Phase Skew
Variations in the characteristic impedance of a differential pair when the two conductors are driven with opposite polarity signals can lead to timing errors. The occurrence of odd mode impedance drift indicates a loss of control over the physical dimensions or the dielectric environment of the transmission line. Control ensures that the circuit handles electromagnetic interference and signal reflections correctly.
Geometric Variation
Changes in the distance between the traces or the height above the reference plane are primary drivers of this electrical instability. As odd mode impedance drift occurs during the manufacturing process, it changes the way the electric field interacts between the two conductors. Variations in the etch profile or the thickness of the soldermask layer also contribute to the shift.
Precise control of the lamination pressure ensures that the spacing remains uniform across the entire panel. Time domain reflectometry is used to identify where these deviations occur along the signal path.
Signal Distortion
Reduced noise immunity is a consequence of an inconsistent impedance profile in high speed links. When odd mode impedance drift becomes excessive, it creates reflections that close the eye diagram at the receiver. This degradation forces the system to run at lower data rates to maintain an acceptable bit error ratio.
Maintaining a tight tolerance on the substrate properties is required for reliable operation in demanding environments.
