troubleshooting coherent optical communication systems

Troubleshooting Coherent Optical Communication Systems

Created by:Michael KoenigsmannApplication SpecialistDigital & Photonic Test Division

May 18th 2015

© 2015 Keysight Technologies

Presented by:Rodrigo VicentiniApplications Engineering ManagerLatin America Region

Agenda

• Market and Technology Trends

• Coherent Measurement Challenges

• Typical Tests in Coherent Transmission Systems

• Typical Impairment Emulation in the electrical Domain

• Tools to generate signals for Coherent Optical Applications

• References

Market and Technology Trends

Drivers for higher Datarates

Troubleshooting Coherent Optical Comms Systems

Market and Technology TrendsBig Data Demand drives Technology Changes

Challenges:• Transmission demand exceeds system capacity• Cost-effective increase of capacity • Pressure to release new technology on time

To maximize transmission within Phy layer, combine & optimize all dimensions for efficiency, cost, power, reach, reliabiliy etc.

• Mobile apps and services, • high resolution devices, • Internet of Things

drive

high speed I/O, network and data center traffic

Modulation

Sym

bo

l Rat

e

Multi level signaling

in digital links,

e.g. C-Phy, PAMx

Parallel lines, subcarrier density

Polarization Mux, OFDM, DMT

Amplitude & Phase

modulation in optical links

e.g. QPSK, QAMx

Maximize Bits per Symbol

Increase Symbol Rate

32 GBd and beyond

Nyquist fs > 2 x fmax

Rule of thumb:

sampling rate = 2.5x Signal BW

Maximize # of carriersBy 2020 there will be around 20 Billion

devices connected to the Internet


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Maximize Transmission within Physical Layer


Different techniques…

Motivation

Higher Order Modulation Formats OFDM

From OOK to Coherent Detection

Time-Domain Pulse Shaping

Polarization Division Multiplex

4


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Application Requirements

Troubleshooting Coherent Optical Comms Systems 6

Coherent Optical Communication

• Requirements for a test instrument:• Flexibilty to address different modulation schemes• Clean signal to test your device and not your instrument• Bandwidth at least 20 GHz• 4 sychronized channels to support dual polarization (= 2 pairs of I/Q signal)

QPSK QAM128 OFDM

Need higher data throughput: 100 G 400 G 1 Tbit/s


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Emulate Optical Distortions in the Electrical Space

7

Pattern Gen

RefTx or golden

Line Card

Error Detector

Network or Test Bed

DUT (100/400G

Rx)

AWG DUT (DSP)

ADC

ADC

ADC

ADC

Digital Coherent Receiver DSP Test

Error Detector

DUT (100/400G

Tx)AWG

- or -

Traditional setup

Test setup with AWG

Deterministic and precise testing with complex impairmentse.g. Phase Noise, OSNR, Polarization, PMD


OSNR Gen.

DUT (100/400G

Rx)AWG Error

DetectorE/O

- or -


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AWG’s in Coherent Optical Applications

– Endless modulation formats for higher data throughput

– Generates clean signals at 32 GBd and beyond

– Provides 4 independent analog outputs for dual-polarization optical applications

• Uses out of the box and in-situ calibration to achieve a clean signal at the device under test

• Emulate optical distortions in the electrical space

8

Cost-effective increase in capacity

16 QAM at 32 GBd with < 4% EVM

QPSK Signalwith addedphase noise;emulating a2 MHz laserline width


Optical Modulation

Generator TooI


Coherent Measurement Challenges

• Clean Signal generation (electrical & optical)• Clean Signal at defined test points• High datarates 32 GBd and beyond• Accurate and Repeatable Test Signals• Distortion Emulation• Flexible Modulation Formats

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Accurate and Repeatable Test Results

Out-of-the-box calibration ensures clean signal at the frontpanel connector

In-situ calibration – extend clean signal to the receiver test point

• S-Parameters of channel are embedded or de-embedded

• Frequency/Phase response is measured in-system andthen de-embedded

11

QPSK, 32 GBd PRBS 6 Gbit/s

Without correction With correction Without correction With correction



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Out-of-the Box Calibration & In-situ calibrationPre-distortion techniques for frequency and phase response compensation

12

AWG

Optionally: optimize the

second I/Q pair at the same time Troubleshooting Coherent Optical Comms Systems

or

iqtools Optical Modulation SW

phase and frequency response

data

I/Q I/Q

Vector Signal Analysis (VSA) softwareto determine frequency and phase response

Infiniium SeriesOscilloscope


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In-situ-calibration

13

End-to-end equalization

Measure frequency/phase response as determined by the coherent receiver End-to-end calibration of the whole transmission system This includes the characteristic of the receiver as well !


M8195A

I

Y-Polarization

Q

I

Q

X-Polarization

π/2

π/2

Tunable Laser

N4391A or N4392A

AWGMZMAmplifiers

Coherent Receiver

Optional Fiber & Optical Amplifiers

Optical ModulationGenerator Tool phase and

frequency response data


Typical Tests inCoherent Transmissions


Test application: system and link stress test

TXelectronic

IQMod.

CarrierLaser

ROADMWSS

OpticalCoherent IQ det.

RXSignal

Processing

LOLaser

Typical test requirements: BER (EVM) vs. Power or OSNR Spectral behavior over 1-2 neighbor channels Signal integrity along the link (time domain modulation analysis) Nonlinear characteristics (PMD, PDL, CD,…)

N4391A or N4392A


15

ROADM – Reconfigurable Optical Add-Drop MultiplexerWSS - Wavelength Selective Switching


Test application: Rx stress test (optical)

Multi-format Reference Transmitter oroptical AWG with or without stress generation

M8195A + OMFT + Tunable Laser+ Optical Modulation Gen. Tool


RXSignal

Processing

LOLaser

Stress generationon Optical Signal- Loss- Polarization - PMD- OSNR- Non-Linearities


16© 2015 Keysight Technologies

Typical requirements: Availability of numerous modulation formats for research and development High quality PAMx, (D)QPSK and QAMx signal for manufacturing Arbitrary constellations for advanced research required (arbitrary stress) BER vs various stress parameters Some customers need RZ DQPSK Support of up to 56 GBd rates for >> 100G research PRBS test pattern, Pre-defined or User-defined Test of receiver algorithm robustness with distorted signals

Test application: Rx stress test (electrical)

Typical requirements: Availability of numerous modulation formats for research and development High quality PAMx, (D)QPSK and QAMx signal for manufacturing Arbitrary constellations for advanced research required (arbitrary stress) BER vs various stress parameters Some customers need RZ DQPSK Support of up to 56 GBd rates for >> 100G research PRBS test pattern, Pre-defined or User-defined Test of receiver algorithm robustness with distorted signals

Multi-format Reference Transmitter oroptical AWG with or without stress generation

M8195A + OMFT + Tunable Laser+ Optical Modulation Gen. Tool


RXSignal

Processing

LOLaser

Stress generationon Optical Signal- Loss- Polarization - PMD- OSNR- Non-Linearities

Stress generationon Electrical Signal- Loss- Polarization - PMD- OSNR- Non-Linearities


17

M8195A + Optical Modulation Gen. Tool


Test application: IQ modulator

Typical test requirements: Cut-off frequency for each path In-application test to specify best EVM and other measures Polarization behavior Phase Response, Skew Modulator Bias Control

Multiformat TXDAC electrical output !

Analysis ofmodulation quality andpolarization behaviour

Frequency responseof modulator

TXelectronic

IQMod.

CarrierLaser

I

Y-Polarization

Q

I

Q

X-Polarization

π/2

π/2

TunableLaser

M8195A + Tunable Laser+ Optical Modulation Gen. Tool

LCA

N4391A or N4392A



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Typical Measures IndicationEVM & Q-Factor General figure of merit for Signal Quality

EVM percentile Signal Quality w/o the effect of IQ Gain Imbalance and Quadrature Error

IQ Gain Imbalance Different amplitudes for I- and Q-Signal

IQ Skew Timing skews between I- & Q-Signal

IQ Offset Wrong bias setting or DC offset at modulator

Quadrature Error IQ phase shifter, wrong bias point of 90 º phaseshifter

Frequency Error Carrier Frequency Offset between Tx & Rx, EVM

Symbol Rate Error, V-shape in ErrorVector vs Time Mismatch in Symbol Rate between Tx and Rx

Dual Pol: IQ Skews, X-Y Skew & X-Y Imbalance Skew & Gain differences between Polarizations

Bit Error Results Indentify Bit Errors per Polarization or acrossPolarization

Missing Transitions in Constellation De-correlation issue between I- & Q-Signal

Coherent testingTypical Measures


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EVM & Q-Factor

20


Error Vectorconnects the measuredVector and the expectedvector, Error VectorMagnitude or EVM is magnitude of this vector

EVM Q-factor1% ~ 100

2% ~ 50

5% ~ 20

10% ~ 10

The Q-Factor describes the signal-to-noise ratio at the decision points. It is calculated from the EVM. The formula is proportional to 1/EVMa and the result is converted into dB. It is calculated from the Eye-Diagram

aEVM

1Q


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EVM percentile

21


EVM_pctlEVM_pctl defines the radius of a circle around a group of measured constellation points centered at the reference constellation point.In contrast to EVM_pctl, the classical EVM value from the VSA software includes I-Q Imbalance and Quadrature Error and is a RMS averaged value. The “Hit Ratio” multiplied with the number of points is equal to the number of points outside the circle. For Gaussian Noise dominated impairments, the classical EVM and the EVM_pctl are equal if a “Hit Ratio” of 0.3173 is selected. The "Hit Ratio" can be set in the EVM_percentile algorithm of the OMA Software.


ConstellationReference

EVM_pctl

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IQ Gain Imbalance

22


IQ Gain Imbalancecompares the amplitudeof the I signal with the amplitude of the Q signal and shows the difference in dB.The effects of IQ gain imbalance are best viewed in constellation diagrams where the width of the constellation diagram is different than its height. Rectangular shape of constellation, different amplitudes in I- and Q-Eye


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IQ Skew

23


IQ SkewTiming skew betweenI- and Q-signals distorted IQ Plot, Eye-

Diagrams are shiftedwith respect to eachother

45° transitions open up Increased EVM


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IQ Offset

24


IQ OffsetDC offsets at the I- or the Q-Signals cause I/Q or origin offsets as shown. I/Q offsets can also resultin carrier feedthrough.It is a measure for the shift between the origin of the measured constellation with regard to the origin of the reference constellation (yellow).Without DC offsets, the carrier feed through as well as the IQ offset becomes zero (-infinity dB). Vertically shifted Eye-

Diagram traces Carrier Feedthrough EVM

carrier feed through

Individual I- and Q-

Imbalance available


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Quadrature Error

25


Quadrature Errorindicates the orthogonal

error between the I and Q Quadrature-Phase. Ideally, I and Q should be orthogonal (90 degrees apart). In the screenshot to the right a quadrature error of 22.91 degrees means I and Q are 67,09degrees apart instead of 90 degrees.This could result from wrong bias point setting for the 90° phase shifter in the Mach-Zehnder-Modulator distorted IQ Plot,

distorted Eye-diagrams EVM

π/2

I

Q


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Frequency Error

26


Frequency Errorshows the carrier's frequency error relative to the VSA's center frequency displayed in Hertz. It is the amount of frequency shift from the VSA's center frequency that the VSA must perform to achieve carrier lock. The maximum allowable Frequency Error depends on the Modulation Format used. Here’s a list of formats:

2 GHz

Modulation Format Maximum frequency offsetQPSK 9.6% symbol rate

16-QAM 4.8% symbol rate







2048-QAM 0.1% symbol rate*

4096-QAM 0.1% symbol rate*


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Symbol Rate Error

27


Symbol Rate ErrorIf the Digital Demodulator is only able to recover the clock phase but not the clock rate, a wrongsymbol rate shows up as typical „V“ shape whenlooking at EVM vs timeplot. Symbols start to spreadall over the constellation High EVM

Measured SymbolRate ≠SymbolRate setting


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Dual Pol: IQ Skews, XY Skew and XY Imbalance

28


Optical Signal Summary ScreenDisplay of- Timing skew betweenI- and Q-Signals for X-and Y-Polarization- Timing skew betweenX- and Y-Polarization. - Gain Imbalance between X and YPolarization- Symbol Rate Display- Frequency Error between Tx and Rx (in this case OMA)- EVM and Q-Factor


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Bit Error Results

29


Bit Error Result Screen- Actual BER- Cumulated BER - BER (EVM)- Bit & Error Counts- Delay between X & Y- PRBS generation details


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PRBS Pattern Sources

30


Effects of IQ PRBS Delay in bit (PRBS length 215-1)

0 bit

2 bit

4 bit

1 bit

3 bit

5 bit

Effects of Delay issues between I- & Q-Data Source

Effect of PRBS delayUsing the same PRBS sequence for I and Q and decorrelating it with a toosmall delay might lead to missing transitions and asymetric spectralcontent !Screenshots were takenbased on a PRBS 215-1with different delaysbetween I and Q showingresult in the correspondingconstellation and spectrum.


Typical Impairment Emulation in the electrical Domain

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Example Impairment


Phase Noise in Optical Modulation Generator Tool


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Carrier Phase Noise

33


Analysis of Carrier PhaseNoise

PhaseNoise Analysis of modulated signals.Display of Carrier Phase,Lorentzian Linewidth, Flicker & Random Noise.Graphical result of Phase Spectrum and corresponding Phase Spectrum Model

Upper Constellationshows performancewithout PhaseTracker.

Lower Constellationshows result after PhaseTracking Algorithmusing Kalman Filters.

500kHz Linewidth

2MHz Linewidth

5MHz Linewidth


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Emulation of PolController

34


Polarization Pattern: Great Circle


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Emulation of PolController

35


Polarization Pattern: Slicer


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Stokes Space Analysis in OMA: Great Circle & Slicer

36


Analysis of Stokes Space Trajectory

The OMA allows to analyzepolarization changes. Two patterns„Great Circle“ and „Slicer“ are generated with the AWG electrically. This electrical signal can be used for receiver stress testing.Here the electrical signal is directlyconnected to the OMA inputs.

Great Circle Slicer


35ps PMD, 991ps^2 SOPMD


Example Impairment: PMD (1st and 2nd order)PMD in Optical Modulation Generator Tool


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Example Impairment: PMD (1st and 2nd order)

38


PMD Analysis in OMA

The OMA allows to compensate and analyze the PMD.An electrical signal is generated with the AWG and analyzed by the OMA.This signal could beused for receiver stress testing.


Tools to generate signals for Coherent Optical Applications


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Go Where you Have Never Been Able to Test Before

– Sample rate up to 65 GSa/s per channel

– 20 GHz analog bandwidth

– 1, 2 or 4 differential channels per 1-slot AXIe module

– 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 (*)

– Amplitude up to 1 Vpp(se) (2 Vpp(diff.)), 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

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How about Optical IQ Modulator ?

I

Y-polarization

Q

I

Q

X-polarization

π/2

π/2


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Optical IQ Modulator:


Optical Multiformat Transmitter: OMFT by IDPhotonics / HHI

• Symbol rate 46 GBd for binary formats such as

BPSK, QPSK 34 GBd: for multi-level formats

such as 16QAM • typically >23 GHz BW Dual polarization I-Q modulator

with typically 28 GHz bandwidth 40 GHz linear RF amplifiers

• C+L band• Manual as well as automatic bias

setting (automatic for QPSK and 16-QAM only)

• In combination with DAC/AWG:pre-distortion to compensate for nonlinearities of amplifiers and modulator

• 19“ form factor

OMFT frequency response (S-parameters) and RF cables de-embedded in AWG

In combination with M8195A AWG


OMFT performance verified in combinationwith M8195A AWG

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Optical Modulation Generator Tool with E/O


E/O Modulator pre-distortion

E/O converter in Optical Mod. Gen Tool• Apply pre-distortion to compensate

modulator non-linearities• Includes settings to compensate for amplifier

gain and Vp• Provides compensation of E/O modulator

frequency response (S-parameters)


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AWG software tools for optical applications

44


Optical Modulation Generator Tool *• Dedicated for optical applications

• Generates 2 x I/Q simultaneously

• Optical impairments in electrical domain

• Signal Path De-Embedding using S-Parameters, VSA Equalizer and factory cal

* based on MATLAB

Optical Modulation Generator Tool (1)


Waveform Input page

Basic Setup Amplitude

Pulse Shaping

Page


45


Optical Modulation Generator Tool *• Dedicated for optical applications

• Generates 2 x I/Q simultaneously

• Optical impairments in electrical domain

• Signal Path De-Embedding using S-Parameters, VSA Equalizer and factory cal

* based on MATLAB

Optical Modulation Generator Tool (1)

Compensation of Cable Skews & S-Parameters


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* based on MATLAB

Optical Modulation Generator Tool (2) Optical Modulation Generator Tool *Electrical Emulation of Optical Signal Properties in Real Time (1):

- Phase Noise & Laser Linewidth

- Polarization Rotation

- 1st & 2nd order PMD

(1) with Rev 2 of AWGin Oct 2015


Polarization

Phase Noise

PMD

Tools for Coherent Signal Analysis

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N439xA Optical Modulation Analyzer Family

N4391A Optical Modulation Analyzer• 4 x 33 GHz in a single scope• 4 x 63 GHz as dual instrument

setup special (call factory)• 4 x 80 GSa/s sampling rate (160 Gsa/s for 63 GHz) • Up to 2 GSa/Ch memory• Lowest noise floor (typ. 1.5% EVMrms)


For coherent transmission research & Mfg

N4392A integrated Optical Mod. Analyzer• Compact, fully integrated, no external cabling• 4 x 23 GHz in a single instrument for dual

polarization optical analysis• 4 x 23 GHz electrical inputs for dual polarization

ICR testing• 4 x 63 GSa/s sampling rate • 16 kSa/Ch memory• Noise floor (typ. 2 % EVMrms)


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Smart Setup in OMA Software


„Smart setup“ - the easiest way to setup an OMA

0 Enter carrier wavelength or frequencyEnter symbol rateCheck modulation formatSelect between back to back and link measurement

-> done

1

2

3

4

1234


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Tabular Measurement results, well structured


Per Polarization results Cross Channel results

X-Polarization

Y-Polarization


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Widest Selection of Modulation Formats


No limits … Custom APSK & OFDM

It‘s your choice:• 39 pre-defined modulation formats

or• Custom APSK demodulation

o Up to 8 amplitude levelso In up to 256 constellation pointso More to come, stay tuned ...

• Custom OFDM demodulation

For now and the future !


Custom APSK

Custom OFDM

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Summary Optical Communication Solutions

52

Keysight’s Offering

Wideband AWG

Tools for Coherent Signal Generation

Accurate and Repeatable Impairments

Tools for Coherent Signal Analysis


N4391A OMA

M8195A AWG

Opt.Mod.Gen.Tool


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Further References

53


Publication title Publication No.N4391A Optical Modulation Analyzer – Data Sheet 5990-3509ENN4392A Integrated Optical Modulation Analyzer – Data Sheet 5990-9863ENInfiniium Z-Series Oscilloscopes - Data Sheet 5991-3868ENM8195A 65 GSa/s Arbitrary Waveform Generator 5992-0014EN

Metrology of Advanced Optical Modulation Formats - White Paper 5990-3748ENKalman Filter Based Estimation and Demodulation of Complex Signals – White paper 5990-6409EN

Vector Signal Analysis Basics - Application Note 5989-1121ENDigital Modulation in Communications Systems - An Introduction – Application Note 5965-7160EEssentials of Coherent Optical Data Transmission - Application Note 5991-1809EN

Contact: [email protected]

More Information is available from these Jumpstations:Arbitrary Waveform Generator M8195A andOptical Modulation Generator Software: www.keysight.com/find/M8195AOptical Modulation Analyzer: www.keysight.com/find/OMALightwave Component Analyzer: www.keysight.com/find/LCA


http://www.keysight.com/find/M8195A

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

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

Questions?

Comments?

54


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Backup

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Overview: AWG Software tools for coherent optical applications

Soft Front PanelOptical

ModulationSoftware

IQTools

Availability Comes with M8195A Firmware

Purchased separately

Available free of charge

Requires - MATLAB runtime

Modulation Formats

QPSK, QAMx andmany more QPSK, QAMx QPSK, QAMx,

OFDM

Dual I/Q Yes Yes No

Built-in Cal Yes Yes Yes

In-system Cal Manual Automatic 1, 2 Automatic 1, 2, 3

I/Q Skew Manual Manual Automatic

Optical impairments

(incl. support for real-time DSP)

No Yes No

1 Keysight Infiniium Series Scope needed 2 using VSA Equalizer 3 using own cal routine

MATLAB



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N4391A & N4392A core specification comparison


Parameter N4391A N4392AAnalog Bandwidth 4x 33 GHz 4x 23 GHzEVM noise floor typ. 1) < 1.5 % rms < 2 % rmsSample rate 4x 80 GSa/s 4x 63 GSa/sMax. BaudRate 63 GBd 46 GBdSensitivity -20 dBm -22 dBmSample Memory Up to 4x 2 GSa 4x 16 kSaSoftware platform VSA/OMA 4.xWavelength accuracy +/- 5 pm +/- 2 pmWeight ~ 48 kg (106 lbs) 13 kg (28.7 lbs)Portability no yes

1) @ reference conditions


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Debug your Tx by identifiyingthe path of each data stream

Full flexibility to setup the link from pattern source to tributary

Select Pre-defined orUser PRBS files

Easy intuitive setup

Individual BER Test on each Tributary


Debug your transmitter faster


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New Measurements and Result Tables


7 new measurement traces and tables related to eachpolarization plane

4 new measurement traces and tables for cross-channelmeasurement results

Get new measurement results in the same way as you are used to.


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Improved Training Tools


12 Demo traces in VSA demo environment for self paced learning 32 GBd demo files measured at

5 power levels Selection of 5 generic files with

various modulation formats Selection of PAM 4 recordings

New help system Dynamic Help

Demo Video


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OMA Software 4.1 (1)

Get deeper insight to behavior of your carrier laser

relative to used local oscillator

by seeing IQ offset resolved in I and Q part to speed up transmitter alignment

by analyzing X-Y imbalanceto see imbalances in power between polarization planes

Carrier phase display– Residual carrier phase

calculated from Kalmanphase tracking


I - Q resolved IQ offset– Offset of I component of

the modulator

– Offset of Q component of the modulator

X - Y Imbalance– Basic test for PDL

analysis by analyzing X-Y imbalance over time


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OMA Software 4.1 (2)

Get deeper insight In long term signal distribution

by percentile EVM analysis

By requesting more analysis points for your own algorithm

Adding software Brick Wall Filter

Plotting a scalar value over number of screen updates to analyze long term effects

EVM percentile– Calculate the percentile of

a selectable hit ratio

– Resolved to polarization plane or full signal


More Algorithms– Brick Wall Filter

– Request more points

Value over time– Plot any scalar measured

value over selectable number of frames


OpticalCoherent

Rx

RXSignal

Processing

LOLaser

Typical Tests inComplex Transmissions

Loss, PDLCD, PMD

TXelectronic

IQMod.

CarrierLaser

Carrier Laser Line-width Phase Noise

Transmitter Signal Quality (EVM,

Skews…)

Received Signal Signal Quality (EVM, BER,…) Distortion caused by link impairments

Carrier Laser Line-width Phase Noise

Coherent Rx assembly Phase Angle vs l Skew Relative gain Polarization S-parameter CMRR

Modulator S-parameter S21, Bandwidth El. ReturnLoss

Skews, Crosstalk

Substitute TX Electronic Multiformat AWG M8195A Optical Impairment Emulation Clean Signal Wide Bandwidth

Substitute Tx with “Golden Tx” based on AWG M8195A and Reference Modulator

N4391A / N4392A

N4391A / N4392A

N4391A / N4392A

N4392A

M8195A + OMFT

M8195ALCA

LCA


Typical AWG requirement (electrical)

65

Company Confidential



32 GBd, 16QAM EVM ~ 3%rms

32 GBd, 16QAM EVM ~ 3%rms

… up to the high end you can achieve


66

32 GBd, 64 QAM = 192 Gb/s

56 GBd, QPSK = 112 Gb/s
