损耗增大。所以紧耦合没有想象的那么好！

作者: yejialu 时间: 2012-6-18 11:28

自然有缺点，看你关注那点

作者: saky66 时间: 2012-6-18 14:43
外层差分走线奇模式与偶模式之间的传输差异是不是不可避免？这么说来差分线不适合外层走？

另外，问个问题：内外层走线阻抗在制板工艺上，哪个更容易控制？

作者: Ivan_GONG 时间: 2012-10-16 15:33
高手现身吧

作者: hewin 时间: 2012-10-21 12:39
我觉着要看实际情况吧，如果外层走松耦合的时候，差分线的旁边正好有一根单根线是不可避免的，那你还会选择松耦合吗？小GONG!

作者: 大胡子 时间: 2012-10-26 17:07
你的说法不对，差分线难道会存在串扰？

作者: Ivan_GONG 时间: 2012-10-27 09:59

大胡子发表于 2012-10-26 17:07 + ]: v& t" E$ H
你的说法不对，差分线难道会存在串扰？

请仁兄发表自己的看法和证据，重在讨论，呵呵。

作者: stevenzhao 时间: 2012-11-21 11:19
给个公式吧，从Intel文档上看的，diff Coupling=(4XZcm-Zdiff)/(4XZcm+Zdiff)<8%,7%最好推荐松一些的耦合，从加工可控程度上也是松耦合要好的

作者: Ivan_GONG 时间: 2012-12-26 17:30

stevenzhao 发表于 2012-11-21 11:19 ; k+ n8 o. Q) B6 ^
给个公式吧，从Intel文档上看的，diff Coupling=(4XZcm-Zdiff)/(4XZcm+Zdiff)

楼主这个公式非常有参考价值如果能多些说明为什么会要控制在7%以内，是否是考虑的串扰问题感觉上没有串扰就没有耦合，哪这个耦合是好处大呢还是益处较大呢。

作者: Ivan_GONG 时间: 2013-1-4 16:45

milkgreen1980 发表于 2013-1-2 17:13 8 m, R2 m$ T) P: Z3 m
差模串擾常常看的是一差分對與另一差分對,怎麼大家都在討論差分對裡P與N的串擾. 不論內外層,不論串擾多寡," ...

那么能否给出PN之间串扰被抵消的证明呢？求仁兄能否进一步解释！

作者: Ivan_GONG 时间: 2013-1-4 17:36
串扰大小相等可是方向却是相反的，也就是说正负相反，减不掉的。

作者: two1405 时间: 2013-1-23 11:20
{:soso_e179:}

作者: honejing 时间: 2013-1-23 20:46
Eric 大師在 DesignCon 2013 community 的這篇發文正好有說明Tighter coupling and Looser coupling 問題。
http://www.designconcommunity.com/author.asp?section_id=2607

Coupled Differential Pairs ：
Before you read on, think about your answer to the question, which is better for differential pairs: tight or loose coupling? When I teach classes to a large group, I always ask this question and the answers never cease to astound me. There are just as many strong proponents for each answer, and some of them are very adamant. When I ask why they choose one or the other, the answers fall into three categories:

1. It’s what we’ve always done, it worked in the last design, it’s what our experts tell us, we changed it once nd the design didn’t work so we changed it back.
2. Tighter coupling reduces cross talk, tighter coupling reduces EMC problems, tighter coupling makes it easier to match lengths.
3. Looser coupling has higher bandwidth, looser coupling has less manufacturing variation, looser coupling is easier to route, looser coupling is easier to add compensation lengths.
The first category of reasons is never a good reason for a design rule. Every design is custom. It may not be true that because one design rule worked in the previous design that it will also work in the next design. In fact, the same design may not even work with just a die shrink change in one of the chips.

The real answer to the question of which is better, tight or loose coupling, has only one answer. The most common answer to all signal integrity problems is "it depends." The only way to answer "it depends" questions is to "put in the numbers" with rules of thumb, approximations, or numerical simulations.

All engineers should be empowered with the knowledge, skill, and tools to answer "it depends" questions. At the very least, it’s important to know what the answer depends on. With experience and confidence, all engineers can answer "it depends" questions.

While all the reasons listed above for or against tight or loose coupling have some element of truth, when you put in the numbers, there are only two first-order factors driving the answer.

For microstrip or stripline transmission-line structures, tighter coupling will always result in higher interconnect density. That allows for fewer board layers or a smaller board size.

If you care about cost, tighter coupling should always be your first choice. It’s not about cross talk, but about interconnect density.

The downside to tighter coupling is that to achieve the target differential impedance, tighter coupled pairs requires a narrower line width than if all features were kept the same and the lines were pulled apart as loosely coupled. The downside of tighter coupling is higher resistive loss from the narrower line width, which means higher attenuation.

If you care about attenuation, you’ll want to consider loosely coupled lines.

How much is the line width reduced by tight coupling? Only a 2D field solver can really answer this question.

As the two lines in a differential pair are brought closer together, the differential impedance of the pair will decrease. To raise the impedance back to the target value, the line width needs to be decreased. Using a 2D field solver, we can literally map out design space for a differential pair: how the line width needs to change as the line to line spacing changes, to maintain a constant differential impedance.

In Figure 1, we show the design space for a stripline differential pair that was designed for 100Ohm target impedance when the two lines were far apart. The conditions were: total thickness between the planes of 13 mils, half ounce copper and Dk of 4. As long as you always walk this line (that’s why I refer to this as the “Johnny Cash” principle), your differential pair will always have a 100Ω differential impedance.
Between loosely coupled and tightly coupled pairs, the line width decreases from about 5 mils to 3.5 mils. This is a 30 percent decrease in line width, which would contribute to a 30 percent increase in conductor loss. When conductor loss dominates, this can contribute to a 30 percent decrease in a channel's interconnect signal bandwidth.

If loss is important, you’ll want to consider tweaking every design knob to decrease loss. That’s when loosely coupled lines should be considered.

Before you make an important design decision, put in the numbers to see just how large or small the “bang for the buck” might be.

作者: Kaso 时间: 2022-3-15 09:52

yejialu 发表于 2012-5-30 09:56
% A3 N( R3 A1 V; E8 S u! J' R4 z目前我们做差分线都倾向于紧耦合，就是线间距不超过线宽的2倍为紧耦合，3倍以上为松耦合，没区分走外层还 ...

紧耦合＜3W，松耦合＞3W，那么松耦合线距没有一个区间范围吗？比如说小于多少才能耦合

作者: qinqinstart 时间: 2022-9-7 09:05
目前正在研究这个问题

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