Meaning
Differential signal routing on printed circuit boards relies on identical physical geometry for both conductors to maintain balanced propagation. When geometric differences occur between the two traces, microstrip asymmetry develops and disrupts the balance of the differential signal. This condition converts part of the differential-mode signal into an unwanted common-mode signal.
Electromagnetic Emission
Balanced lines carry equal and opposite currents whose magnetic fields cancel each other in the far field. When microstrip asymmetry exists, this cancellation is incomplete, resulting in high levels of electromagnetic radiation from the trace. This radiated energy can cause the product to fail regulatory compliance tests for electromagnetic compatibility.
Furthermore, the unbalanced trace becomes more susceptible to external electromagnetic interference, which compromises the reliability of the system.
Phase Discrepancy
Unequal path lengths or trace widths cause the signals on the two lines to arrive at different times. This time delay converts a portion of the high-speed transition into a common-mode voltage spike. This skew limits the usable data rate of the interface because it shrinks the available timing budget.
Design Compensation
Engineers address routing imbalances by utilizing length-matching structures. These serpentines compensate for microstrip asymmetry by adding small, controlled loops to the shorter trace. Design software automates the insertion of these matching segments to ensure that propagation delays are kept identical.
This compensation restores the differential balance and preserves the integrity of high-speed data streams.
