Energy Transformation
Signal phenomenon where energy from a differential signal is transformed into a common-mode signal or vice versa. The occurrence of mode conversion typically results from physical asymmetries in a transmission line or connector. It represents a loss of signal balance that degrades performance.
Causal Factor
Differences in trace length or dielectric constant between two conductors in a pair trigger the shift. In mode conversion, the phase or amplitude balance is disrupted, causing a portion of the intended signal to return through the ground path. Even a small skew in the arrival time of the two components creates a common-mode voltage.
System Impact
Increased electromagnetic emissions are the most significant consequence of this imbalance. Because mode conversion generates common-mode current, the cable behaves like an antenna and radiates noise into the surrounding environment. This energy can interfere with sensitive radio receivers or cause a failure in regulatory compliance tests.
Furthermore, common-mode noise from external sources can convert into differential noise, directly polluting the data signal and reducing the signal-to-noise ratio.
Control Metric
Standards for high-speed data transmission specify maximum allowable limits for this parameter. Designers minimize mode conversion by using tightly coupled traces and ensuring that any impedance discontinuities are symmetrical.
