Watching the currents flow

摘要

It was late Sunday evening, and we were still trying to trace why the embedded DSL modem was feeding a strange 50-MHz carrier into the network-much stronger than -40-dB noise allows. And no 50-MHz signal existed on the design! Jeremy, the board designer, did his best to lay out a six-layer board and placed many bypass capacitors as numerous reviews suggest. He also kept power and ground planes everywhere on separate layers and kept the analog traces as far from noisy digital parts as he could. We had a faint hope that the board was picking up a spur from a clock line 1 in. away; our hope disappeared when the second, improved board revision made things worse.