Geometric Symmetry
Signal transmission design requires two coupled conductors to carry complementary signals of equal amplitude and opposite phase. This configuration, known as differential pair routing, maintains a consistent spacing between the traces to ensure electromagnetic coupling along their entire path. When noise is coupled equally into both lines, the receiver cancels the common-mode component to extract the clean signal.
Maintaining this configuration requires keeping both traces on the same layer of the circuit board to ensure identical dielectric properties. Changing layers forces the signals to traverse vias, which introduces parasitic capacitance and inductance that disrupts the impedance of the pair unless matched return vias are placed nearby.
Impedance Matching
Trace separation dictates the degree of coupling and the differential impedance of the transmission line. When the traces run close together, they benefit from tighter coupling and higher noise immunity, but they also require a narrower trace width to maintain a target impedance of one hundred ohms. Widening the gap reduces the coupling, making the pair more susceptible to board-level crosstalk from adjacent single-ended signals.
Phase Distortion
Propagation delay differences between the two traces generate phase mismatch and convert differential signal energy into common-mode noise. Length matching is achieved by adding small serpentine bends to the shorter trace as close to the source of the mismatch as possible. If the mismatch exceeds a small fraction of the signal rise time, the rising and falling edges no longer align, causing severe signal degradation at the receiver.
This delay disparity must be restricted to less than five picoseconds in multigigabit per second channels.
