Design_World_-_Internet_of_Things_Handbook_April_2020

24 DESIGN WORLD — EE NETWORK 4 • 2020 eeworldonline.com | designworldonline.com

INTERNET OF THINGS HANDBOOK

Protocol Decode of USB4 Signaling. In parallel,

it's necessary to view, trigger, and decode the

high-speed 10G and 20G lanes.

Design simulation, protocol decode, USB-PD, RF, channel characterization, SBU, thunderbolt, DP

Transmitter Test

Active Cable Test

Interconnect Test

Return Loss Test

Receiver Test

Active Cable Test

SW

HW

DUT

Fixture

Tx

Cable

Tx

Rx

Test fi xture

M8000 Integrated protocol

J-BERT <200FS RMS

E5080B ENA with S96011A

enhanced TDR so ware

Automated

standards test

so ware

Automated

standards test

so ware

UXR

Infi niium

Scope

<1mV RMS

<25fs RMS

Tx test fi xture

Cable/connector

test fi xture

Test matrix for the Type-C ecosystem

purpose of reducing intersymbol interference. Because

there is little signal margin, the Rx equalization used during

the Tx compliance testing must accurately refl ect the

specifi cation and should also refl ect how the silicon Rx is

implemented.

The Tx specifi cation has the common test parameters

like voltage, eye diagram, SSC, and rise/fall times. There are also the

familiar jitter parameters like UI, TJ, and DDJ. However, there are

also requirements for UJ or uncorrelated jitter. In the traditional jitter

decomposition model, TJ was split into RJ and DJ. In this case, TJ is

split into correlated jitter (DDJ) and uncorrelated jitter. UJ can further

be decomposed into RJ and UDJ.

One reason for this fi ner distinction is a large cross-talk element

when 20-Gbps lines run on four differential pairs in parallel over tiny

structures and cables. If the jitter decomposition is not implemented

precisely per the spec, incorrect jitter analysis results.

A new requirement that did not exist in the USB 3.2 compliance

test specifi cation is the return loss test. If the impedances don’t

match, the signal from the Tx silicon will never make it to the Type-C

connector; nor will the Rx signal going into the Type-C connector

make it to the Rx silicon. This test should always take place before

Tx or Rx testing. If it fails, there is no point proceeding with the Tx

and Rx testing.

TRANSMITTER EQUALIZATION

There has always been the need to characterize the transmitter

equalization (Tx Eq). However, in the USB 3.2 spec, there was just

one Pre-shoot at 2.2 dB and 1 de-emphasis at -3.1 dB. (As a quick

review, pre-shoot and de-emphasis refer to boosts to the signal just

before and just after a signal polarity inversion respectively.) For

USB4, there are now 16 presets with different combinations of pre-

shoot and de-emphasis.

In addition to testing each of the 16 presets, USB4 requires

optimization of the Tx Eq for the optimal eye opening. It is common

to over-look the optimization of the Tx Eq or set it incorrectly in the Tx

silicon. At 20 Gbps, it is common to fail signal integrity because there

are only one or two Tx Eq settings that will work for each specifi c loss

channel implementation.

The various Tx tests discussed so far were at TP2 where the use

case is a cable with an embedded retimer, redriver, or an optical

cable. But USB4 has the notion of a 0.8-m lossy, passive cable. This

is a much more demanding use

case. The measurements are

essentially like TP2, but must

allow for the 0.8-m-cable loss

model. This is probably the most

diffi cult test to pass for Tx testing

at 20 Gbps.

Like the Tx test cases, Rx

testing also has the short-channel

use-case and long-channel test

cases. Case one is the short-

channel test case where the

PG stressed cocktail is applied

directly to the Type-C connector.

Case two is the signifi cantly more

complex use and test case where

the BERT-stressed cocktail now

must go through the 0.8-m 20-G

or 2-m 10-G use case. Improper

set-up of the calibration channel

or calibration of the stress

cocktail will cause either under-

stressing or over-stressing the Rx.