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The Challenges of Measuring PAM4 SignalsPanelists:

Doug Burns, SiSoftStephen Mueller, Teledyne LeCroyLuis Boluña, Keysight Technologies

Mark Guenther, Tektronix

Jose Moreira, AdvantestMartin Rowe, UBM (Moderator)

January 21, 2016

The Challenges of Measuring PAM4 SignalsDouglas Burns Signal Integrity Software Inc. (SiSoft)

PAM4 PAM4 vs NRZ

– An old Story PAM4 Features

2 bits of data per UI (4 discrete logic levels) Gray Code data encoding may be used May require Forward Error Correction (FEC)

Multiple codes available, needs validation with Hardware PAM4 Measurements:

Measurement problem not really changed Except we are working with 3 smaller eyes, not 1 eye.

Eye height at 3 separate measurement levels

Vertical Bathtubs Eye Width for 3 eyes

Horizontal bathtubs

Vertical Bathtubs

Shows Eye Height as a function of

probability

Easier to understand than a

probability color chart

Great alternate view from Eye

Diagram

Horizontal Bathtubs

Shows Eye Width as a function of

probability

Easy to see offsets and Eye width

variations– Each Eye can have its own sampling

time

Great alternate view from Eye

Diagram

Encoding

Goal: reduce the number of bit errors when there is a symbol error – Symbols contain 2 bits of data

Gray Code Popular– Code style can be programmable

Noise is More Important than JitterStephen Mueller, Teledyne LeCroy

Rj = 10.0 psRj = 4.0 ps12 Gbaud NRZ 6 Gbaud PAM4

Comparison showing an NRZ and PAM4 signals at the same bit rate and how the eyes close with

increasing amounts of Rj applied. The PAM4 signal shows little effect from the jitter relative to NRZ.

How Jitter Closes Eyes

Rj = 0 psRj = 1.6 psRj = 2.4 psRj = 3.2 ps

Rj = 3.2 ps

12 Gbaud NRZ 6 Gbaud PAM4

Comparison showing an NRZ and PAM4 signals at the same bit rate and how the eyes close with

increasing amounts of Rn applied. The effect on the NRZ and PAM4 signals are similar.

How Noise Closes Eyes

Rn = 0 mVRn = 6 mVRn = 12 mVRn = 18 mVRn = 24 mV

Rn = 32 mVRn = 32 mV

PAM4 vs NRZ Slew Rates

NRZ PAM4Slew Rate = 4.95 V/ns

Small increase in noise causes small

increase in horizontal eye closure

Slew Rate = 1.41 V/ns

Small increase in noise causes large

increase in horizontal eye closure

ΔV

Δt

ΔV

Δt

NRZ PAM4

PAM4 vs NRZ, 8 mV Rn, 0 ps RjSlewNRZ = 4.95 nV/sRn = 8 mVRj = 8 / 4.95 ps = 1.62 ps

SlewPAM4 = 1.61 nV/sRn = 8 mVRj = 8 / 1.61 ps = 4.97 ps

Due to low slew rate, PAM4 is relatively insensitive to jitter but very sensitive to noise!

The Challenges of Measuring PAM-4 Signals

Luis Boluña, Keysight Technologies

We need to understand PAM-4 more deeply than we do now.

Challenges of PAM-4 Impairments• New Tx output topology may create new types of distortion

• Linear output stage• Analog summing two level weighted NRZ streams to create PAM-4• Other new topology???

• New Output measurements needed to quantify these new distortions

• New Rx Input circuits may be susceptible to new types of impairments

• Triple input level detectors• Transition qualified phase detector in clock recovery• Multi-level replacement for DFE• Other new topologies???

• Stressed input testing will need to emulate new impairments which cause bit errors

• Impairments of levels and eye levels can be hard to emulate at speed.

• An Arbitrary Waveform Generator is suitable to provide such capabilities.

Generating and Measuring PAM-4 Impairments

PAM-4 Validation

PAM-4 Stimulus

QSFP28 3 meter cable DUT

Output of lossy channelwith no EQ

12.5 Gbps (6.25 GBaud), PRBS7Tx with 3-tap FFE PAM-4 at Rx

Download Simulated Tx Waveform to AWG

Compare simulation with measurement

Keysight ADS 2016.01 Channel Simulation

TxRx

DUT

AWG Tx with FFERX:PAM-4 at DCA

AWGSamplingOscilloscope

DUT:QSFP28 3m

PAM-4 Simulation to Measurement Correlation

Measured

Simulated

Channel Output

Measured Simulated

2/3 Width 72.55ps 69.70ps

1/2 Width 74.70ps 67.90ps

0/1 Width 62.60ps 62.95ps

2/3 Height 86.2mV 87.6mV

1/2 Height 84.6mV 80.0mV

0/1 Height 106.2mV 103.0mV

Validation Summary:Comparison inside of FlexDCA

Comments on FEC

• Past and current implementation of Forward Error Correction (FEC) have dealt with an independent FEC per lane.

• Future work is implementing striping packets of data across multiple lanes.

• Challenges arise when you stripe FEC across SerDes lanes-meaning all cumulative errors (random and burst) need to be accounted for across all lanes not just the one you are monitoring.

Oversimplified Diagrams(transcoding omitted)

(Frame Loss Ratio)

The Challenges of Measuring PAM4 SignalsMark Guenther, Tektrronix

Compliance Goal

PAM4 TECHNOLOGY MATURITY Dynamic Standards Environment Dynamic Tools Environment

Design / Debug Goal

Measurements per Transition Type

NEW MEASUREMENTS & TECHNIQUES

Relative Eye Position Measurements

▪ Vertical Linearity▪ Horizontal Offset

Clock Recovery Options

▪ “Conventional” ?▪ Selected Edges ?▪ Noise-Tolerant ?▪ IBIS Model ?▪ Spec-Compliant ?

Example: Rise & Fall Times

ADDITIONAL PAM4 CHALLENGES

▪ The Importance of Equalization and De-Embedding

▪ New Challenges to Equipment Fidelity▪ With PAM4’s greatly reduced height for each eye, the noise performance

(effective bits) of measurement hardware is at a premium

The Challenges of Measuring PAM4 SignalsJose Moreira, Advantest

NU

MBE

R O

F TE

STED

UN

ITS

TIMER&D/ LAB CHARACTERIZATION

PRODUCTION RAMP

VOLUME PRODUCTION

LOOPBACKAT-SPEED?AT-SPEED TESTING

• Expectation is that volume production is handled by loopback testing even for first generation devices (failure coverage and customer returns will determine if this is true)

• How will the PAM-4 production ramp be implemented (loopback or at-Speed, bench/rack or ATE, plain wire loopback or parametric ATE loopback, golden device)

Volume Production for PAM-4 Devices

AT-SPEED PATTERN

AT-SPEED LOOPBACK

• ROI is not there for dedicated PAM-4 ATE pin-electronics cards.

• For at-speed testing the best approach would be an extension of current 28/32 Gbps NRZ solutions:

• Power combiner or DAC on the stimulus side.• Single comparator with three independent

functional tests to cover all PAM-4 levels.• Main challenges are:

• Signal integrity• Pattern (standard ATE PRBS engines cannot

handle some PAM-4 patterns forcing the use of memory base patterns)

• test time• Cost

Challenges for ATE

32 Gbps at-speed NRZ ATE System

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Thank you!Questions?

If you’d like a copy of these slides, contact Martin Rowe, martin.rowe@ubm.com

or leave your business card at the end of the session.

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Interested in contributing to EDN or EE Times?

EDN: Technical articles, tutorials, “how to.”

EE Times: News, opinion, tech trends, or quizzes.Contact martin.rowe@ubm.com