Keysight High Speed Digital Seminar Series PAM-4 … · NRZ (Non-Return-to-Zero) vs. PAM (Pulse Amplitude Modulation) 3 NRZ (PAM-2) PAM-4 • 2 amplitude levels • 1 bit of information

Keysight High Speed Digital Seminar Series PAM-4 Stressed Pattern Generation April 2015

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Agenda

Introduction – Switch from NRZ to PAM-4

Challenges to implementing PAM-4 Serial Data Links

Solutions for Input (Rx) Characterization (Stressed Pattern Generation)

PAM-4 Analysis Solutions for Output (Tx) Testing

Demonstrations

• PAM-4 Simulation and Analysis Tools (DCA-X Oscilloscope Software)

• PAM-4 Waveform Generation (AWG Waveform Generation Tools)

Q&A

2 © Keysight Technologies 2015 PAM-4 Stressed Pattern Generation

Canada HSD Seminar Series

Page © Keysight Technologies 2015

NRZ (Non-Return-to-Zero) vs. PAM (Pulse Amplitude Modulation)

3

NRZ (PAM-2)

PAM-4

• 2 amplitude levels

• 1 bit of information in every symbol

• 28 Gbaud NRZ = 28 Gb/s

• 4 amplitude levels

• 2 bits of information in every symbol

2x throughput for the same Baud rate

28 Gbaud PAM-4 = 56 Gb/s

• Lower SNR, more susceptible to noise

• More complex TX/RX design, higher cost

PAM-4 Stressed Pattern Generation



Enabling the next step in link data rate

56 Gb/s lane data rate will be the principle enabler for 400GbE

Two contenders for implementing 56 Gb/s lane data rate:

• 56G NRZ

• + No new science – linear evolution from 25/28G lanes

• - Difficult to manage channel loss & link jitter budget

• 28 Gbaud PAM-4

• + Channel loss problems were solved with 28 Gb/s NRZ

• - Linear Tx & Rx - more complexity, chip real estate & power

• - Lose 9.6 dB usable SNR

• - Lots of new challenges – little experience to draw from

• Both signaling technologies will be utilized to enable 400 GbE

4 PAM-4 Stressed Pattern Generation


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Typical PAM-4 Communication Systems

Keysight Technologies 2015

Transmitter, Channel, Receiver

5

Channel

e.g. CEI-56G_VSR

Chip (e.g. VSR)

Optical Transceiver

Module

Fiber

Link

Optical Transceiver

Module

Channel

e.g. CEI-56G_VSR

Chip (e.g. VSR)

km of SM fibre

Electrical • Chip-to-Module (c2m)

Optical e.g. CEI-56G-VSR-PAM

Backplane Channel

(e.g. 50G-LR )

Conn Conn

Da

ug

hte

r Ca

rd

Da

ug

hte

r Ca

rd

Electrical Backplane e.g. CEI-50G-LR using PAM

Backplane IC

Backplane IC

Channel e.g. CEI-56G_VSR

Chip (e.g. VSR)

Chip (e.g. VSR)

Chip-to-Chip (c2c)

Chip-to-Module Chip-to-Module




Moving from NRZ to PAM-4

Jump from 10G NRZ to 25G introduced many new concepts....

• Still a linear transition (more or less)

Multi-Level signaling changes all the rules – in place for 50+ years!

• Saturating to linear output stages

• More complex (and precise) level threshold detection for

inputs

• Finite rise time creates inherent ISI

• How to implement clock recovery

• How will DFE need to change

• ....

• ....

6

Revolutionary – not Evolutionary

0 – 3 crossing 0 – 2 crossing




How will these map into testing?

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




Implementing PAM-4 links = “New Science”

8

Inherent ISI requires receivers to be less susceptible to pattern dependent jitter

Some of the other challenges learned so far include...

• Eye time skew from linear drive of VCSELs

• Upper eyes arrive sooner than lower transitions

• Each eye may need to be sampled with independent delay




Other impairments that challenge PAM-4 receivers

• Non-linearity - Amplitude compression in lower eyes

• Non uniform effective SNR across individual eyes

Receivers sensitive to additional artifacts beyond “traditional” jitter types in NRZ

• Still learning what impairments cause problems

• New measurements WILL be defined for Tx Outputs

• New stress types WILL be defined for Rx Input testing




Status of Standards using PAM-4

Implemented

• IEEE 802.3bj clause 94 (25.78 Gb/s as 13.6 GBaud PAM-4 in 1m

backplane)

• Low adoption rate – limited advantages over clause 93 – 25.78 G NRZ

Under development/discussion

• OIF CEI 4.0 (56G-XSR, VSR, MR, LR – lane enabler basis for standards

below)

• IEEE 802.3bs / CDAUI-8

• 64G Fibre Channel

• InfiniBand 600G HDR




Change in 400G testing requirements is likely

OIF/CEI-56G and 802.3bs baseline drafts reuse output and input testing from

100G-KP4 (802.3bj clause 94)

100G-KP4 had little (any?) test chips evaluated for interoperability during its

development

100G Standards based on 25G NRZ lanes had significant changes late during

draft stages

• Added Channel Operating Margin measurement and RSIT

• This happened after first test chips were evaluated!

400G baseline drafts lack any input tests with linearity or eye skew tests

Likely to see additional output measurements and input tests added during

standards development



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Keysight PAM-4 Solutions

PAM-4 End-to-End Link Simulation

• Keysight Advanced Design System (ADS) Software

PAM-4 Transmitter Characterization

• Oscilloscope-based measurements

using Keysight Z-Series Real Time Oscilloscope

or DCA-X Sampling Oscilloscope.

Channel Characterization

• Performed using Network Analyzer or TDR/TDT

PAM-4 Receiver Characterization

• M1895A 65GSa/s Multichannel Arbitrary Waveform Generator

(AWG) and Bit Error Ratio Test (BERT) Products

Keysight Technologies 2015 12

7 Apr 2015 PAM-4 Stressed Pattern Generation


PAM-4 Pattern Generation for Input Testing


Input Testing – BERT Focus

The Bit Error Ratio Tester will continue to be the principle tool for Input

testing

Emerging 400G standards will dictate that host chips have built in error

measurement capability - error checkers and eye mappers

• BERT will focus on precision stressed pattern generation capability

• Optical modules likely too power and space constrained for BIST

• Error analyzers still important for initial development and will be offered

BERT Pattern

Generator

BERT Pattern

Generator

BERT Error

Detector

DUT

Rx

IN Tx

OUT Error

Checker

14




Input (Rx) test – PAM-4 generation options

Combines two amplitude weighted NRZ signals through combiner (power divider)

+ Longer user patterns

+ “Drives like a BERT” – responsive stress controls

+ Faster rise time, built in De-emphasis

- Limited stress types (SJ, RJ, BUJ, Even/Odd, SI, RI, etc.)

• Long lead time for new stress capability – if possible as all

- Higher cost – requires two high performance channels for each PAM-4 signal

15

Conventional NRZ BERT Pattern Generators

NRZ

Pattern #1 6 dB

+ NRZ

Pattern #2

(equal

delay)

2 channel

BERT





Replaces passive combiner with DAC

+/- Same advantages and limitations of passive combiner except:

+ Digital rather than analog inputs – deskewed with reclocker

+ Amplifier isolates load reflections from analog weighting network

- No De-emphasis

16

Conventional NRZ BERT PG driving DAC

NRZ

Pattern #1

NRZ

Pattern #2

2 channel

BERT

Clk

6 dB

+ (equal

delay)

PAM DAC





Latest generation AWG can create PAM-n up to 32 Gbaud

• + Not limited to PAM-4 – can generate any amplitude modulated signal

• + Can generate any stress type – jitter and amplitude

• Future proof – Standards WILL require new stress types

• Zero lead time for new stress – create in math simulator

• + Smaller form factor – lower cost

• Up to 4 channels in single blade

• - Limited user pattern lengths

• Sample memory length generally exact power of 2

• - Slower rise times

• Fast enough to emulate channel, but not golden transmitter

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Arbitrary Waveform Generator




Is an AWG really fast enough?

Recent DAC technology gives 5X speed increase over last

generation

• 65 GS/s

Fractional re-sampling allows generation of fine resolution jitter and

other impairment distortions




Integer vs. Fractional Re-sampling

0 10 20 30 40 50

-1

-0.5

0

0.5

1

Integer re-sampling

Factor

3

Fractional re-sampling

0 10 20 30 40 50

-1

-0.5

0

0.5

1

Factor

3.2

Slightly lower jitter for clean signals,

but cannot add infinitesimally small

amount of timing distortions

Intrinsic jitter is slightly higher, but

impairment distortions can be

added smoothly




...and not just Jitter!

The “Arbitrary” nature of the AWG allows it to generate an endless

variety of impairment stresses

• Linearity, skew, etc. – amplitude related in addition to timing jitter

De-emphasis is directly incorporated into pattern waveform

Cable/Fixture de-embedding and channel emulation easily created

without additional hardware in signal path to distort waveform

Transmitter

(TX)

Receiver

(RX)

Channel TP1 TP2 TP3

0 2 4 6 8 10-20

-15

-10

-5

0

Frequency (GHz)

Loss (

dB

)




Time resolution in AWG vs. Traditional BERT

Timing units in traditional BERTs are Unit Intervals

• Full integer for bits, de-emphasis tap width, etc.

• Fractional for timing jitter (mUI)

Timing resolution in AWG is not linked to Unit Intervals

• Pre-distortion emulates timing resolution << sample interval

• De-emphasis tap width is not limited to integer UI!




OIF CEI-56G demo at OCF - 25.78 Gbaud through 770 mm channel

Channel De-embedding

Magnitude and phase calibration through channel precisely opens closed eyes

• Uses 300 frequency tones – not limited to 2, 3, or 4 de-emphasis taps

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Not limited to nTap De-emphasis resolution



27 dB loss at Nyquist (12.9 GHz)

Open eye – Noisy, but correct shape!


M8195A 65 GS/s AWG PAM-n Pattern Generator

23

Supports PAM-4, PAM-8 and other higher-order modulation formats

Supports coherent OFDM, QAM-N

Future proof – new stress types can be generated with FW update

1 – 4 channels

65 GS/s – 8 bit

20+ GHz analog bandwidth

Intrinsic RJ: <200 fs typ. (32G NRZ)

New GUI “Drives like a BERT”




… Higher Order PAM Modulations…

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PAM4

PAM8

PAM16 PAM-4 Stressed Pattern Generation



AWG Control Software (GUI)

Product ships with IQTools

• Compiled Matlab data generation tool

Integrated into M8070A BERT Control SW in July



Mainframe for M8020A BERT or AWG

BERT Control Software AWG Software

AWG Soft Front Panel IQ Tools Software


Fast New Stress Creation

PAM-4 links are new science – still learning what impairments create errors

in receivers

User problem – new receiver design seems to have high error rate

• How do you isolate the cause (impairment), and verify your assumption

is correct?

DCA-X with FlexDCA + M8195A AWG allows users to create and generate

new stress types in minutes!




Simulation Using FlexDCA Software Tool to quickly add math processing to waveforms and see results

Runs inside the DCA sampling oscilloscope or standalone

Waveform source can be saved measurement, or simulated




Simulated PAM-4 Eye

28





Four Easy Steps!

1. Using DCA-X, study the current pattern eye for anomalies

2. Using FlexDCA – create a new stress to amplify the anomaly – or create

an entirely different one

Using DCA-X, Flex DCA, M8195A

Example:

Device appears to be

sensitive to DC wander

Create DC wander by

summing attenuated

average of data amplitude





3. Download the new waveform file into the M8195A

4. You are ready to test ALL of your sample chips with the new stressed

PAM-4 pattern!

Using DCA-X, Flex DCA, M8195A




Assuring “what you see is what you get”

Math simulators can create waveforms too perfect to exist in real life

• Including at AWG Output!

Use function block to emulate M8195A transfer function to predict

output waveform

• View waveform after function block, take waveform data to AWG

before

Built in M8195A transfer function block under development

Stress Creation

Math M8195A

Transfer Function

Waveform

Source



PAM-4 Analysis Solutions for Output (Tx) Testing

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PAM-4 Analysis: 86100D DCA-X Test Solutions

Electrical – Highest Precision (includes built-in clock recovery and precision timebase)

Keysight 86100D DCA-X with 86108B • Channels: 2

• Bandwidth: 50 GHz

• Jitter: <45 fs rms typ.

• Electrical Clock Recovery – integrated HW Clock Recovery

works with PAM-N signals up to 32 Gbaud

Note - PAM software works with any DCA module

(optical and electrical)

Optical (add Electrical/TDR remote heads)

Keysight 86100D DCA-X with 86105D-281 • Channels: Up to 2 optical per module, 8 electrical

• Bandwidth: 34 GHz (optical), 60 GHz (electrical)

• Jitter: < 85 fs rms typ. (with 86100D-PTB)

• N1070A Optical Clock Recovery (external)

• 32 Gbaud Single Mode

• 14 Gbaud Multimode

• Electrical Remote Heads

• N1045A 60 GHz Electrical Only

• N1055A 50 GHz Electrical with TDR/TDT

Electrical and Optical solutions to 32 Gbaud


Canada HSD Seminar Series Keysight Technologies 2015

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FlexDCA Eye/Mask Mode Measurements Level Mean/Thickness/Skew; Eye Mean/Height/Width/Skew



Page 35

FlexDCA Scope Mode Measurements Level, Noise

Level/Noise measurements can be

performed on any single-valued

pattern (not just the linearity

pattern).



Page Keysight Technologies 2015

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PAM-4 Analysis: Real-Time Oscilloscope Test Solutions

Keysight 63 GHz Z-Series • Channels: 2 (63 GHz), 4 (33 GHz)

• Bandwidth: up to 63 GHz

• Intrinsic Jitter: <100 fs rms typ.

• Software Clock Recovery

Keysight Infiniium V-Series and Z-Series Oscilloscopes with N8827A PAM-4 Software

Keysight 33 GHz V-Series • Channels: 2 (33 GHz), 4 (16 GHz)

• Bandwidth: up to 33 GHz

• Intrinsic Jitter: <100 fs rms typ.

• Software Clock Recovery



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Keysight N8827A PAM-4 Analysis Software


Infiniium PAM-4 Software Tool provides Eye and Level based measurements

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Clock Recovery:

• PAM-4 FOPLL, SOPLL, constant, explicit, explicit FOPLL, explicit

SOPLL, equalized

• Specify data rate or symbol rate

• Provide edge filtering for 03, 30 / 12, 21 / Non-symmetric edges.



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PAM-4 real-time eye setup.

• PAM-4 aware SDA wizard

• PAM-4 aware bit qualification.



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PAM-4 real-time eye measurements:

• Eye width, eye height, eye skew (relative) for each PAM-4 eye

• Level mean, RMS, and “thickness” for each level

PAM-4 Waveform measurements:

• Level mean, RMS, and “thickness” for each level

• Data TIE for each threshold

• Rise/Fall times for each of 6 PAM-4 transition types

• Support for CTLE & FFE Equalization




Summary PAM-4 signaling will be an enabler for 400G links

Transition from NRZ to PAM-4 is revolutionary

• Many new challenges - New science

Required Tx measurements and Rx stress types will change

Rx Characterization Tool:

• Keysight M8195A AWG with N1010A FlexDCA

Tx Characterization Tools:

• Keysight 86100D DCA-X Wide-Bandwidth Oscilloscope

• Keysight Infiniium V-Series and Z-Series Oscilloscopes

• PAM-4 Analysis Software



Questions?

Keysight High Speed Digital Seminar Series PAM-4 … · NRZ (Non-Return-to-Zero) vs. PAM (Pulse Amplitude Modulation) 3 NRZ (PAM-2) PAM-4 • 2 amplitude levels • 1 bit of information

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