I'm tweaking the design of my usb-audio device to pass the usb high speed eye pattern test. Due to the positioning of key components, the design features quite long d- d+ traces, 139mm (5.47") to be exact. In a previous iteration of the PCB, I just routed a differential trace on the default settings and got the usb funtion to work just fine. But, the length seems pushing it to meet usb compliance. That's why I want to match the impedance as best as possible while keeping transmitted and rf noise to a minimum to keep the analog part quiet.
The pcb is a 4 layer board, the part where the traces are running looks like this:
I have a few options:
- The option pictured below: route the traces as microstrip on the top layer (red) with a ground fill on the layer below it (yellow). This has the advantage of not having to use any vias near the PHY and the tvs-diodes near the connector. Another advantage is that, with my board-house's stackup, this option matches the 90 ohms impedance the best. The disadvantage is the wierd plane "stick-out" needed to ensure the traces reach the phy without interruption in the ground plane below. I'm not quite sure what the effect of this is. (The yellow plane surrounding the "stick-out" is 3.3v, changing that would be a lot of work.) Also, my guess is that a microstrip radiates more than a stripline.
- Route the traces as microsrip on the bottom layer (green). This has the advantage of having a continuous ground plance (pink, invisible) above it. Disadvantage: I'd need introduce two sets of vias if I were to keep all components on top.
- Stripline. Less radiated rf(?). But also again vias, and not able to achieve 90 ohms diff / 45 ohms single ended spot on with the existing stack-up (but within 5% tolerance).
Which option would you think is best?