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KhoiNguyen_CSC , 06-20-2024, 07:32 PM

HI, I am designing an Ethernet passthrough board that will pass 4 pairs of Ethernet signal through. Upon the design, I just have questions regarding the differential impedance without reference plane beneath. I have used Saturn PCB toolkit to calculate the differential pair but their model has a reference underneath as shown. How should I approach this design then

Robert Feranec , 06-21-2024, 04:18 AM

Make H large so plane is far away. This video may help: https://youtu.be/pXSK9KlNsW4

KhoiNguyen_CSC , 06-21-2024, 04:02 PM

Thank you @Robert Feranec for your prompt response. Upon watching the video, I understand that differential pairs without ground plane still exhibit controlled impedance. However, theirs impedance seem to be higher than that with ground plane.
With that in mind, I am unsure how to calculate the differential impedance for my case (without reference plane). I tried Saturn PCB with H large but it just require a significant increase conductor width. AS you can see in the picture, with H = 1000mm, the trace width W =10mm only resulted in 243 Ohm!

Robert Feranec , 06-21-2024, 04:34 PM

I dont have the saturn on this computer, did you try also H=100 or H=10 to see if you still get similar results? I dont think you need to put there 1000. Also did you play with spacing etc?

KhoiNguyen_CSC , 06-21-2024, 05:47 PM

Hi Robert, thank you for the quick response. I tried H = 10mm and it gives 5mm conductor width!
In addition, I found an application note online about this but not sure how reliable it is : https://www.hagtech.com/pdf/impedance.pdf . I think this one model after a twisted pair model but with modification
What do you suggest I should try next?