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I have seen multiple designs that reference the RF on layer 1 to layer 3 with a cutout on layer 2, on an 8 layer board. All the calculations work out (using Qucs and Saturn) so the impedance is 50R. My question is why not made the L1-L2 distance a little larger so the sums work out?

It would simplify the layer stack considerably especially when adding RF on other layers using stripline.

On the same topic, what are the constraints when deciding track impedance? There are a lot of combinations of track width, layer gap and plane pullback that will give 50R. Do I set the desired track width and go from there? Any papers on this topic would be appreciated.Example of cutout on L2 to use L3 as reference plane

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  • \$\begingroup\$ Could you explain the picture? \$\endgroup\$
    – Andy aka
    Apr 9, 2020 at 9:28

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There are a couple of reasons to do this.

  • You often have things like high speed digital lines on the top layer that you want to be thin so you can fit them and want a sane impedance for them (often 50R) so you need the layer stack to give you that as L1/L2. However your RF line is somewhat lossy if you make it thin, mainly a skin effect thing at high frequency, so a larger line geometry is indicated.
  • It is helpful if your RF line and the geometry of pads required for connectors and component footprints more or less match, it avoids lots of faffing about with relieving grounds under caps and resistors. An 0603 cap for example will have a footprint that to match the width of its feed line needs a fairly wide track.
  • Sometimes dielectric losses are a concern (mainly at power), more spacing means less heat in the substrate.
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  • \$\begingroup\$ As we use 0201 as standard we typically have thin RF tracks to match and live with the skin effect loss. It occurred to me in the small hours last night the it may be due to the L1-L2 microvias. A quick check this morning and it's hard to get the 1:1 aspect ratio needed for the microvia while maintaining 50R impedance. I still have a whole lot to learn about layer stacks(and everything else) but in this instance I think my question has been answered, it has been bothering me for the better part of a week. Thank you for taking your time to reply to my question. \$\endgroup\$ Apr 10, 2020 at 19:51
  • \$\begingroup\$ Microvia aspect ratio ratios would explain it. \$\endgroup\$
    – Dan Mills
    Apr 10, 2020 at 23:15

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