Precision Time-Domain Reflectometry:

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Precision Time
-
Domain Reflectometry:
Helping to solve today’s difficult signal
integrity/transmission problems

October 2003

TDR Customer Presentation, Oct ‘03

Page
2

Agenda

1. Brief TDR review

Some new things and some old things seen in a new way

2. Advanced calibration techniques unique to Agilent

3. New techniques for improved 2
-
event resolution and
impedance accuracy

4. S
-
parameter results from the TDR


Where to find out more: New Application Note
(see last slide)

TDR Customer Presentation, Oct ‘03

Page
3

1. What is TDR?


Time domain reflectometry


Analyze the quality of high
-
speed components and
channels for transmission
quality


Are there any reflections
due to impedance
discontinuities?


How big are they?


Where are they?

Zo
Z

Zo
Z

Incident energy

Transmitted energy

Reflected
energy

TDR Customer Presentation, Oct ‘03

Page
4

TDR: Launch a fast step into the DUT and
measure anything that reflects back

Z
L

E
i

E
r

STEP GENERATOR

OSCILLOSCOPE

TRANSMISSION SYSTEM

UNDER TEST

Typical Step
: 200 mV, 250 kHz

‘square wave’ with 35 ps rise time

Zo
Z

Zo
Z

Incident energy

Transmitted energy

Reflected
energy

TDR Customer Presentation, Oct ‘03

Page
5

What is TDR?


Launch a fast pulse into
the device under test


Measure what reflects
back from the DUT


The
size

and
polarity

of
any reflections
indicates the
magnitude

of any
discontinuity


The
time

it takes for the
reflection to return is
used to indicate the
location

of any
discontinuity

Input pulse

Reflected pulse(s)

Transmission
lines

with changing
impedance

TDR Customer Presentation, Oct ‘03

Page
6

Displaying impedance in the Time Domain:

TDR provides “Instantaneous Impedance”

Typical TDR result


A: 50 Ohm cable


B: Launch to microstrip


C: 50 Ohm microstrip


D: 75 Ohm microstrip


E: 50 Ohm microstrip


F: “open” circuit

A

B

C

D

E

F

Compare to a network analyzer which provides
impedance as a function of frequency

TDR Customer Presentation, Oct ‘03

Page
7

2. Some important advantages of the 86100 TDR


Would you buy a network
analyzer without a calibration
kit? No!



Without calibration we are
forced to rely completely on
the raw performance of the
instrument and have no ability
to remove error causing
mechanisms outside the
instrument that are in the
measurement path

DUT

TDR Customer Presentation, Oct ‘03

Page
8

Systematic TDR measurement errors can be
removed through simple calibration


A simple concept:


By placing known
reflections on the
system, the
measurement errors
can be identified and
removed


Simple to perform:

Connect a short and a
load at the reference
plane


Errors caused by
cabling, attenuation etc.
can be removed from
the measurement


Agilent is the only
provider to use
Normalization.

Test Fixture

Device Under Test

Blue Trace
-

Normalized

Green Trace
-

Standard

Error = 2.3 ohms

TDR Customer Presentation, Oct ‘03

Page
9

What arguments might you hear against this?


“We don’t need fancy calibrations. We have a precision airline
inside the TDR”


But what can you do for error mechanisms beyond the TDR
output?


Agilent doesn’t need to do a calibration either, unless there
is something beyond the TDR output that degrades the
results (which, in real life, there almost always is)


“Calibration is a weak excuse for bad hardware”


Calibration techniques are a proven route to a better
measurement


Can you imagine doing network analysis without a good
calibration process?

This is all explained in more detail in the new Application Note (see last slide)

TDR Customer Presentation, Oct ‘03

Page
10

3. Some problems the industry faces…..


Data speeds are getting faster in
electrical circuits


Devices are getting smaller and
more complex


As edge speeds increase,
more high
-
frequency energy
is present


More difficult to control
impedance

TDR Customer Presentation, Oct ‘03

Page
11

The edgespeed of the TDR step sets two important
measurement levels

The
two
-
event resolution


(how close can two reflections be
and still be seen as separate
events)


Closely spaced reflections can
get blurred together

Two
-
event resolution set by
material velocity and TDR system
risetime


How
accurate

is the measurement
of the
reflection magnitude


As step speeds increase,
more high frequency content


Reflections often get worse


Reflections for a 20 ps edge
can be much larger than a 35
ps edge


2
risetime
c

TDR Customer Presentation, Oct ‘03

Page
12

A 35 picosecond step is insufficient to see
closely spaced reflections


With a 35 ps step, all you know is the device is there


If there is more than one reflection, we can’t tell

35ps

TDR Customer Presentation, Oct ‘03

Page
13

High resolution allows your customers to see
what they could never see before


At 9 ps step speed, we see
5 separate reflections


Each event is easily seen
and quantified

9ps

V
-
connector

pin
-
collette

V
-
connector

pin
-
collette

hermetic

feedthrough

coaxial

feed
-

through

microstrip

transmission line

coaxial
-
microstrip

launch

TDR Customer Presentation, Oct ‘03

Page
14

A faster step often yields a higher reflection
magnitude


At
35 ps
, the reflections
look very small (~52 Ohms)


At
9 ps

the reflections
increase to over 58 Ohms


The 35 ps result isn’t
necessarily wrong and the 9
ps right


Test at an edge speed
similar to how the device will
be used


Some examples


20 to 35 ps for 10 Gb/s


5 to 12 ps for 40 Gb/s

Designers working at the very high

data rates or with very small devices

need a very fast TDR

TDR Customer Presentation, Oct ‘03

Page
15

How can I test faster than the 35 ps the
TDR is specified at? Two choices


Digitally

increase the edge
speed through some signal
processing


“Normalization calibration”
(discussed earlier) can use
DSP to enhance the
effective edge speed


Can decrease the risetime
to less than 20 ps


Electrically

speed up the
pulse


Use external hardware to
produce a much faster
edge

TDR Customer Presentation, Oct ‘03

Page
16

86100/Picosecond Pulse Labs 4020
Measurement capabilities

35ps

<9ps

The Picosecond Pulse Labs 4020 modules takes the 35
Picosecond pulse from the Agilent TDR and increases the speed
to under 9 picoseconds


Two
-
event resolution is improved by a factor of 4!


(1.5 mm ‘air’, less than 1 mm in common dielectrics)

TDR Customer Presentation, Oct ‘03

Page
17

Optimizing

Measurements


You will lose your edge
speed if you have:


Excess or poor quality
cabling to and from
the DUT


The scope receiver
channel has
insufficient BW


Recommend TDR with the
86118A ~75 GHz remote
plug
-
in:


Max. bandwidth


Minimum cabling
distances

4020 Remote

TDR Head

Sampling

Port

Device

Under

Test

54754A
TDR
module

86118A

TDR Customer Presentation, Oct ‘03

Page
18

Configuring a system


86100 A or B mainframe

(3.05 FW or above)



54754A TDR plug
-
in


86118A 70 GHz plug
-
in


Lower BW channels can be used, but
edgespeed and resolution will be
reduced


Cabling between the DUT and the
receive channel degrades TDR
speed


Picosecond 4020 TDR or TDT
enhancement module

TDR Customer Presentation, Oct ‘03

Page
19

Using the 4020 with TDR’s that don’t use Normalization


PSPL 4020 works with other TDRs:


Significant pulse aberrations.
Cannot be calibrated out


If pulse aberrations are not
removed, they can be
misinterpreted as close
-
in
reflections


86100 TDR calibration significantly
improves the 4020 pulse quality


Normalization also provides an
excellent way to eliminate
fixturing errors

TDR without
Normalization
4020 pulse

86100 4020
pulse

TDR Customer Presentation, Oct ‘03

Page
20

4. Frequency domain analysis is critical for
completely

understanding device performance

DUT

Incident wave

Transmitted wave

S
11

S
21

Incident wave

Transmitted wave

TDR

TDT

t

t

DUT

TDR Customer Presentation, Oct ‘03

Page
21

Benefits of S
-
parameter analysis


Some things are just easier to see in the frequency domain


Resonances


Frequency response


Device modeling can be more accurate with frequency
domain data


Some critical measurements of
differential devices

are
better understood as a function of frequency

“There is no fundamental difference in the
information content between the time domain and
the frequency domain”


Eric Bogatin


Chief Technical Officer


GigaTest Labs

TDR Customer Presentation, Oct ‘03

Page
22

Changing the way high speed digital customers
view VNA’s


Everything you ever wanted to know
about your device… and more


DUT

VNA covers all combinations of in, out,
reflections, & crosstalk. All
combinations contained in a 16
element matrix for a 4 port DUT.

TDR Customer Presentation, Oct ‘03

Page
23

For differential circuits, frequency domain
analysis helps even more


Differential are circuits
becoming more
important at high
speeds


Differential behavior can
be much different than
viewing each port
individually (there is
transmission line
coupling

designed in )


Differential circuits
reduce emissions and
are less susceptible to
radiation


Like making the cross
-
talk work for you.

Port 1

Port 2


Balanced (Differential) devices can be
analyzed as pairs (Mixed
-
Mode S
-
parameters)
rather than single ended

Differential Common



Less far field emissions
(crosstalk)



More cancellation of incoming
interference

TDR Customer Presentation, Oct ‘03

Page
24

What About Non
-
Ideal Devices?

Undesirable mode conversions cause emission or susceptibility problems


Differential
-
stimulus

to

common
-
response conversion

+


Common
-
stimulus

to

differential
-
response conversion

EMI Generation

EMI Susceptibility

+

=

=

Imperfectly matched lines mean the electromagnetic fields of the signals are not as well confined as
they should be


giving rise to generation of interference to neighboring circuits.

Imperfectly matched lines mean that interfering signals do not cancel out completely when subtraction
occurs at the receiver. Measured by stimulating common
-
mode to simulate interference.

TDR Customer Presentation, Oct ‘03

Page
25

S
-
Parameters describe differential well.

Four quadrants of differential/common mode S parameters:
S
(response,stimulus,output,input)



Example S
CD21
: Drive port 1 differentially and measure what

has been converted to common mode at port 2

TDR Customer Presentation, Oct ‘03

Page
26

Everything you ever wanted to know…..

Single ended

S Parameters

Mixed Mode

S Parameters

Mixed Mode

S Parameters

Displayed in the

Time Domain

Frequency domain s
-
parameters can be used to gain insight in frequency domain
plots and time domain views.

TDR Customer Presentation, Oct ‘03

Page
27

VNA, TDR, or both?


All of the S
-
parameter data
available using the Physical Layer
Test System (PLTS) is now
available using the 86100 TDR!


N1930A


Controls 86100 TDR


Guided setup and calibration


Automatic deskew


Conversion of TDR data to
complete S parameter results


TDR Customer Presentation, Oct ‘03

Page
28

“True” differential measurements


Some TDRs make a big deal of producing both a negative
and a positive step for doing differential TDR. “True”
differential


Agilent produces only positive pulses and then uses math
to build a differential measurement


A differential system has coupled lines. The electromagnetic fields will be
very different for two positive pulses. How can you get the right impedance
result if you don’t have the correct voltages present?

Agilent’s method provides differential, common mode, cross terms, all with a single,
accurate setup. And this method simplifies the design to allow almost perfect
matching of the two positive pulses giving the most accurate results..

TDR Customer Presentation, Oct ‘03

Page
29

There are very good reasons why we do what we do….


We use superposition techniques to combine the results of multiple
separate measurements


“I learned superposition in my first course in electronics. I believe it
for circuits with wires and resistors…but I’m not sure about
electromagnetics”

From the classic text on electromagnetics, “Fields and Waves in Communications
Electronics” by Ramo, Whinnery, and Van Duzer, (1965, John Wiley and Sons) we read “
It is
frequently possible to divide a given field problem into two or more simpler problems, the
solution of which can be combined to obtain the desired answer. The validity of this
procedure is based on the linearity of the Laplace and Poisson equations. That is






2
(

1

+

2

)

=

2

1

+

2

2






and






2
(k

1

)


= k

2

1




The utility of the superposition concept depends on finding the simpler problems with
boundary conditions which add to give the original boundary conditions
”.



Or…..

TDR Customer Presentation, Oct ‘03

Page
30

Don’t worry….it’s covered in the new Application Note!


Easy to read and digest


TDR is valid only for linear
passive devices (or active
devices configured as linear and
passive). So our technique is
completely valid


We could have built it with a +
and


pulse. Important reasons
we did what we did


Allows almost perfect
symmetry in the stimulus on
each leg


Asymmetry leads to mode
conversion and potentially
critical measurement errors


86100 channel
steps overlaid


almost
exactly the
same

Linear, passive
device to be
tested

TDR Customer Presentation, Oct ‘03

Page
31

By the way…..the VNA analysis is also based on
superposition


The VNA can only stimulate one
port at a time


The VNA uses sinewaves

(doesn’t even use pulses!)


Precision results obtained by
combining results after taking
several individual measurements


No one ever questions the
accuracy of a VNA.

“True differential is best” is a myth, and one that is keeping
you from making the most accurate measurements.

TDR Customer Presentation, Oct ‘03

Page
32

Summary



Review of TDR



Why Calibration gives superior results



Speeding the pulse up significantly for
higher resolution



Using frequency domain analysis from
TDR data to get more insight



Why ‘true differential is better’ is a myth
that may be keeping you from making the
best measurements


New literature


Application Note: High
-
precision Time

Domain Reflectometry: 5988
-
9826EN


Flyer: 86100 and the Picosecond 4020

lit # 5988
-
9825EN


Accessories Flyer: lit # 5980
-
2933EN