pam-4 solutions for transmit and receive design ... · pam-4 solutions for transmit and receive...

PAM-4 Solutions for Transmit and Receive Design Characterization 23 October 2014

Page

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

Page

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

Page

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

NRZ (PAM-2) PAM-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

Page

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



Page

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?

• ....

• ....



Revolutionary – not Evolutionary

Page

Implementing PAM-4 links = “New Science”



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

decision threshold

Amount of switching jitter

Page

Implementing PAM-4 links = “New Science” (2)



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

Page

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



Solutions for Tx characterization

Page

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)



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

Page

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



What can get measured?

Page

Transmitter Linearity Test Pattern



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

Page

QPRBS13 – Eye Measurements

Types of EYE-based measurements:

• Eye Height

• Eye Width

• Eye Skew



Quaternary PRBS13 Test Pattern

• Level – mean, “thickness”, and

skew at the points of minimum ISI.

Page

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.



30 Gbaud, 3 tap LFE (1 precursor) 30 Gbaud, No equalization

Page

Two Tx Characterization Solutions:



Sampling and Real-time Oscilloscopes - which to choose?

Real Time

Oscilloscope

Equivalent-time

Sampling Oscilloscope

Page

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



Page

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



Page

Eye diagrams: Highly synchronous sampling at arbitrary bit locations

PRBS

Reconstructed

Waveform

Trigger Point Sampling

Point

Clock

Trigger

Re-Arm Time

One Bit



Page

Real-time Sampling

Trigger

EventT=1/F

S

T

T



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

Page

Which is the right scope for your application?



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

Page

PAM-N Real Time Tx Test Solutions



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

Page

Where are we today? PAM-N TX Analysis Software



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.

Page

Where are we today? PAM-N TX Analysis Software



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

Page

PAM-N 86100D DCA-X Test Solutions



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)

Page

Live Demo



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.

http://www.keysight.com/find/flexdca_download

Solutions for Rx characterization

Page

Which is the right generator for your application?



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

Page

M8195A 65 GS/s AWG PAM-n Pattern Generator



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

Page

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!



Page


Step 1: Study the current

pattern eye for anomalies



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)

Page


Step 3: Download the

simulated stressed pattern into

the M8195A



Using DCA-X, Flex DCA, M8195A – 4 Easy Steps!

Step 4: New stressed pattern is available for

testing your new design!

N1010A

FlexDCA

Page

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



Questions?

Page

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

Page

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

Page

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

pam-4 solutions for transmit and receive design ... · pam-4 solutions for transmit and receive...

Documents