Reduction of signal line to power plane coupling using controlled-return- current transmission lines

In printed circuit boards, transmission of high-speed digital signals is achieved using single-ended or differential transmission lines, typically using a microstrip or stripline configuration. This requires continuous reference planes, which is difficult to maintain in multilayered IC packages or boards. As a result, the signal return current may need to flow between different reference planes due to a cut-out in the reference plane or a signal via transition. The uncontrolled path of the return current results in signal integrity and electromagnetic interference problems. We propose a new controlled-return-current transmission line design to reduce this problem. Our approach is based on guard traces to help control the return currents.