56+ Gb/s Serial Transmission using Duobinary … · Motivation 10-TH3 – 56+ Gb/s Serial...

56+ Gb/s Serial Transmission

using Duobinary Signaling

Jan De Geest Senior Staff R&D Signal Integrity Engineer, FCI

Timothy De Keulenaer Doctoral Researcher, Ghent University, INTEC-IMEC

10-TH3 – 56+ Gb/s Serial Transmission using Duobinary Signaling

Introduction

Motivation


Standard groups looking into serial data rates of 50 Gb/s and above

IEEE 802.3bs 400 GbE

NRZ and PAM4

OIF CEI-56G-VSR/SR (< 100 mm)

NRZ and PAM4

OIF CEI-56G-MR (500 mm)

PAM4

Duobinary signaling presented as alternative for 56+ Gb/s serial data rates

Overview


Duobinary signaling

Duobinary demonstrator

Design of duobinary chipset

Measurements

Conclusions

Duobinary Signaling

Duobinary Signaling

NRZ signaling

Requires extensive pre-emphasis or equalization

Difficult to scale to data rates beyond 40 Gb/s

PAM4 signaling Reduces spectral requirements compared to NRZ

Requires less pre-emphasis and equalization than NRZ

More complex – multi-level transmitter and receiver

Reduced level spacing – less tolerance to noise

Duobinary signaling 3-level modulation scheme

Leverages passive channel frequency response for signal shaping

Reduces spectral requirements compared to NRZ

Requires less pre-emphasis and equalization than NRZ

2T

A/3

T A/2

A T


Duobinary Signaling

Duobinary transmission system: Data source (binary data transmitter)

Duobinary precoder

Feed-forward equalizer (FIR filter)

Passive channel (backplane)

Duobinary to binary converter (rectifier)

Binary data receiver

Equalization effort reduced Main shaping takes place in the channel

Duobinary spectrum has null at ½ data rate

FIR filter limited to 5-taps


Emphasize high-frequency components

Flatten group delay response

Duobinary Signaling

Duobinary – correlative coding Each symbol conveys information corresponding to

the previous and current bit – partial response

Combination results in 3 level waveform

Forbidden transitions form rudimentary error

detection – not considered here

Requires simple precoding at transmitter

Duobinary PSD Spectrum compressed compared to NRZ

Same spectrum as PAM4

Relaxes channel design criteria and bandwidth

requirements of transceiver IC’s


Duobinary Demonstrator

Duobinary Demonstrator

56 Gb/s PPG

TX board

FFE

Scope/BERT

RX board

56 Gb/s 56 Gb/s 14 Gb/s

CHANNEL

TX

28 GHz clock

RX



56 Gb/s PPG

TX board

FFE

Scope/BERT

RX board

56 Gb/s 56 Gb/s 14 Gb/s

TX

28 GHz clock

RX

Duobinary Demonstrator – Backplane Channel

CHANNEL

18

Figure 18: ExaMAX® backplane demonstrator.

Figure 19: Losses 13.7 in backplane channel only (red) and total losses of backplane

channel + test setup (blue).

• ExaMAX® connector system

• 24 lay – 160 mil backpanel

• 18 lay – 94 mil daughter cards

• Megtron 6 board material

• Daughter card trace length = 6”

• Backpanel trace length = 1.7” to 26.75”

• Total length = 13.7” to 38.75”

• 1.3 dB loss per inch at 28 GHz


18

Figure 18: ExaMAX® backplane demonstrator.

Figure 19: Losses 13.7 in backplane channel only (red) and total losses of backplane

channel + test setup (blue).

Duobinary Demonstrator – Backplane Channel

-29.5 dB

-35.5 dB -37.0 dB

-45.0 dB

56 Gb/s PPG

TX board

FFE

Scope/BERT

RX board

56 Gb/s 56 Gb/s 14 Gb/s

CHANNEL

TX

28 GHz clock

RX

TX board

FFE TX

RX board

RX

Duobinary Demonstrator – Loss

1.05 dB at 28 GHz 1.05 dB at 28 GHz

5.6 dB at 28 GHz

3.8 dB at 28 GHz


Duobinary Demonstrator – Total Channel Loss


COMPONENT LOSS at 28 GHz

TX board 5.60 dB

Coax TX board to channel 1.05 dB

Channel loss IL [dB]

Coax channel to RX board 1.05 dB

RX board 3.80 dB

Total loss IL + 11.5 dB

Chip Design

Chip Design – Overview

Transceiver chain

Transmitter overview

Receiver overview

FFE parameter optimization


Transceiver Chain


Transmitter Building Blocks


Receiver Overview


FFE Parameter Estimation

Channel delays, attenuates and

reduces the bandwidth of the impulse

response

FFE impulse response

calculated from

frequency response

FFE impulse response

after channel propagation


Generating Least-square-error Fit

11

impulse response and the taps or by multiplying them in the frequency-domain and

recalculating the impulse responses.

Using the measured impulse responses, a least-square-error (LSE) fit to the idealized

duobinary response is done. The idealized response consists of two bit-spaced narrow

sinc pulses. The LSE fit matches the 5 normalized FFE parameters as well as the optimal

timing. In this way, it selects the optimal number of pre- and post-cursors in the FFE. The

result of such a fit is shown in figure 8.

Figure 8: Fitting the FFE output to the ideal duobinary response.

It is clear that the FFE is capable of matching the main cursors of the duobinary channel.

There is some remaining error which will result in extra inter symbol interference, which

is evident from the eye-diagram of figure 9.

Figure 9: Simulated eye-diagram of the ideal (left) and fitted (right) duobinary signal.


Making a linear combination to obtain the best match

1-tap FFE Parameters 56 Gb/s NRZ 56 Gb/s Duobinary 56 Gb/s PAM4

Impulse response

desired - obtained

Obtained eye pattern normalized at output of FFE

Desired eye pattern normalized


2-tap FFE Parameters


56 Gb/s NRZ 56 Gb/s Duobinary 56 Gb/s PAM4

Impulse response

desired - obtained






Impulse response

desired - obtained



Chip Design – Conclusions

Design of a 5-tap FFE capable of equalizing a backplane channel into a 56 Gb/s duobinary

channel

Design of a sensitive 56 Gb/s duobinary receiver with built-in DEMUX

Optimized FFE parameters estimation methodology based on fast frequency-domain

measurements and Matlab optimization techniques


Measurements

Measurements – Overview

Test setup

40 Gb/s BER vs. insertion loss

56 Gb/s BER measurements


Test Setup

56 Gb/s PPG

TX board

FFE

Scope/BERT

RX board

56 Gb/s 56 Gb/s 14 Gb/s

CHANNEL

TX

28 GHz clock

RX


BER Measurement Results – 40 Gb/s

40 Gb/s – 13.7” 40 Gb/s – 26.25”


28.0 dB at 20 GHz

BER < 1E-12

41.3 dB at 20 GHz

BER ≈ 1E-9

40 Gb/s transmission across channels ranging from 13.7” to 26.25”


19

Measurements started at 40 Gb/s on the shortest link (13.7 in, 35 cm). The loss at the

Nyquist frequency is 28.8 dB, resulting in a vertical and a horizontal eye-opening of 18.2

mV and 15 ps (0.6 UI) respectively, compared to the maximum output eye-pattern at the

transmitter having an eye-opening of 93.4 mV and 19.1 ps (0.76 UI) at 40 Gb/s. Both

eye-patterns are shown in figure 20. This results in error-free (BER < 1E-12)

transmission when connected to the duobinary decoder.

Figure 20: 40 Gb/s output eye-pattern at the transmitter (left) and after a 13.7 in

backplane channel (right).

Figure 21: Chart showing the BER (blue) and the vertical eye-opening (red) as a function

of the loss at the Nyquist frequency for a 40 Gb/s signal measured across the ExaMAX®

backplane.

In figure 21 it is shown that a total loss (backplane + test setup) at the Nyquist frequency

of up to 36.8 dB can be received error-free (BER < 1E-12), and up to 41.4 dB with a BER


Error-free (BER < 1E-12) up to

about 37 dB total link loss (20”

channel + test setup) at Nyquist

(20 GHz)

BER ≈ 1E-9 up to about 42 dB

total link loss (26.25” channel +

test setup) at Nyquist (20 GHz)


20

below 5E-9, which is considered OK for a link with FEC in the current 25 Gb/s IEEE

802.3bj standard. The 36.8 dB loss corresponds to a total channel length of 22 in, while

the 41.4 dB loss corresponds to a total channel length of 27 in. At 36.8 dB and 41.4 dB

the vertical eye-opening are 11 mV and 5 mV respectively, as shown in figure 22.

Figure 22: Eye-diagrams of the 40 Gb/s duobinary signal across a 22 in (left) and a 27 in

(right) backplane channel.

Moving towards higher speeds leads to more frequency-dependent loss. At 50 Gb/s the

signal after a 13.7 in backplane channel was still received error-free (BER < 1E-12), with

an eye-opening of 6.8 mV as shown on the left in figure 23.

By increasing the speed to 56 Gb/s the loss at the Nyquist frequency increases further,

and the vertical eye-opening at the input of the receiver decreases to about 6 mV as

shown on the right in figure 23. The BER obtained at 56 Gb/s is better than 5E-9, which

is more than sufficient assuming FEC is applied.

Figure 23: Eye-diagrams of the 50 Gb/s (left) and 56 Gb/s (right) signal after a 13.7 in

backplane channel.

4.3 Design of active daughter cards

The performance of the transceiver chipset is limited by the total amount of losses that

can be compensated for by the FFE. In the sections above we have shown that a total of

11.5 dB of losses at 28 GHz are added by the measurement setup. These losses are caused

by the TX and RX test boards and by the coax cables needed to connect the different


56 Gb/s – 13.7”

41.0 dB at 28 GHz

BER < 1E-12

Error-free (BER < 1E-12) operation at 56 Gb/s across 13.7” channel with a total link loss of

about 41 dB at 28 GHz

Conclusions

Conclusions


Selection of duobinary signaling as a modulation format for high-speed serial transmission

Duobinary demonstrator

Design of duobinary transceiver chipset

Measurement results demonstrate 56 Gb/s serial transmission over a state-of-the-art

backplane channel is possible using duobinary signaling Limited equalization: 5-taps FFE, no CTLE or DFE


Visit us at booth 817 for a live demo

56+ Gb/s Serial Transmission using Duobinary … · Motivation 10-TH3 – 56+ Gb/s Serial...

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