Signal Corruption
Signal integrity in high-speed printed circuit board transmission lines is governed by the symmetry of the differential trace pairs. When asymmetry occurs, differential to common mode conversion happens, transforming the intended data signal into unwanted common-mode voltage. This conversion leads to increased electromagnetic radiation and raises the susceptibility of the board to external electromagnetic interference.
Testing for this parameter is performed with a mixed-mode vector network analyzer that measures the S-parameters along the signal channel.
Impedance Asymmetry
Traces on a board must maintain identical lengths, widths, and dielectric environments to prevent skew. In differential to common mode conversion, a minute mismatch in the arrival time of the two opposing signals causes a voltage imbalance that propagates as a common-mode wave. Even the fiberglass weave of the substrate can introduce this delay if one trace runs on top of a glass bundle while the other runs over a resin-rich area.
Engineers utilize trace routing strategies like zig-zag routing to neutralize these local differences.
Shielding Mitigation
Solder joint uniformity and connector pad layouts are additional factors that can degrade high-frequency performance. At high data rates, any unbalanced parasitic capacitance at the pins will excite common-mode energy. The measurement of differential to common mode conversion is therefore an important test for boards carrying multi-gigabit data signals.
Eliminating this conversion ensures the board complies with international electromagnetic compatibility standards. Failure to meet these emission limits will prevent the product from being distributed in commercial markets. During the design verification stage, engineers must trace the route of the returns to make sure there are no breaks in the reference ground plane directly beneath the high-speed differential pairs, since any discontinuity instantly spikes the common-mode conversion rate.
