Troubleshoot and verify 8b/10b encoded signals with a real-time oscilloscope

Chris Loberg, Tektronix
EETimes (4/30/2012 6:53 PM EDT)

Few serial technologies have become more widely adopted than 8b/10b coding, which is now used in standards like PCI-Express, Serial ATA, SAS, Fibre Channel, InfiniBand, FireWire, MIPI M-PHY, HDMI, DisplayPort, CIPRI, OBSAI, XAUI, USB3.0 and others.

Therefore, any designer will eventually need the ability to efficiently analyze 8b/10b encoded signals using common instrumentation such a real-time oscilloscope. The intent of 8b/10b line coding is to achieve DC balance and provide enough state changes to ensure stable clock recovery. Since DC balance is maintained, 8b/10b signals can be transmitted through transformers, optical channels or AC coupled links which have DC offsets at the pins of their integrated circuits.

AC coupled data signals would have DC drifts depending on the data content. A long sequence of 1s will lead into positive drift and many 0s will drift toward negative voltage, as shown in Figure 1 below. Without correction it will cause errors at the receiver side since a fixed threshold is being compared to the drifting voltage level of the data signal.

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