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I am designing a PCB with a couple of transmission lines. I have already calculated the thickness and width of my traces based on the substrate height and dieletric to have a 50 ohm transmission line, so I think I have that covered. But the question that now come to my mind is, is there any requirement on the distance between this transmission line and another traces present on the board? I will surround my TL with GND traces to avoid noice problems, but I have no idea on the distance I should left between my TL and the GND traces. Is there anyway to calculate this or anything else I should consider?

Many thanks!

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  • \$\begingroup\$ What kind of TL structure is it? Stripline? Microstrip? Coplanar? That will influence how far apart you should keep other copper. \$\endgroup\$
    – Krunal Desai
    Commented Jan 18, 2016 at 20:50

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To maintain the characteristic impedance, you should keep all other copper as far away from the transmission lines as possible.

Maintain a clearance of 10x the trace width if you can. 5x will probably be okay if you have to. 3x if you are willing to sacrifice signal integrity for design density.

Guard tracks are not necessary. If they are close enough to change the behavior of the transmission line, they will change the characteristic impedance. If they are far enough away to not affect the transmission line, then they could just as well be left off.

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  • \$\begingroup\$ Argh. I wanted to upvote and my keypad has a mind of its own. I will fix this shortly. \$\endgroup\$
    – Peter Smith
    Commented Jan 18, 2016 at 19:20
  • \$\begingroup\$ Many thanks for your answer. I think I have enough space to mantain a clearance of 10x. Will design my board based on this. \$\endgroup\$
    – cventu
    Commented Jan 20, 2016 at 13:34
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What kind of signal speed are you dealing with? As traces are brought closer to other traces, the characteristic impedance decreases. That is why I generally aim for an impedance 10% higher than the target impedance. The calculators you can find online ignore surrounding objects and traces, and I have found 10% to be a safe estimate. So design your trace widths for 55 ohms instead of 50.

Another tool I highly recommend is Hyperlynx SI, though it is very expensive. If this is for a one-off board, I recommend the trial version: https://www.mentor.com/pcb/product-eval/hyperlynx-si-virtual-lab

Hyperlynx uses Maxwell's equations to more accurately calculate the effect of surrounding objects on the impedance, and will also give you an idea of how much crosstalk you'll see between two traces. Generally you want to keep traces as far apart as is allowed by the board size, shape, and component layout. As mentioned, guard traces are probably not necessary.

There's really no way to say for sure how far away traces need to be from other traces to avoid crosstalk--It all greatly depends on your layout, signal speed, trace size, etc. Just space them as far apart from one another as you can. Then order a few boards you can use for testing, and make sure they work. Then you can produce more boards if desired.

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