practical de-embedding for gigabit fixtures

7/28/2019 Practical de-Embedding for Gigabit Fixtures

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Practical De-embedding for

Gigabit FixturesBen ChiaSenior Signal Integrity Consultant

5/17/2011


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Topics Why De-Embedding/Embedding? De-embedding in Time Domain De-embedding in Frequency Domain De-embedding example Compliance test examples Accuracy consideration

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Terminalolgy De-Embedding Remove fixture effect from the measurement Example: remove probes, SMA+traces

Embedding Add a cable to see what happen to the signal Example: test receiver sensitivity by adding a 4-inch cable between

the driver and DUT

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De-embedding Example

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Measurement Reference Plane

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6-meter cable de-embedding

Does your scope show the real waveform? `6


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Example: De-embedding Probes

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More Example: DDR2 Device Fixture

How to remove this fixture effect?8

DDR2


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DDR2 Fixture De-Embedding

Scope performs de-embedding9


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De-Embedding CalibrationBoth VNA/TDR have firmware to

simplify de-embedding process

Probe de-embedding

Fixture de-embedding

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Calibrate with SMA standard Short Load

Connect Port 1 to SMA to measure S11

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SMA

How to capture S11 at the package ball?

PCB trace

TDR De-embedding Example


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TDR De-embedding Example

Agilent 54754A Option A Step 1: Capture fixture S4P ( Pad+trace +SMA )

Calibrate using SMA standard and Probe standard Measure DUT

Capture S4P of SMA+Trace in Time Domain

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SMA

Port1: Pad Probing

Port2: Probing


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Probing Example

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TDR De-embedding

Option A Step 1: Capture fixture S4P for trace with SMA at one end and

PCB pads at the other end?

Step 2: De-embedding fixture trace using ADS

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TDR De-embedding

Option B De-embedding including SMA using on-board calibration short

and load standard

Measurement

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DUT

short

50 ohm load


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Return Loss De-embedding

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De-Embedding using VNA

Equipment : Agilent ENA Calibration : SOLT DUT: Port 1, 2 - SMA Port 3, 4 - USB connectors

Require TRL-like PCB calibration standard Fixture delay estimation use port extension to measure fixture delay

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SMA USB

Port1,2 Port3,4


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USB3.0 De-embedding Example

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A

C

HowtomeasureS4PfromAtoC?


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USB3.0 De-embedding Example

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HowtomeasureS4PfromAtoC?

A

C


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De-embedding fixture andCalibration board

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De-embedding fixture

A

C

Calibration Board


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Calibration Fixture

Calibration Standard Open/Short/Load Port1, 3 use coaxial standard Port 2,4 use calibration board

Delay/through Port 1-2 use coaxial standard Port 3-4 use 2x fixture trace Prot 1-3, 2-4, 1-4, 2-3 use 1x fixture trace

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Open/Short/load1x fixture trace

1x thru trace

2x thru trace


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Example : Return Loss Measurement

Introduction Return loss measurement When return loss is not passing Return loss fixture removal

How to fix the return loss problem Summary

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Sata Interconnect

23Return Loss Specification limits the reflected energy


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Return Loss Definition

Slide

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Return Loss = or

where

Zo is the Tx/Rx differential output/inputimpedance

ZL is the link differential impedance

Vi is the differential voltage incident upon Rx/Tx

Vr is the differential reflected voltage from Tx/Rx


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Common Mode vs. Differential Mode

Slide

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Return Loss Measurement

Many design can not meet return loss requirement Return loss accuracy and repeatability become crutial Both TDR ( time domain ) and VNA (frequency domain) can

be used for return loss measurement

Recommend VNA for marginal design Accuracy noise floor and calibration Repeatability - calibration standard

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Return Loss Element

Termination and IO circuit Ask circuit designer to reduce mismatch in both differential mode

and common mode

Differential driver circuit state Design special circuit to set IO to static 1 or 0 state Capable to set static de-emphasis state

Pattern Blocking Capacitor

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Blocking Caps

The 100nF off chip coupling capacitor leads to a coupling timeconstant of 10 us. Such a long time constant is needed to limitbaseline wander with scrambled data

Make sure DC blocking capacitor is in placed for activetransmitter return loss measurement - TDR/VNA port maygradually get damaged

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How to Pass Return Loss

Small IO capacitance less reflection Uniform impedance less reflection Shorter and uniform impedance fixture May not require fixture de-embedding if margin is large enough

Long fixture trace and via will have better return loss numberbecause reflection are attenuated by the long trace Yougot wrong result

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Vector Network Analyzer

Frequency Domain Equipment Single ended measurement Convert to differential mode and common mode with

software

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How Return Loss Are Measured today

VNA Setup 0 dbm (1mw)

Tx Setup One driver on AC high state ( < Voh) Another driver on AC low state (> Vol)

Rx Setup Activate terminator

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Return Loss De-embedding

Simulation to remove the SMA, PCB trace, via and fingers. The return loss after de-embedding are usually worse than the

VNA data measured at the SMA or PCB fingers

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SMA Anti-pad Optimization Antipad diameter under trace Antipad diameter above trace Signal via radius Trace moved to different layer Ground via distance

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Optimized Anti-pad Design

SMA

Optimization critera-- 50 ohm-- low return loss


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Return Loss Tuning Strategy : Simulation

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PkgVia

PkgTrace

Pkg Stub

BondWire

RiCi

Rterm

Agilent ADS or any spice tool can make this simple model


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Transmit Port B Return Loss Including PCB

Red: ModelBlue: Measured

An Example: XAUI


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Receive Port B Return Loss Including PCB

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Red: ModelBlue: Measured


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Transmit Port B Return Loss

Not Including Socket


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Receive Port B Return Loss

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Not Including Socket


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Pink: Measured Rx Black: Model Rx

Slide39

Violations at 3Ghz

What was wrong in this design?


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Circuit for Return Loss Calculation

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S11

Z0

CL

RL RS

CL: Cell Capacitance RS: Cell Series Resistance

RL: Termination resistance Z0: Reference Resistance for S11 Calculation


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Rx Return loss model vs measured

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83ps

SMA

Pcbvia

PackagePad/ball/via

Ci


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Rx Return Loss Debugging

Identify the elements from S11 plot 3GHz = 333ps Reflection at 180 degree 165ps Round trip delay 83ps Delay from PCB pad to Pakage pad ~83ps

Fix: Increase the impedance to 50 ohm at the PCB via and package

via in the model

Slide42


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Pink: measured Rx Black: modified Rx model

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Replace PCB via andPackage pads to 50 ohm


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TX : Increase PCB and Pads to 50 ohm

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Return Loss Summary

Measurement based return loss provides accurateresults after de-embedding SMA and lead-in traceson the text fixture

SMA Impedance discontinuity can affect return losssignificantly

Optimized SMA anti-pad design can improve returnloss accuracy

Return loss measurement can be verified by carefulmodeling the package and PCB interconnecttransistion

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Hands-on Lab

USB Compliance test Configuration Different fixture design comparison Insertion loss Eye diagram

Conclusions

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USB 3.0 Tx Compliance Channels

Compliance Channels are definedto test Transmitter for worst caseconditions.

Worst Case Channel for Hosts 5 Device PCB Trace 3M Cable

Worst Case Channel for Devices 11 Device PCB Trace 3M Cable

Host Tx

TP0

Device TxTP0

Host Tx

TP1

Device TxTP1

3 Meter

USB 3.0

Cable

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GRL Case Study:Three USB 3.0 Probing Setups for Device Testing

USB-IF Fixture with 4USB3.0 Cable

Agilent Fixture with 4USB3.0 Cable

LiTek Fixture with 0Cable

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Scope

TP0

TP0

TP0

MeasurementPlane

LITEK_TIER2.S4P

FixtureX.S4P

Measurement

PlaneTP0

(Device Connector)

INTEL_TIER2.S4P

AGILENT_TIER2.S4P


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Compliance Test Point

Embed of USB 3.0 Cable + HostChannel

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Transmit ChannelTP1

TP0

CTLEIn Scope SW

cascade_cable_back.s4p

100mV

Eye Mask


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Differential Insertion Loss Comparison

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2.5GHz Nyquist Frequency


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Results

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Agilent + 4 Cable Intel + 4 Cable

@ Scope

Inputs

LiTek + 0 Cable

@ TP0DUT

Connector

@ TP1-EqCompliance

Point

250mV

234mV 228mV


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Conclusions

LiTek with 0 cable provides 6-9% Higher Voltage Margin than4 Cable based fixtures.

Intel Fixture, which is recommended by the USB-IF provides theleast amount of margin and represents the worst case of the

three fixtures tested.

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Thank You

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Contact Information

Corporate Headquarters

3500 Thomas Road, Suite ASanta Clara, CA 95054 USA

GRL Company Confidential

Johnson Tan, CEO

[email protected]

Mike Engbretson, Chief Technology [email protected]

practical de-embedding for gigabit fixtures

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