Keysight High Speed Digital Seminar Series PAM-4 … · NRZ (Non-Return-to-Zero) vs. PAM (Pulse...
Transcript of Keysight High Speed Digital Seminar Series PAM-4 … · NRZ (Non-Return-to-Zero) vs. PAM (Pulse...
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
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NRZ (Non-Return-to-Zero) vs. PAM (Pulse Amplitude Modulation)
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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
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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
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Typical PAM-4 Communication Systems
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Transmitter, Channel, Receiver
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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
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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
• ....
• ....
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Revolutionary – not Evolutionary
0 – 3 crossing 0 – 2 crossing
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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
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Implementing PAM-4 links = “New Science”
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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
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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
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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
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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
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PAM-4 Pattern Generation for Input Testing
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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
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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
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Conventional NRZ BERT Pattern Generators
NRZ
Pattern #1 6 dB
+ NRZ
Pattern #2
(equal
delay)
2 channel
BERT
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Input (Rx) test – PAM-4 generation options
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
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Conventional NRZ BERT PG driving DAC
NRZ
Pattern #1
NRZ
Pattern #2
2 channel
BERT
Clk
6 dB
+ (equal
delay)
PAM DAC
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Input (Rx) test – PAM-4 generation options
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
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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
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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
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...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
)
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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!
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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
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27 dB loss at Nyquist (12.9 GHz)
Open eye – Noisy, but correct shape!
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M8195A 65 GS/s AWG PAM-n Pattern Generator
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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”
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… Higher Order PAM Modulations…
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PAM4
PAM8
PAM16 PAM-4 Stressed Pattern Generation
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AWG Control Software (GUI)
Product ships with IQTools
• Compiled Matlab data generation tool
Integrated into M8070A BERT Control SW in July
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Mainframe for M8020A BERT or AWG
BERT Control Software AWG Software
AWG Soft Front Panel IQ Tools Software
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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!
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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
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Simulated PAM-4 Eye
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Fast New Stress Creation
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
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Fast New Stress Creation
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
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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
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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
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FlexDCA Eye/Mask Mode Measurements Level Mean/Thickness/Skew; Eye Mean/Height/Width/Skew
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FlexDCA Scope Mode Measurements Level, Noise
Level/Noise measurements can be
performed on any single-valued
pattern (not just the linearity
pattern).
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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
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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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Keysight N8827A PAM-4 Analysis Software
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Infiniium PAM-4 Software Tool provides Eye and Level based measurements
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PAM-4 real-time eye setup.
• PAM-4 aware SDA wizard
• PAM-4 aware bit qualification.
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Keysight N8827A PAM-4 Analysis Software
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Infiniium PAM-4 Software Tool provides Eye and Level based measurements
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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
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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
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Questions?