High Speed Digital

Design & Verification

Seminar

Measurement fundamentals

Agenda

Sources of Jitter, how to measure and why

Importance of Noise

Select the right probes!

Capture the eye diagram

Why measure Jitter?

Determine the probability of how often a design will meet a bit error rate specification (10-12 BER typically desired).

Understand the source of jitter in order reduce jitter to meet a timing budget requirement.

Test for compliance to ensure compatibility between components from multiple vendors.

Ensure that digital designs have the timing margins to operate reliably 24 hours a day.. 7 days a week.. without crashing!

Where does jitter come from?

Transmitter Receiver

•Thermal Noise (RJ)

•DutyCycle Distortion (DCD)

•Power Supply Noise (RJ, PJ)

•On chip coupling (PJ, ISI)

•Lossy interconnect (ISI)

•Impedance mismatches (ISI)

•Crosstalk (PJ)

•Termination Errors (ISI)

Total jitter components

TJ: Total Jitter (convolution of RJ (based on BER) & DJ, and measured in peak-to-peak.

RJ: Random Jitter (rms)

DJ: Deterministic Jitter (peak-to-peak).

• PJ: Correlated & uncorrelated Periodic Jitter (caused by cross-talk and EMI)

• DDJ: Data Dependent Jitter

• DCD: Duty Cycle Distortion (caused by threshold offsets and slew rate mismatches).

• ISI: Inter-Symbol Interference (caused by BW limitation and reflections).

Jitter probability: BER

random tic determinis pk pk n J J s ´ = -

=

How do real time ‘scopes measure jitter?

NRZ

Serial

Data

Recovered

Clock

Jitter

Trend

Jitter

Spectrum

Units in Time

Units in Time

Jitter

Histogram

Important settings for accurate results in

eye diagram & jitter analysis

• Clock signal edge position are compared against reference Clock edges

• Data signal edge position are compared against reference Clock edges

Is my serial system using an explicit clock I can probe ?

• Yes-> Probe the clock and configure scope to use it!

– If receiver apply a multiplier -> scope must be configured to to the same

– If receiver apply a PLL on explicit Clock -> scope must do the same

• No? -> Derive the clock from the data using software PLL emulation

• Software clock recovery must be flexible to imitate Receiver Clock recovery

• Jitter Observed on Oscilloscope depend on Soft PLL Bandwidth Parameter

• Too Big PLL bandwidth transalte into lower observed Jitter

• Too Small PLL bandwidth translate into higher observed Jitter

• The good software PLL BW Value is the one from YOUR Reciever

What is real-time jitter analysis?

The timing variation measurements can be…

• Data: Time Interval Error (TIE), sometimes called “phase error”

• Clock: Period, Cycle-to-Cycle, N-Cycle

• Parametric: Frequency, rise time, setup time, hold time in a synchronous system

The views includes…

• Eye Diagrams (repetitive volts vs time)

• Histograms (N vs time error)

• Trend (time error vs time)

• Spectrum (time error vs frequency)

• RJ/DJ separation (rms/p-p tabular)

• Bathtub curves (BER vs eye opening)

Real-time jitter analysis consists of a collection of successive

timing variation measurements displayed in multiple views

where…

•Can measure data rates up to >28Gb/s

•Can measure jitter frequencies up to 1/2 the data rate

•Can use software for ideal clock recovery (no added jitter)

• Can correlate jitter to other signals (power supply, cross-talk)

• Can measure jitter on live signals with active probes

• General purpose tool, useful for other tasks

• 1ps@12GHz / 150fs@33GHz rms jitter noise floor typical

Weakness of Real Time Oscilloscope Jitter Measurements

Measuring low frequency jitter requires lots of Memory

Real time Oscilloscope trade-offs

Benefits of Real Time Oscilloscope Jitter Measurements

Compliance

• Component Manufacturers

• 50 ohm fixtured

• Driven by standards work

Debug

• PCB/System Engineering

• Live Signals

• Logic Analyzers

• Jitter Identification!

Real time ‘scope versus other jitter solutions

Verification

• Probing Sweatshops

• Live Signals

• Ease of use, reliability

RT Scopes -

Active Probes

Needed

BERTs, TIAs,

DCAs,

Generators

Agilent EZJIT jitter measurement application

Signal

Trend

Histogram

Spectrum

Best shown live

on scope (NB This note disappears in

slideshow mode)

Why is Noise important?

Noise eats up margin

•By adding vertical noise

•By increasing jitter

Noise is a significant concern in real-time scopes:

• Wideband front end

• High-speed digitizer

Why is vertical noise floor important ?

Let’s consider theoretical signals with Zero jitter and fixed voltage noise

with three different edge speeds and crossing a Threshold at 50%

1)Voltage noise translates directly to Timing uncertainty (Jitter)

2)Higher Vertical Noise Floor translates to higher Timing uncertainty

3)At constant amplitude noise floor, slower edge speed translates to higher

Timing uncertainty

Noise sources

Noise

Probe Attenuator A/D Amplifier

Probe Noise Quantization Noise

Quantify your scope noise!:

• AC Vrms Measurement

• Histogram-standard deviation

• FFT

Auto measurement of noise: AC Vrms

Best shown live

on scope (NB This note disappears in

slideshow mode)

Histogram: standard deviation ~ AC Vrms

Best shown live

on scope (NB This note disappears in

slideshow mode)

FFT

Best shown live

on scope (NB This note disappears in

slideshow mode)

FFT- ADC spurs

Best shown live

on scope (NB This note disappears in

slideshow mode)

FFT-harmonic distortion

Best shown live

on scope (NB This note disappears in

slideshow mode)

Noise minimization technology

Patented, proprietary Agilent Faraday Shield packaging

• Means measurement accuracy for HSD

Why are probe loading and response Important?

Probe loading affects your device under test.

You want to observe as nearly as possible the

operation of your circuits without the probe.

You want to see as nearly as possible what is happening at

the probe tip. You don’t want to make assumptions or

guesses.

Probe selection fundamentals

The probe becomes part of the circuit under test and may

change your circuit’s operation (ZPROBE Infinity)

The probe may or may not accurately reproduce the signal

under test

Circuit Under Test

Simplified probe loading model

Zsource

CPROBE RPROBE

LGROUND

LSIGNAL

• Resistive, capacitive and inductive

loading effects must be considered

Probe impedance versus frequency

R

C

L

|Z|

f fRES

fRES 1

2 LC =

Dominates Z At Low Frequencies

Resistive loading

Effects

• Bias change

• Amplitude reduction

• Offset shift

For Errors Less than 10%

RPROBE 10 RSOURCE >

Dominates Z At Mid-Band Frequencies

Capacitive loading

Effects

• Risetime Change

– Could cause circuit to START working

– Could cause circuit to STOP working

• Propagation Delay Increase

• Bandwidth Reduction

Minimize probe tip capacitance to improve

timing measurement accuracy

Dominates Z At High Frequencies

Combined inductive-capacitive loading

Effects

• Resonant LC Tank

• Low Input Impedance

Minimize the length of connection

accessories (inductance = 1nH/mm)

fRES 1

2 LC =

Compensated High-Resistance Passive Dividers

Passive probes

6-14 pF

9 MW Adjustable Compensation

Capacitor

Probe Tip

Cable Oscilloscope Input

1 MW

Features:

Higher input resistance

High dynamic range

(>100V)

Benefits:

Applications: Tradeoffs:

Rugged vs. an active probe

Low cost

Small probe tip

General purpose probing

High resistance

nodes (<100kW)

Lowest bandwidth

(<500MHz)

Heaviest capacitive loading

50W Terminated Resistive Dividers

Passive probes

Probe Tip

450 W 950 W

Oscilloscope Input

50 W

50 W Cable

Input C < .2 pF

Features:

Low capacitive loading

Highest bandwidth ( 6GHz)

Benefits:

Applications: Tradeoffs:

Timing measurement

accuracy

Low cost vs. active probe

Low Z systems

Transmission line

systems

Heavy resistive loading

Active probing

The performance of an active

probe is dominated by the

connection to the point being

probed

+

-

Oscilloscope

ZO = 50W

50W

50W 50W 25KW

200 fF

ZO=50W

50W RF Connector

+sig

ZO=50W

200 fF

-sig

50W 25KW

Probe Cable ~ 5 mm ~ 10 cm (~4 in) Probe Amplifier

The InfiniiMax I Technology

Response and loading

The secret

Differential or single-ended?

EVERY signal is differential.

“Ground” is a convenient fantasy.

Differential probes offer:

•Higher CMRR, especially at high frequencies

•Immunity to “Outside mode” effects

•Higher impedance at high frequencies

Probing Methods for Differential Signals

• InfiniiMax Active Differential Probes • 13 GHz Solder-in, Socket, Browser,

SMA, ZIF

• Differential or Single-ended

• Use with Real-time Scope, DCA-J &

BERT

• Soft Touch Probes: • No PCB layout modification required

• No remaining stubs or sockets

• Use with 16800/16900 Series Logic

Analyzers, E2960 Series PCI Express

Tools

• DDR2/3 BGA Probes: • Access Command & Data signals

• Use with Scopes & Logic Analyzers

Active probes

Probe Amplifiers Agilent 1169A Agilent 1168A Specified Bandwidth 12 GHz 10 GHz

Characterized Probe Tips Yes Yes

Noise Referred to Input 2.5 mV rms 2.5 mV rms

Attenuation 3.45:1 3.45:1

Differential Dynamic Range 3.3 V p-p 3.3 V p-p

DC Offset Range +/- 16 V +/- 16 V

Maximum Voltage +/- 30 V +/- 30 V

Agilent offers excellent

bandwidth, characterized

performance for various

probe tips, low noise,

low attenuation, good

dynamic range and small

size in its InfiniiMax II

series probes

N5380A 12 GHz Differential

SMA Adapter Probe Head

N5381A 12 GHz Differential Solder-in Probe

Head: 210 fF input capacitance, 50 kOhm input resistance

N5382A 12 GHz Differential Browser: 210 fF input C, 50 kOhm input Resistance

Capturing an eye diagram

The easiest way to get an overall idea of the quality of the serial signal

Trigger on Clock signal (if available) as the rough first pass to build Eye Diagram

Eye Diagram is the superposition in the middle of the screen of 3 bits

Multiple case combined form the Eye (000,001,010,011,100,101,110,111)

Preceeding bits might impact the ones you’re seeing - this is called Inter Symbol Interferences

Use Clock Recovery with PLL Emulation on 8B/10B signal and memory folding to build eye

101 Sequence

Building an Eye diagram the synchronous way: Explicit Clock used as Trigger

011 Sequence Overlay of all combinations

Best shown live

on scope (NB This note disappears in

slideshow mode)

What represents “good enough”?

The eye-mask is the common industry approach to measure the eye opening

Failures usually occur at mask corners

• But what is cause of failure?

Violating USB FS 12Mb/s Eye Diagram Good Displayport Eye Diagram

Best shown live

on scope (NB This note disappears in

slideshow mode)