Why Differential Pairs Still Need a Return Plane

Differential pairs don't need a return plane. You may have heard that before. It's wrong. This video proves it with simulation. Using the SATA 100Ω differential pair from the previous video, we run the impedance workflow in Sigrity X Aurora with the return plane intact, then remove it entirely and rerun. With the plane in place, single-ended impedance holds around 48Ω as designed. Remove it and that same trace jumps to 139Ω average, with peaks hitting 300Ω differential. Three times the target. Then we add a slot void directly underneath the pair to show exactly how instantaneous the impedance discontinuity is at the void boundary. A differential pair is still two transmission lines. The electromagnetic field still needs a return reference. At SATA, USB3, PCIe, and DDR speeds, the consequence of missing that reference is not small. Route over a continuous ground plane. Always.