BERT ups the ante on PAM

Data centers are now moving to a new world of faster data rates, with lane rates of 56 Gbits/s enabled not only by use of 28-Gbaud, quad-value phase amplitude modulation (PAM-4) but also with 56-Gbit/s non-return-to-zero (NRZ) signaling (which some are now referring to as “PAM-2”). And the centers also know that in a few years, perhaps six or seven, PAM-8, 16-QAM, DMT, or some other format will invade their world. Indeed, full scale development of those formats will likely be underway in 18 months or less.

And it’s not as if those fast rates are only seen on one type of interconnect; the IEEE 802.3 bs standard and the Common Electrical Interface 56G (CEI-56G) standard of the Optical Internetworking Forum (OIF) define five different types of paths — long, medium, short, very-short, extra-short, and ultra-short reach at bandwidths to 400 Gbit/s — that can be found on and between the boards — optical transceiver modules, line cars, backplanes, and switch cards — in the data center’s racks. Supporting this type of complexity could significantly hamper a data center’s ability to keep up with performance demands, what with having to test to tighter margins for timing, channel loss, non-linearity, level interference, and crosstalk effects.

Recognizing this dilemma, Keysight has just introduced the AXIe-based M8040A high-performance bit-error-ratio testers (BERTs), the latest members of its M8000 series. Capable of testing devices that operate up to 32 or 64 Gbaud (PAM-4) or Gbits/s (NRZ), depending on model, the instruments simplify test setups while providing repeatable and accurate results, so engineers in validation as well as R&D labs can rapidly characterize receivers on the physical layer for next-generation data center interconnects.

One key to the tester’s ease of use is its pattern generator module, which provides built-in de-emphasis, jitter injection, and an optional second channel. Through the M8070A system software, users can select PAM-4 and NRZ and not have to bother with external combiners and cabling and the de-skew involved to provide PAM-4 signals. Separately operating dual channels are an option. Remote heads permit high-quality signal sourcing by reducing the distance to the device under test. Further, the analyzer module provides true PAM-4 error analysis in real-time for long pseudorandom-binary-sequence (PRBS) and quaternary PRBS (QPRBS) patterns. This allows proofing even low bit-error ratios and symbol-error ratios with the required confidence. Users can control the M8040A from a graphical and remote control interface.

The M8040A BERT will initially be released in a 32-Gbaud version, with the 64-Gbaud to follow shortly. Prices start $323,000 for a 32-Gbaud PAM-4 BERT with one channel, including the M8070A software.