Meaning
Electromagnetic interaction between adjacent signal paths on a printed circuit board defines the unwanted transfer of energy from a driver line to a quiet line. This crosstalk coupling occurs through mutual capacitance and mutual inductance when conductors run parallel over long distances. The intensity of the interaction drops rapidly as the spacing between traces increases.
Physical Origin
Mutual capacitance injects a current proportional to the rate of voltage change on the active line. At the same time, mutual inductance induces a voltage proportional to the rate of current change. These two mechanisms combine to produce forward and backward noise on the victim trace.
Backward noise flows toward the source of the victim line and remains constant over a long line length. Forward noise travels with the signal pulse, creating a short, high-amplitude spike at the victim receiver.
Board Layout
Designers reduce the signal interference by maintaining a separation distance of at least three times the trace width. Solid ground planes under the traces provide a low-impedance return path, which confines the magnetic fields. Using stripline routing instead of microstrip also helps because the homogeneous dielectric balances the inductive and capacitive contributions.
Signal Measurement
Oscilloscope measurements evaluate the induced voltage spike on the quiet line under worst-case switching conditions. The peak amplitude of the disturbance must remain below the noise margin of the receiving logic gate. A receiver fail can happen if the noise exceeds the threshold of the input pin.
