Low Frequency Passive and High

Frequency Active Probing Techniques

and Tradeoffs – What to Use and Why

Mark Lionbarger

Field Applications Engineer

Agenda

Probe Specifications Bandwidth

Risetime

Noise

Loading Parasitics

Examples

Dynamic Range Single Ended vs. Differential

Probe Connectivity Solder-In

Browser etc.

Probe Connectivity Examples – DDR Case Study

Virtual Probing and VP@RCVR

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Probing Overview - What is a probe

Probe - Any controlled impedance structure that conducts

Electromagnetic (EM) fields that can sample a portion of a

signal with minimal impact and divert it to a measurement

device.

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Controlled impedance: Circuit has a known loading value

(resistance, capacitance, inductance)

Conducting: Need to be able to carry an EM field (current/voltage

signal)

Sample: Probes needs to sense a small portion of the signal and

divert the energy to a measurement device (possibly adding gain)

?

Connectivity Challenge

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Oscilloscopes have coaxial inputs (SMA, BNC, etc.)

Circuit Boards usually do not have coaxial connectors

Strategies

Add Surface Mount Coax connectors

Costly

Price

Board Area

SMP or MMCX can be higher density

Design Time

Fixturing

Fan out high speed connections to SMA connectors

Great for characterization

Need to know interface or custom build for ASIC

Need to de-embed fixture

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USB test Fixture

SATA test Fixture

Debug

What about general circuit debug?

Non-standard Interface

Probing on components

Interposers

Coax cable solder center conductor to probe point

Simple

Destructive

Termination mismatch

High Loading Effect

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R_Load_Effective = 25 Ohm

33 % Gain Error

(Might be ok for a narrowband signal)

Frequency Domain View

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Sine Waves have narrow band frequency content

Narrowband matching is much easier

Data Patterns have very broad frequency content

Broadband matching is challenging

Oscilloscope Probes

Oscilloscope probes solve the connectivity problem

High Bandwidth

Controlled Impedance (over broadband)

Low loading/minimal impact on signal

Variety of form factors

Solder-in

Hand held browser

Non-Destructive

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Ideal Probe vs. Real Probe

Ideal probe:

Perfectly flat magnitude

response

Perfectly linear phase response

No loading (infinite impedance)

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Real probe:

Non-ideal magnitude response

Non-ideal phase response

Some loading (finite

impedance)

Vsignal Vmeas

Wide Variety of Probes to Choose From

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Wide Variety of Accessories to use with those Probes

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Wide Range of Scopes for any Application

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Different Probes for Different Applications

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Passive - Low Bandwidth, High Impedance 500 MHz Bandwidth

10 MΩ Impedance

Active Single – Medium Bandwidth, High

Impedance

Up to 2.5 GHz Bandwidth

200 kΩ – 1 MΩ Impedance

Active Differential - High Bandwidth4 GHz - 25 GHz Bandwidth

1 kΩ Impedance

High Voltage Differential

Up to 120MHz Bandwidth

DC – 7kV Differential Voltage Range

8 MΩ – 48 MΩ Impedance

Current Probe Up to 100MHz Bandwidth

700A Peak Current

Active Voltage Rail

4GHz Bandwidth

50kΩ Impedance

±30V Offset

± 800mV Dynamic Range

Probe Specifications

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Key Specifications

Key Specifications

Bandwidth (Frequency Response)

Risetime (Step Response)

Dynamic Range

Gain/Attenuation

Noise

Loading Impedance

AC vs. DC.

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Analog Bandwidth

f0

.707

FREQUENCY

VOUT

VIN

1 Bode Plot

Analog bandwidth is the frequency at which

the ratio of the amplitude displayed on the

scope to the input amplitude is -3dB or .707

All oscilloscopes and probes are specified with

an analog bandwidth

Analog Bandwidth

Example 1: A 3 GHz oscilloscope measures a 1 GHz sine wave

f0

.707

FREQUENCY

VOUT

VIN

1

1 GHz input

f0

.707

FREQUENCY

VOUT

VIN

1

Example 2: A 3 GHz oscilloscope measures a 3 GHz sine wave

3 GHz input

1 GHz, 1 V sinewave is

input into oscilloscope

1 GHz, 1 V sinewave is

measured by oscilloscope

3 GHz, 0.707 V sinewave is

measured by oscilloscope

3 GHz, 1 V sinewave is

input into oscilloscope

Fourier Expansion of a Square Wave

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𝑥 𝑡 = 𝑠𝑖𝑛𝜔0𝑡 +1

3𝑠𝑖𝑛3𝜔0𝑡 +

1

5𝑠𝑖𝑛5𝜔0𝑡 + ∙ ∙ ∙

Analog Bandwidth vs. Pulse Shape – Fundamental

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Fundamental Plus 3rd harmonic

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3rd harmonic is out of phase

with the fundamental at center

of the eye – Can decrease eye

opening

Fundamental Plus 3rd and 5th Harmonics

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5th harmonic is in phase with

the fundamental at center of the

eye – May open the eye height

Analog Bandwidth and Bitrate

General Rule : Scope Analog BW ~ 2.5 x Bitrate

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Harmonic Content 25Gbps Square Wave

1st

3rd

5th

nth

12.5GHz 37.5GHz 62.5GHz

Bandwidth Effect on the Signal – 2.5GHz Clock

Bandwidth Effect on Rise Time: 2.5GHz Clock Example

Analog Bandwidth for Edge Characterization

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Signals that appear to be low speed can still have high frequency content and require high sample rate.

Digital signals can have a low bit rate but a very fast rise time

125kb/s has UI of 8us and a 75kHz Fundamental Frequency

Analog bandwidth required to characterize signal:

Bandwidth = 0.45 / Rise Time

125 kb/s CAN signal with 21ns rise time21ns ~ 300x faster than UI!!!!!

Frequency content up to ~ 21.5MHz

Scope Probe System Bandwidth

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-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

0 2 4 6 8 10 12 14 16 18 20

Mag

nit

ud

e (

dB

)

Frequency (GHz)

1 Stage Filter

2 Stage Filter

14 GHz System

20 GHz

Scope20 GHz Probe

Cascading Elements Without compensation Reduces Total System Bandwidth

System Frequency Response

Frequency compensation applied in the design and during calibration at

manufacturing time to restore total system bandwidth

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-10

-9

-8

-7

-6

-5

-4

-3

-2

-1

0

1

2

3

4

0 2 4 6 8 10 12 14 16 18 20

Mag

nit

ud

e (

dB

)

Frequency (GHz)

1 Stage Filter

2 Stage Filter

Compensator Response

Loading

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Loading

We have to take some energy from the signal to measure it

This means the probe tip must have a finite impedance across the frequency range of interest

Obviously we want to keep this impedance as high as possible

Loading caused by the effective resistance, capacitance, and inductance of the probe leads and input impedance

Impact and Error determined by how much the probe’s input impedance loads down the circuit under test

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0

200

400

600

800

1000

1200

1400

0.01 0.1 1 10 100

Imp

ed

ance

Mag

nit

ud

e (

Oh

ms)

Frequency (GHz)

LeCroy Probe ImpedanceDxx05 Probe with Dxx05-SI

LeCroy Dxx05 with Dxx05-SISolder-in Tip (Differential)

LeCroy Dxx05 with Dxx05-SISolder-in Tip (Single-Ended)

Passive Probe Ground Lead Effects

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Optimal Probe Frequency Response

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Transfer Function, 𝑉𝐶

𝑉𝑆= 𝐻 of the series

RLC circuit is given by

𝐻 =𝑍𝐶

𝑍𝑐 + 𝑍𝐿 + 𝑅

Using the Definitions

𝑍𝐶 =1

𝑗𝜔𝐶

𝑍𝐿 = 𝑗𝜔𝐿

We get the transfer function

𝐻 =1

𝑗𝜔𝑅𝐶 − 𝜔2𝐿𝐶 + 1

Choosing Optimal Values for R and L

R = 45Ω

L = 12nH

C = 9.5pF

3dB BW = 520MHz

Ideal Passive Probing Example

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Low ground blade inductance from

10nH – 20nH results in the probe

achieving > 500MHz frequency

response

Copper foil provides a nearby

ground connection to reduce the

length and inductance of the entire

ground loop

Insulating cap on probe tip

provides electrical isolation

between test points to eliminate

the risk of shorting

Not so Ideal Probe Frequency Response

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In most cases, the alligator lead is used

as the return path

Using alligator lead can result in a 10

inch ground loop

20nH/in as a rule of thumb results in a

loop inductance of 200nH!

Choosing realistic values for R and L

R = 45Ω

L = 200nH

C = 9.5pF

3dB BW = 176MHz

Resonant peak at 125MHz!

40% error even at 65MHz!

Ground Loop Error Real World Example

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65MHz signal measured on

probe with test jig and probe

with a long ground lead

Long ground lead probe

exhibits 40% error as predicted

Must keep the signal frequency

down to 35MHz if we want to

keep error under 10%

Passive Probe Accessory Kit

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PK007-030 HF-Compensated Ground Lead

PK007-014

Copper Pad

PK007-024

Tip Ground Lead

with 0.8mm SocketPK007-016

Ground Spring

PK007-013

Ground BladePK007-005

Spring Tip

PK007-017

Single Lead

Adapter

PK007-018

Dual Lead

Adapter

PK007-020

Micro Clip

Long 0.5 mm

PK007-021

Micro Clip

Short 0.5 mm

IC CapsPK007-026

Ground Lead

with Mini Clip

PK007-007

Insulating Cap

PK007-027

Ground Lead

with 0.8 mm socket

Passive Probe Applications – Micro Grabber

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Passive Probe Applications – SMT boards

Sharp spring loaded tip maintains constant contact with probe point

2/23/2017

Company Confidential

37

Bayonet ground stretches to ground point

Differential probe loading model

Differential probe loading model

2/23/2017 38

Differential Impedance

Differential Input Impedance Plot

Impedance varies as a function of frequency Zmax = 1.2 kOhm at DC

Zmidband = 500 Ohms at 12.5 GHz

Zmin = 120 Ohms at 25 GHz

2/23/2017 39

Loading Impedance

DC vs. AC Impedance

Impedance varies as a function of frequency

Near DC, resistance dominates

At high frequencies, reactance dominates

Frequency dependent measurement error*

Error correction methods described later

Look carefully at impedance specs.

Not all manufacturers spec AC and DC impedance separately

Some only specify DC impedance (resistance)

LeCroy publishes probe loading in the manuals and datasheets

A high DC resistance does not imply a high AC impedance

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Why do we care about loading impedance?

2/23/2017 41

Frequency dependent error due to

loading is compensated in software.

If we compensate, why do we care

about loading?

Still need a good loading over

frequency so not to load down

driving circuit

Want high impedance across the

entire frequency band to avoid

loading effects over entire probe

bandwidth

Dynamic Range

2/23/2017 42

Dynamic Range

Three elements of Dynamic Range

Input Differential Mode Dynamic Range

Input Common Mode Range

Input Offset Range

Ensure your probe has enough of each type of range to be able to be

able to get your signal on screen

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Differential Mode Range

Differential Mode Range (DMR)

Sometimes casually called “The Dynamic Range”

Need to look at common mode and offset range as

well

Maximum voltage between + and - inputs.

Directly Verify on scope.

Differential Mode Range

Maximum voltage

between inputs

Common Mode Range

Common Mode Range (CMR)

Maximum voltage between either pin and

(scope) ground

Normally not seen on screen

Verify by grounding one input at a time.

Common Mode Range

Maximum voltage from

either input to ground

Offset Range

Offset Range Not all inputs are sitting at the same non-zero common-

mode voltage

Offset range is maximum differential offset a probe can

apply to the input signal to bring in within the differential

mode dynamic range of the scope.

VCM

Vip

Vin

Voffset = Vip - Vin

*Terminals are

referenced to ground

Dynamic Range

Example Voltage Conditions for LeCroy D2505 25 GHz Probe

1.6 Vp-p (+/- 800 mV) differential dynamic range

+/- 4 V Common Mode Range

+/-2.5 V Offset Range

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Internal Gain/Attenuation

2/23/2017 48

Inside of the differential amplifier, matching circuits are

used to match impedance of the probe amplifier and the

probe tip.

Also equalizer circuits are required to flatten the

impulse response of the probe.

Passive Matching circuits and Equalizers have a nominal

attenuation

Amplifier adds back gain to compensate for attenuation in

matching circuits.

Probe datasheets list effective gain/attenuation

Noise

2/23/2017

Ideally, want the probe to add a minimal amount

of noise

Tradeoffs between gain and noise

Higher Gain (lower volts per division) adds

more noise at output of diff amp.

Some vendors specify probe only noise

Scope+Probe System Signal-to-Noise ratio

is what counts!

*Baseline SNR calculated with no signal applied to the probe49

Noise Comparisons

2/23/2017

Noise comparison between competitive probe (dark blue line is LeCroy D1605)

Tradeoffs between gain and noise Also, higher bandwidth

probe has higher noise.

Some probes do not have enough gain to enable high sensitivity measurements Probe in light blue cannot go below

50 mV/div

*Baseline SNR calculated with no signal applied to the probe50

𝐵𝑎𝑠𝑒𝑙𝑖𝑛𝑒 𝑆𝑁𝑅 𝑑𝐵 = 20 ∗ log𝑀𝑎𝑥𝑖𝑚𝑢𝑚 𝑆𝑖𝑔𝑛𝑎𝑙 𝑅𝑀𝑆 𝐴𝑚𝑝𝑙𝑖𝑡𝑢𝑑𝑒

𝑅𝑀𝑆 𝑁𝑜𝑖𝑠𝑒

Types of Tips

Variety of Interconnection Options

2/23/2017 51

Solder In (SI)

Highest Bandwidth (Up to 25 GHz)

Best Signal Fidelity

Solder Connection Required

Positioner Tip (PT)

Aka “Browser”

Very High Bandwidth (Up to 22 GHz)

Excellent Signal Fidelity

Convenience of Moving

(No Solder Required)

Quick Connect (QC)

Lower Bandwidth (Up to 6 GHz)

Good Signal Fidelity

Solder resistor test points

Slip on Tip

Square Pin (SP)

Lowest Bandwidth (3.5 GHz)

Decent Signal Fidelity

Easy to Use with headers

Best Practices High Speed Active Probing:

DDR Case Study

2/23/2017 52

Recommended DDR probing method 1: Hands-free probe holder mounted in reverse positionStrain relief for the probing connection can be provided by utilizing this counter-weight configuration

The typical use model for a hands-free

probe holder is designed to place weight

at the tip of the probe

This reverse mount acts as a counterweight, removing

force from the probe tip, and providing strain relief for

the probing connection during DDR testing

Not recommended probing

configuration for DDR

Recommended probing

configuration for DDR

Recommended DDR Probing Method 2: Gooseneck strain relief Mount adhesive base on nearby chip, strain relief is provided to the solder tips

Adhesive base

mount

Tips soldered

to chip pins Probing discrete

components

A probe with flat geometry and

rubberized flex circuit lead can be

easily secured in place

Probe Placement and Coupling

2/23/2017 55

DQ is noisy• DQ is probed single ended and is more

susceptible to noise coupling than DQS

and CK which are probed differentially.

• The Physical placement of the DQ probe

is very close to the CK probe. You can see

from the shape of the noise that it is

crosstalk from CK signal.

• Ideally the DQ probe should have been

positioned closer to a 90 degree angle

from the other probes.

CK#

DQ

Recommended DDR Probing Method 3: Chip clip secures probe to board or chassisA chip clip prevents movement of the probe platform cable assembly when mounted on a board edge or chassis

Chip clip mounted on corner of

DDR board

Chip clip mounted on edge of

computer chassis

Best practices example: using Kapton tape and signal labels on the board

Signal labels on the

board are recommended

Kapton tape is

recommended on the

probe interconnect lead

Taping under the probe lead tip can prevent

accidentally shorting or coupling to nets

underneath.

DDR probe heads hot glued to the back of a single-sided DIMM

Probe tips hot glued to the back side of

the BGA ball out of this single-sided

DIMM. This allows for secure connections

during card insertion into the DDR slot.

Note: always hot glue the top,

not the bottom of the probes

Probe leads soldered and taped to DDR4 DIMM before inserting into slot

Be careful not to tape on the sides of a DIMM as there are

notches that accept the clips from the connector. If these are

taped over, the DIMM may not seat properly in the slot.

Dealing with less-than-ideal probing situations

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VP@Rcvr – Compensating for reflections

The Virtual Probe@Receiver Math operator (“VP@Rcvr”) enabled by

the EyeDoctor II package is designed to quickly compensate for signal

reflections due to a termination impairment.

Does not need S-parameters of DUT or probes

Builds a transmission line model to virtually move the probing point

closer to the receiver

Sim option can be used to verify the model

2/23/2017 61

Typical LPDDR2 signals

LPDDR2 667

667MT/s

Clock rate

333 MHz)

2/23/2017 62

Clock

Strobe (DQS)

Data (DQ)

Zoom in a little (…OK …a lot)

Sometimes the

signals look

really nice, like

this:

2/23/2017 63

Clock

Strobe (DQS)

Data (DQ)

Zoom in a little (…OK …a lot)

But other times

they look lousy,

like this:

It’s hard to make

accurate timing

measurements

when the edges

are not clean

2/23/2017 64

Clock

Strobe (DQS)

Data (DQ)

Look at Eye Diagrams in DDR Debug Tool Kit – Read Eye

Read eye:

2/23/2017 65

DQS

DQ

Look at Eye Diagrams in DDR Debug Tool Kit – Write Eye

Write eye:

2/23/2017 66

DQS

DQ

What’s Happening Here?

2/23/2017 67

Read Write

DQS

DQ

DQS

DQ

What gives?

So what’s going on here?

Controller DRAM

Z0 = 50Ω

RT >> 50Ω

VA VB

VA

VB

T1 T2 T3

Measure the propagation delay

We’re using the

signal itself as a

TDR pulse

2/23/2017 69

Strobe (DQS)

Data (DQ)

Now we have a simple model

2/23/2017 70

Z0 = 50Ω

RT >> 50Ω

VA VB

TD = 411ps

Controller DRAM

Testing Strategy – VP@Rcvr Example

12/4/2013 71

DQS

Before

DQS

After

DQ

Before

DQ

After

Reflections at Vref have been removed

We can create a virtual probe point

2/23/2017 72

PCB

Memory Controller

DRAM

What do our virtually probed signals look like?

Write burst

signals at

original probe

point:

…vs new

virtual probe

point:

2/23/2017 73

DQS

DQ

Original Virtual

What do our virtually probed signals look like?

Write burst eye

diagrams at

original probe

point:

…vs new

virtual probe

point:

2/23/2017 74

DQS

DQ Original Virtual

End result: the correct signal, at the correct probing point:

2/23/2017 75

Read WriteDQS

DQ

VP@Rcvr Example: Interposer Shortcoming

2/23/2017 76

• Signals from end user’s

large FPGA

• Interposer probe point

1.5 inches from receiver

• VP@RCVR can

compensate for the

distance between actual

probe point and desired

probe point

VP@Rcvr Example 2: Mid-bus Probing

Probed mid-bus.

Reflections from

RX

Probed mid-bus.

VP@Rcvr applied

Probed at RX

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Virtual Probe

IC System Board PCIE Connector Plug-In Card IC

Signal degrades over long transmission path and connectors

Virtual Probe – Emulation with S-Parameters

2/23/2017 79

Virtual Probe

2/23/2017 80

Virtual Probe Example

2/23/2017 81

One Stage Virtual Probing

Two Stage Virtual Probing

Virtual Probe with Equalizer

Questions?

2/23/2017 85