Option lets oscilloscope monitor link-training signals

Article By : Martin Rowe

The HSSLTA decimates some 6 million Ethernet frames down to those where equalisation changes take place.

Connection speeds continue to increase, and now 100Gbit/s lanes are coming to 400Gbit/s Ethernet. Although 400GbE links will be fibre, their 100Gbit/s per-lane signals must still travel over copper backplanes and interconnects.

Such high data rates require equalisation (pre-emphasis and de-emphasis) of signals to compensate for transmission losses. Because each link is unique, a transmitter and receiver must negotiate how the transmitter will launch a signal into the channel to optimise BER (bit-error ratio). To let you monitor and analyse the link-training process, (Figure 1), Tektronix has added a 100GbE Link-Training option (HSSLTA) to its 70GHz DPO70000SX oscilloscope. The option lets you see how well the negotiating process works and where it needs optimising.

[Tektronix PAM4 measure-system (cr)]
__Figure 1:__ *The HSSLTA option lets an oscilloscope monitor the link-training process in high-speed serial links.*

The HSSTLA option gathers PHY-level and link-level data on the negotiation process. It then decimates some 6-million Ethernet frames down to those where equalisation changes take place (typically 50-60) by triggering acquisitions on those equalisation data changes. From the captured and time-stamped frames, the oscilloscope software can analyse the data or let you can download the data for analysis and storage.
The screen image (Figure 2) shows how the software takes negotiation data from a 100GbE signal. The markers at the top indicate frames where negotiation takes place. Each frame (marked "FM") contains four hex words that define the negotiation data. The table in the screen image displays state changes in the process, including columns that indicate equalisation coefficients. You can export the table data to a spreadsheet for analysis and storing test results.

[Tektronix DPO70000 Link-Training (cr)]
__Figure 2:__ *HSSLTA software lets the oscilloscope capture and analyse link-training data.*

To make the measurements, you need to connect probes to your DUT, Figure 3 shows how to connect differential probes, using two oscilloscope channels per transmission lane. Typically, you need look at just one lane because the others should use the same equalisation coefficients.

[Tektronix PAM4 probeconnection (cr)]
__Figure 3:__ *Two oscilloscope channels are required to capture differential signals.*

The HSSLTA option will available for download sometime in November 2016. You'll need to obtain a licence key, which costs $11,900. It works on the DPO70000SX series only.

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