Using Differential Pairs in Allegro PCB Editor
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Dynamic Phase Control for Differential Pairs
Differential pair technology has evolved where more stringent checking is required in
the area of phase control. This is evident on higher data rates associated with parallel
buses such as QPI, SMI, PCI Gen 2, DDR, QDR and Infiniband. In the simplest of
terms, differential pair technology is sending opposite and equal signals down a pair of
traces. Keeping these opposite signals in phase is essential because it ensures that
they function as intended. As the current Static Phase is limited to a onetime check
across the entire Driver-Receiver path, a new Dynamic Phase check was introduced in
SPB 16.3 that performs phase checks at bend point intervals across the differential pair.
The Dynamic Phase check is designed to meet the guidelines suggesting that the path
lengths of the true and complement signals within the differential pair must differ by no
more than "x mils" along the entire path of the net. If at any point on the net, the skew
between true and complement exceeds "x mils", the mismatch needs to be
compensated within "y mils". Representative values for x and y might be x = 20 and y
=600.
