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When we are matching the length of positive and negative tracks of a differential signal in a PCB, is the position of matching important? Does it matter we match the length immediately after the transmitter or the receiver? If yes, which side is preferable and why? It seems there is no discussion about it in the Internet!

I have drown an image to clarify the question: Which one is preferable ?

Edit: The length difference is mainly due to the connection of tracks and pads as shown in this picture: connection

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    \$\begingroup\$ The point of running the signal in parallel is not only to match the length but to reduce a common-mode noise, and also keep the same phase on both conductors. So the way you are doing it (with this blip) is not a very good idea. If you have a length difference due to the "right-turn", then equalize it with a "left turn" elsewhere. \$\endgroup\$
    – Eugene Sh.
    Feb 22, 2018 at 16:24
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    \$\begingroup\$ Also, AFAIK you length match after the "unmatching" happens. If you put the extra length close to the transmitter, the signals will be unmatched until the length difference is corrected. \$\endgroup\$
    – Wesley Lee
    Feb 22, 2018 at 16:26
  • \$\begingroup\$ Actually the length difference is mainly due to connection of tracks and pads. I'm adding an image to the post to clarify this... It should also be noted that I am routing a 10 Gbps signal. \$\endgroup\$
    – M.H
    Feb 22, 2018 at 16:33
  • \$\begingroup\$ Then you better approach the pads from a different angle, such as it would perpendicular to the line connecting the two pads. \$\endgroup\$
    – Eugene Sh.
    Feb 22, 2018 at 16:41
  • \$\begingroup\$ @EugeneSh. It's impossible due to other problems... \$\endgroup\$
    – M.H
    Feb 22, 2018 at 17:08

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The key idea behind differential signal routing is that any noise source that affects one wire affects the other wire equally, effectively cancelling out the noise.

Therefore, it is important that the noise enters each wire at roughly the same place along its length - otherwise, the two "copies" of the noise will be at different phases when they reach the receiver, and will not cancel as effectively.

With this in mind, imagine an extreme case where the "noise" source is at 10GHz (say it's another 10Gbps signal on your board). Lets say that at this frequency, 0.35 inches is 1/2 wavelength. Now, pretend that your two traces are 0.35" different in length, and that you put a 0.35" length correction at one end. Now, a large section of your positive trace is 0.35" "misaligned" from your negative trace - and now, instead of the 10GHz noise cancelling at the receiver, it adds constructively, doubling the noise. The best practice in this scenario is to add tiny length compensations along the entire length of the pair.

This is, obviously, an extreme case. If your most annoying noise source is, say, 100MHz, the wavelength (and thus alignment tolerance) is much longer. Also, even in the 10GHz case, 0.35" of initial length mismatch is pretty extreme.

My conclusion? You're probably fine either way (although you may want to avoid the right angle bends in favor of curves or 45 deg bends). There's not much difference in putting the length correction at the transmitter or the receiver. But if you really want to do it right, the length corrections should be distributed along the line, matched up with the places where the length errors happen.

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