AN 1517 PCB Layout Guide for USB Hubs

http://electronics.stackexchange.com/questions/48467/2-layer-usb-2-0-high-speed-routing

USB Hardware Design Guide AN0046 - Application Note

INTEL, High Speed USB Platform Design Guidelines

Tira cálculos reales en cuatro y dos capas: http://www.electronic-products-design.com/geek-area/electronics/pcb-design/general-pcb-design/pcb-track-impedance

Presenta una técnica para bajar el ancho de pista: http://www.focusembedded.com/blog/high-speed-usb-in-a-two-layer-pcb/