Yet another differential pair question - Page 1
Forum section: electronics / projects

author: DragonRider
author_rank: Newbie
timestamp: October 24, 2024, 09:37:16 pm
content: Hello, first post here, I've been reading here, but was reluctant to post.Anyway, I finished a design for a 6-layer board with a multiple differential pairs (not too sensitive, 3 x 100Mbit Ethernet and 5 x USB 2.0) and relatively densely populated in some areas. So I have a few questions about diff-pair routing, since I'm having some doubts and are second-guessing myself since I'm not too experienced with PCB design (mechanical/aeronautics engineer, I've done one 4-layer design and 6-7 simpler 2 layer boards).I plan to use JLCPCB for the PCBs and have chosen their JLC06161H-2116A 6-layer stackup with differential pairs routed on the top and bottom layers. I wanted tightly coupled ground planes for EMI reasons. The layers are arranged like this:1. Signals (including Ethernet plus a lot of other slower things. Most ICs are on this side of the board.- Prepreg (~4.5 mils)2. GND- Core (~5 mils)3. Signals (slow stuff, UARTs, I2C, control signals and such)- Prepreg (~4.5 mils)- Core (~27.5 mils)- Prepreg (~4.5 mils)4. Power plane (to big areas and a little bit of routing too)- Core (~5 mils)5. GND- Prepreg (~4.5 mils)6. Signals (various signals, including the Ethernet and USB differential pairs). Many power components are on this side of the board, near to the connector.I routed the USB entirely on the bottom side (layer 6), but Ethernet had to go from Layer 1 through vias to Layer 6. Most traces are not too long (~50-60 mm).The differential pairs are routed as 6 mil traces with 10 mil separation for USB (7 and 10 mils for ETH).Since the stackup has a thin prepreg between layers 1-2 (and 5-6) the distance from diff-pair traces to the ground plane is smaller than to each other. Is this OK? Could this lead to some noise or EMI issues? I guess this is what a loosely-coupled differential pair is, and should be fine, but I'm still having doubts.Also, it was necessary to route Ethernet lines beneath a power module (enclosed DC-DC converter module from Traco Power, namely the THN30WI-2411). Since this component is on top of the traces, would this alter their impedance or lead to noise, EMI issues etc? This DC-DC converter is only mounted on this side, but has its routing on the layer on the opposite side of the PCB (and all the accompanying components are placed there). The ethernet traces are beneath the converter, but close to the edge with the pins 1,2,6 (not in the center, where I assume the internal inductor/coil is). I didn't even think about this when doing the design. Can I expect issues with this?

author: RoGeorge
author_rank: Super Contributor
timestamp: October 25, 2024, 06:37:23 am
content: See if the video links from this post help:https://www.eevblog.com/forum/chat/fun-for-nerds/msg5583117/#msg5583117It is mostly based on the book(s) "Grounding & Shielding Techniques by Ralph Morrison"Other bibliography recommended by Rick Hartley:Quote

author: DragonRider
author_rank: Newbie
timestamp: October 26, 2024, 08:25:43 pm
content: Thanks, I've watched that video (and a few others with Rick Hartley and Eric Bogatin too). I'll try to get a hold of the book as well.Still not totally sure about the effect of the power module, I guess we'll see when the prototype boards arrive.