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Differential pair routing

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chrischina

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I am a bit lost about how to establish the parameters to establish differential pair routing. I am currently trying to route several differential pair and I am a bit new to this. I have searched online some tutorial but I don't find any really explaining **easily and out of the theory** with sipmple succession of steps about how to do it

I have downloaded saturn pcb software to see if it can help but I don't understand well how the soft is working because what it ask me to input is typically what I am searching for (it looks it is reverted... !!!)

Assuming the differential pair I am routing is requiring 90 ohm (this is what the hardware guideline say "Route the USB differential pair on the top layer with a trace width and differential spacing tuned to the
PCB stack-up for 90Ω differential impedance ").

So in Saturn PCB I have indicated in target zdiff : 90
Conductor spacing: I don't know (Am I supposed to arbitrary settle this, then accomodate the other parameters ?)
Conductor width: same
Conductor height: this I suppose I need to check my layer stack up. In this case I suppose I have to decide the spacing between top track and ??? first prepreg or core ?

I would really appreciate if someone who have experience could explain their methodology via a "1./ 2./ 3./ 4./ points methodology" to be able to achieve any differential pair routing.

Thank you in advance for your help.
 

chrischina said:
I am a bit lost about how to establish the parameters to establish differential pair routing. I am currently trying to route several differential pair and I am a bit new to this. I have searched online some tutorial but I don't find any really explaining **easily and out of the theory** with sipmple succession of steps about how to do it

I have downloaded saturn pcb software to see if it can help but I don't understand well how the soft is working because what it ask me to input is typically what I am searching for (it looks it is reverted... !!!)

Assuming the differential pair I am routing is requiring 90 ohm (this is what the hardware guideline say "Route the USB differential pair on the top layer with a trace width and differential spacing tuned to the
PCB stack-up for 90Ω differential impedance ").

So in Saturn PCB I have indicated in target zdiff : 90
Conductor spacing: I don't know (Am I supposed to arbitrary settle this, then accomodate the other parameters ?)
Conductor width: same
Conductor height: this I suppose I need to check my layer stack up. In this case I suppose I have to decide the spacing between top track and ??? first prepreg or core ?

I would really appreciate if someone who have experience could explain their methodology via a "1./ 2./ 3./ 4./ points methodology" to be able to achieve any differential pair routing.

Thank you in advance for your help.
Click to expand...

The first thing you need is your board stackup. Unless the stackup is undefined? The stack up would provide dielectric material properties (Dk, Df) and vertical spacing/thickness of dielectric layers above and below your transmission line. These are important parameters needed for determining appropriate trace width and spacing to meet Differential impedance of 90 ohms.

There are many TLINE editors out here that would allow you to input all the necessary parameters for accurate impedance calculations, but you should at minimum understand your stackup dimensions and material first. Then you can adjust trace and space accordingly.

In general. The wider the trace, the lower the impedance. The more tightly coupled two transmission lines are (edge to edge spacing) the lower the differential impedance will be. The larger the dielectric spacing above and or below the transmission line, the larger the impedance. Dk also impacts impedance for a given differential trace geometry and stackup.
 

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