Meaning
High-speed digital traces must be matched in length to ensure that complementary signals arrive simultaneously. The differential skew represents the time difference between the arrival of the positive and negative signals of a differential pair at the receiver. This value must be minimized to prevent common-mode noise and signal degradation in high-speed lanes.
It limits the throughput of interfaces like USB and PCIe.
Signal Integrity
When the two signals in a pair do not arrive at the exact same moment, the differential eye diagram closes, which increases the likelihood of data errors. The differential skew creates unwanted electromagnetic radiation by converting the differential signal into a common-mode signal. This radiation can interfere with sensitive radio receivers mounted on the same circuit board, compromising the overall performance of the wireless transceivers.
Solder mask variations and board fiber weaves can also contribute to this timing difference.
Board Layout
Routing paths must include compensation bends to ensure that the length of both traces remains equal. Designers control the differential skew by adding trombone or serpentine routing to the shorter trace of the pair. This balancing must occur close to the mismatch point to keep the electromagnetic fields in balance across the board.
Receiver Margin
The transceiver specification dictates the maximum allowable phase mismatch the input buffer can tolerate. If the differential skew exceeds this budget, the receiver cannot reconstruct the clock signal correctly. This failure leads to packet loss and forces the system to drop to a lower transmission rate.
