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PAM-4 Solutions for Transmit and Receive Design Characterization 23 October 2014
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Agenda
Challenges we have learned about implementing PAM-4 serial data links
Solutions for output (Tx) characterization
Solutions for input (Rx) characterization
Answer your questions
2 PAM-4 Solutions for Transmit and
Receive Design Characterization
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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 56Gb/s lane data rate:
• 56G NRZ
• + No new science – linear evolution from 25/28G lanes
• - Difficult to manage channel loss & channel reflections
• 28 Gbaud PAM-4
• + Channel loss problems worked out with 28 Gb/s NRZ
• - 30% chip real estate, 35+% more power
• - Lose 9.6 dB usable SNR
• - Lots of new challenges – little experience to draw from
• Both signaling technologies will be utilized to enable 400GbE
PAM-4 Solutions for Transmit and
Receive Design Characterization 3
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NRZ (Non-Return-to-Zero) vs. PAM (Pulse Amplitude Modulation)
NRZ (PAM-2) PAM-4
PAM-4 Solutions for Transmit and
Receive Design Characterization 4
• 2 amplitude levels
• 1 bit of information in every symbol
• 4 amplitude levels
• 2 bits of information in every symbol
(2x throughput for the same Baud rate)
• Lower SNR, more susceptible to noise
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Status of the Standards using PAM-4
Very early in Standards development
• 802.3bj clause 94 (25.78 Gb/s as 13.6 Gbaud PAM-4 in 1m backplane)
• Low adoption rate – no advantage over clause 93 – 25.78 G NRZ
• OIF CEI-56G-VSR draft v2
• Very complete early draft
• Basis for other standards – Ethernet, Fiber Channel, Infiniband, ...
Under consideration:
• 64GFC
• Other OIF standards: CEI-56G-MR, CEI-56G-LR
• 400Gb Ethernet
PAM-4 Solutions for Transmit and
Receive Design Characterization 5
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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?
• ....
• ....
PAM-4 Solutions for Transmit and
Receive Design Characterization 6
Revolutionary – not Evolutionary
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Implementing PAM-4 links = “New Science”
PAM-4 Solutions for Transmit and
Receive Design Characterization 7
Inherent ISI requires receivers to be less susceptible to pattern dependent jitter
decision threshold
Amount of switching jitter
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Implementing PAM-4 links = “New Science” (2)
PAM-4 Solutions for Transmit and
Receive Design Characterization 8
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 needs 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
PAM-4 Solutions for Transmit and
Receive Design Characterization 9
Solutions for Tx characterization
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PAM Test Patterns
PAM-4 transmitters must be capable of generating:
• JP03A Test Pattern - Repeating 0,3 sequence
• JP03B Test Pattern - 0,3 repeated 15 times, 3,0 repeated 16 times
03030303030303030303030303030330303030303030303030303030303030
• Transmitter Linearity Test Pattern
- The transmitter linearity test pattern is a
repeating 160-symbol pattern with a sequence of
10 symbol values each 16 UI in duration.
–1,–1/3,+1/3,+1,–1,+1,–1,+1,+1/3,–1/3
- 10 consecutive symbols mitigates the impact
of ISI on “Level” measurements (VA, VB, VC, VD)
PAM-4 Solutions for Transmit and
Receive Design Characterization 11
Reference: IEEE Std 802.3bj-2014, Amendment to IEEE Std 802.3™-2012 as amended by
IEEE Std 802.3bk™-2013
Will these be leveraged into future IEEE/OIF
PAM4 Standards?
• Quaternary PRBS13 Test Pattern (QPRBS13)
- The QPRBS13 test pattern is a repeating 15548-symbol (338 training frame words) sequence
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JP03B Test Pattern
JP03B is a 62 bit clock pattern with phase reversal
- 0,3 repeated 15 times, 3,0 repeated 16 times
03030303030303030303030303030330303030303030303030303030303030
JP03B is an ideal pattern to measure:
• Random Jitter (RJ)
• Periodic Jitter (PJ)
• Even-Odd (F/2) Jitter
PAM-4 Solutions for Transmit and
Receive Design Characterization 12
What can get measured?
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Transmitter Linearity Test Pattern
PAM-4 Solutions for Transmit and
Receive Design Characterization 13
160-symbol pattern with a sequence of 10 symbol values each 16 UI in duration
Measure each “Level” (VA, VB, VC, VD)
Mean voltage (or power, if optical)
Noise
Plus more…
Analyze Transmitter Linearity
Per IEEE Std 802.3bj™-2014 (Amendment to IEEE Std 802.3™-2012 as amended
by IEEE Std 802.3bk™-2013), measure:
Minimum signal level, Smin
Effective symbol levels, ES1 and ES2
Level separation mismatch ratio, RLM
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QPRBS13 – Eye Measurements
Types of EYE-based measurements:
• Eye Height
• Eye Width
• Eye Skew
PAM-4 Solutions for Transmit and
Receive Design Characterization 14
Quaternary PRBS13 Test Pattern
• Level – mean, “thickness”, and
skew at the points of minimum ISI.
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Other Measurement Considerations
Clock Recovery (CR) – used to track out low-frequency jitter, trigger the scope
• Real-time oscilloscopes use SW CR
• Contact Keysight
• Sampling oscilloscopes use HW CR
• Existing Keysight HW clock recovery designs work on PAM-4 signals
(Example: 86108B 50 GHz module).
Equalization
• FFE/LFE
• CTLE
• DFE (likely required)
PAM-N Simulation
• Generate PAM signals, embed a channel, de-embed fixtures
• N1010A FlexDCA application software with PAM-N simulator, contact Keysight.
PAM-4 Solutions for Transmit and
Receive Design Characterization 15
30 Gbaud, 3 tap LFE (1 precursor) 30 Gbaud, No equalization
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Two Tx Characterization Solutions:
PAM-4 Solutions for Transmit and
Receive Design Characterization 16
Sampling and Real-time Oscilloscopes - which to choose?
Real Time
Oscilloscope
Equivalent-time
Sampling Oscilloscope
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Equivalent-Time Sampling Extremely wide bandwidths at low sample rates
A sample is taken, the data pattern
repeats and the next sample is taken
at a slight delay compared to the
previous sample
In practice, samples are very close together
(can be less than 100 fs apart). Through
multiple passes of the signal, the waveform
can be precisely reconstructed
PAM-4 Solutions for Transmit and
Receive Design Characterization 17
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Sampling Scope Bandwidth is Independent of Sample Rate
Measurement bandwidth is
affected by how narrow the
sampler control pulse is (can
be just a few picoseconds)
Since only one sample is
taken, the A-D process can
be very high resolution (up to
16 bits) with very low noise
S Sampler input
Sampler control pulse
Sampler pulse: Low bandwidth High bandwidth
PAM-4 Solutions for Transmit and
Receive Design Characterization 18
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Eye diagrams: Highly synchronous sampling at arbitrary bit locations
PRBS
Reconstructed
Waveform
Trigger Point Sampling
Point
Clock
Trigger
Re-Arm Time
One Bit
PAM-4 Solutions for Transmit and
Receive Design Characterization 19
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Real-time Sampling
Trigger
EventT=1/F
S
T
T
PAM-4 Solutions for Transmit and
Receive Design Characterization 20
Sample entire waveform in one acquisition
Nyquist criterion obeyed: Fs > 2*BW of signal
Interpolation is used to precisely fill in points in between
actual sampled points to yield better resolution
S(t)
Could
Trigger Here….
Or Here
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Which is the right scope for your application?
PAM-4 Solutions for Transmit and
Receive Design Characterization 21
Compare Real-Time (RT) vs Equivalent-Time (Sampling) Oscilloscopes
Real-Time Scopes
• Best for troubleshooting scenarios
Captures one-time events
No explicit trigger needed, advanced
trigger features
• Fastest sample rate (160 G Sa/s), large
record length (deep single-shot memory)
• Does not require repetitive signals
to generate pattern waveforms
• Highest Compliance App coverage
Sampling Scopes
• Highest Overall Signal Fidelity
Wider bandwidth (> 90 GHz)
Lowest timebase jitter (RJ < 45 fs rms)
Lower noise floor (< 0.25 mV)
Higher A/D resolution (16 bits)
• Lower sample rate (kSa/s), but deep sub-
sampled memory (up to 2^23 bits long, 4096 samples/bit)
• Modular platform (Electrical, Optical, TDR)
• Roughly half the price for same BW
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PAM-N Real Time Tx Test Solutions
PAM-4 Solutions for Transmit and
Receive Design Characterization 22
Electrical – Achieve New Extremes
Keysight DSO/DSA Z634A • Channels: 2 @ 63 GHz / 4 @ 33 GHz
• Bandwidth: Up to 63 GHz
• Software Clock Recovery
• 1st order PLL
• 2nd order PLL
• Equalized
• Fixed
• Jitter: <80 fs rms typ.
Note - PAM software works with any RT Infiniium oscilloscope
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Where are we today? PAM-N TX Analysis Software
PAM-4 Solutions for Transmit and
Receive Design Characterization 23
Software for the RT-Scopes
Working TODAY:
• Capture PAM measurements on the RT
• Beta Measurements software:
Eye Width (@BER)
Eye Height (@ BER)
Equalization (FFE/CTLE)
Coming Soon: Full PAM4 SW Option will be added to
Infiniium RT User Interface including:
• TIE
• Rj/Dj
• Noise analysis
SW algorithms designed to accommodate
severely degraded eye diagrams.
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Where are we today? PAM-N TX Analysis Software
PAM-4 Solutions for Transmit and
Receive Design Characterization 24
Software for the 86100D DCA-X (demo to follow)
Working TODAY:
• Capture PAM measurements on the DCA
• Beta Measurements in MS Office Excel:
Eye Width (@BER)
Eye Height (@ BER)
Eye Contours (@ BER)
Level (voltage/power)
Random Noise
Q
Linearity/Skew Plot
Equalization (FFE/CTLE)
• FlexDCA PAM-N Simulation (Beta)
Coming Soon: PAM4 SW Option will be added to
the FlexDCA user interface. SW algorithms designed to accommodate severely
degraded eye diagrams.
LAN
or
GPIB
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PAM-N 86100D DCA-X Test Solutions
PAM-4 Solutions for Transmit and
Receive Design Characterization 25
Electrical – Highest Precision (includes built-in clock recovery and precision timebase)
Agilent 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)
Agilent 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 (contact Keysight for 56 Gbaud solutions)
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Live Demo
PAM-4 Solutions for Transmit and
Receive Design Characterization 26
N1010A FlexDCA running on a laptop
(simulating actual HW measurements).
TRY THIS YOURSELF! Download FlexDCA: www.keysight.com/find/flexdca_download
Using N1010A FlexDCA Simulator (same UI used by 86100D DCA-X)
PAM-N signals imported into a workbook for
analysis.
Solutions for Rx characterization
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Which is the right generator for your application?
PAM-4 Solutions for Transmit and
Receive Design Characterization 28
Compare BERT vs Arbitrary Waveform Generator
J-BERT M8020A
High Performance BERT
• Best signal performance for digital
signals, highest signal bandwidth
• Up to 32 Gbaud NRZ and PAM4
• Fast, but fixed transition times
• Standard jitter types SJ, RJ, BUJ, F/2
• Combines 2 NRZ signals through
combiner (power divider)
M8195A 65 GSa/s AWG
• High channel density, up to 4 channels per
module
• Up to 32 Gbaud PAM 4 and other
modulation schemes e.g. 16QAM, DMT
• Adjustable transition times
• Can create any jitter type – jitter and
amplitude
• Future proof – standards WILL require
new stress types
• Zero lead time for new stress –
create in math simulator
NRZ
Pattern #1 6 dB
+ NRZ
Pattern #2
(equal
delay)
2 channel
BERT
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M8195A 65 GS/s AWG PAM-n Pattern Generator
PAM-4 Solutions for Transmit and
Receive Design Characterization 29
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)
Up to 2 Vpp (diff) into 50 Ω
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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 I isolate the cause (impairment), and verify it is really the cause?
DCA-X with FlexDCA + M8195A AWG allows users to create and generate
new stress types in minutes!
PAM-4 Solutions for Transmit and
Receive Design Characterization 30
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Fast new stress creation
Step 1: Study the current
pattern eye for anomalies
PAM-4 Solutions for Transmit and
Receive Design Characterization 31
Using DCA-X, Flex DCA, M8195A – 4 Easy Steps!
Step 2: Using FlexDCA – create a new stress
to amplify the anomaly
– or create an entirely different one
(DC wander in signal)
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Fast new stress creation
Step 3: Download the
simulated stressed pattern into
the M8195A
PAM-4 Solutions for Transmit and
Receive Design Characterization 32
Using DCA-X, Flex DCA, M8195A – 4 Easy Steps!
Step 4: New stressed pattern is available for
testing your new design!
N1010A
FlexDCA
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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
Tx characterization tools:
• Equivalent-time Sampling Oscilloscopes:
• Keysight 86100D DCA-X (plus optical and/or electrical plug-in modules)
• Keysight N1010A FlexDCA Offline/Remote Access Software
• Real-time Oscilloscopes:
• Keysight DSAZ634A
• Keysight DSOZ634A
Rx characterization tool:
• BERT: J-BERT M8020A High Performance BERT with N1010A FlexDCA
• AWG: M8195A 65 GSa/s AWG with N1010A FlexDCA
PAM-4 Solutions for Transmit and
Receive Design Characterization 33
Questions?
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Keysight M8000 BER Series of BER Test Solution
M8195A 65 GSa/s AWG 35
Fast, accurate receiver characterization
16 Gb/s J-BERT M8020A, 4 channel
M8195A > 32 Gbaud multi-level generator,
4 channel*
16 Gb/s J-BERT M8020A, 1 – 2 channel
32 Gb/s J-BERT M8020A, 1 channel
M8070A Software
* Integration in M8070A planned
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J-BERT M8020A High Performance BERT
Accurate PAM-4 receiver
characterization:
Adjustable amplitude/offset
Calibrated jitter built-in
De-emphasis 8 taps ± built-in
Interference with built-in
superposition
Up to 32 Gbaud and 16 Gbaud
Memory and PRBS (up to 231-1)
Add-on to 32G and 16G J-BERT
M8020A
PAM-4 signal at 25.78 Gbaud, PRBS 27-1
measured with DCA-X (N1055A-54F)
Keysight Restricted 36
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Go Where you Have Never Been Able to Test Before
Sample rate 54 GSa/s to 65 GSa/s per channel
20 GHz bandwidth
> 32 GBaud symbol rate
1, 2 or 4 differential channels per 1-slot AXIe
module
M8195A 65 GSa/s AWG 37
In Speed, in Bandwidth and in Channel Density
Explore your possibilities
New M8195A 65 GSa/s AWG
– Up to 16 GSa of waveform memory per module (*)
– 8 bits of vertical resolution
– Amplitude up to 2 Vpp(diff.) or 1 Vpp(se),
voltage window -1.0 … +3.3V
– Ultra low intrinsic jitter
(RJrms < 200 fs @ 32 Gb/s PRBS 211-1)
– 16-tap FIR filter in hardware for frequency
response compensation (*)
– Multi-module synchronization up to 16 channels
per 5-slot AXIe chassis
(*) Rev 2