< I have made double-sided boards that ran at 100MHz+. The trick was to us < (a) one side entirely unetched as a ground plane, (b) design all the < tracks as striplines, (c) know the characteristics of the PCB material, < (d) use 100% SMD components with the shortest possible ground < connections, (e) use carefully designed twisted pair lines to carry < signals about if you couldn't route the tracks, (f) terminate everything < properly and (g) take a lot of care and think about what you are doing. (h) accept lower component density on the board to do that. It's very doable but, a 4-6 layer makes it so much less painful for a production system. Allison