Data Mining S-Parameters
or
Forensic S-Parameter Analysis
copy available for download
from www.beTheSignal.com
Dr. Eric Bogatin
Bogatin Enterprises,
www.beTheSignal.com
and
Teledyne LeCroy
www.TeledyneLeCroy.com
Oct 1, 2013
1
Fastest Way to Solve a Problem is to Identify its Root Cause
If you have the wrong root
cause, you will only fix the
problem by luck
2
The Real World: Interconnects are Not Transparent
Driver Package Board Backplane Board
PackageReceiver
1.
We use SS-parameters to describe the behavior of the interconnects
2.
We can answer some “why”
why” questions from the SS-parameters
3
S-Parameters as a “black box” behavioral model of the interconnect
Don’t need to know anything about the Sparameters to use them as a “black box” model
4
Two Valuable Channel Simulations: 1) Single Bit Response (SBR)
(described by S-parameter matrix)
200 psec UI
ISI
Invert pulse shape
to get tap
coefficients for
FFE
“echos of bits past”
5
Two Valuable Channel Simulations: 2) PRBS and Eye
Diagram
5 Gbps PRBS signal as
input
5 Gbps PRBS signal
after passing through
the S-parameter file
5 Gbps PRBS signal as
eye diagram
6
System Simulation with S-Parameter Behavioral “Black Box” Models
TL1
TL3
TL5
V1
J2
1
2
CMOS,3.3V ,FA ST
J3
TOP
Port1
Port2
Port3
Port4
83.5 ohms
447.547 ps
3.000 in
TL2
Stackup
m1_1234.s4p
83.5 ohms
447.547 ps
3.000 in
Stackup
BO...
83.5 ohms
447.547 ps
3.000 in
Coupled Stackup
TL4
Port2
Port3
Port4
83.5 ohms
447.547 ps
3.000 in
Stackup
TL6
m1_1234.s 4p
83.5 ohms
447.547 ps
3.000 in
Stac kup
Differential Response
0
-10
-20
-30
-10
-20
-30
-40
0
-40
0
1
2
3
4
5
6
7
8
9
U2
Port1
Port2
Port3
Port4
1
2
m1_1234.s4p
CMOS,3.3V ,FA ST
83.5 ohms
447.547 ps
3.000 in
Stackup 0
0
Differential Response, dB
J4
Port1
Differential Response, dB
U1
10
1
2
3
4
5
6
7
freq, GHz
8
9
10
-10
-20
-30
-40
0
1
2
3
4
5
6
7
8
9
10
freq, G Hz
freq, G Hz
Turn S-parameter Behavioral Model into a
SPICE compatible model using pole-zero
model of S-parameters which any SPICE can
use:
5 Gbps @ RX
“broad band SPICE”: Simbeor, HyperLynx,
ADS, Sigrity, SiSoft D
7
Opening the Lid to the Black Box
What treats lay within?
8
S-Parameters: How Precision Reference Signals Scatter Off a DUT
Incident Wave
Time Domain t
Transmitted wave
Wave
cted
Refle
t
Frequency Domain
TDR
parameter =
S11
waveform out from a port
waveform in to a port
TDT
S21
9
Keeping Track of the Going-in Port and Coming-out Port
VERY IMPORTANT:
S-parameters are about the response of
interconnects to Signals
S31
S11
3
1
|S21| also referred to as
“Insertion Loss”
2
S21
4
S11 also referred to as” “Return Loss”
S41
S jk =
Sout in
sine wave out from port j
sine wave into port k
S22
S43
S12
10
Port Identifications: Single-ended and Differential
Diff port 2
Diff port 1
Single-ended S-parameters
Differential, mixed mode,
balanced S-parameter matrix
S11
S12
S13
S14
SDD11
SDD12
SDC11 SDC12
S21
S22
S23
S24
SDD21
SDD22
SDC21 SDC22
S31
S32
S33
S34
SCD11
SCD12
SCC11 SCC12
S41
S42
S43
S44
SCD21
SCD22
SCC21 SCC2
Each term: magnitude, phase, at each frequency value
11
S-Parameters From Many Sources: Measurement, Circuit Simulation, EM Simulation
Measurement
LeCroy SPARQ
Circuit simulation: QUCS
Electromagnetic simulation: ADS
Agilent VNA
Agilent ADS
Polar Instruments 2D field solver
Simbeor 3D Field Solver
12
Don’t Think Frequency or Time Domain,
Think Frequency AND Time Domain,
Single-ended AND Differential S-parameters- all equivalent, all look different
Frequency Domain
Single ended S-parameters
Differential S-parameters
Measurement
Circuit simulation
Electromagnetic simulation
Single ended T-parameters
Time Domain
Differential T-parameters
13
Bogatin’s 10 Rules: (PCD&F Magazine, Aug 10 , 2010
or www.beTheSignal.com, BTS218))
1.
Answer “it depends” questions by “putting in the numbers”
2.
Separate myth from reality by “putting in the numbers”
3.
Watch out for the whack-a-mole effect
4.
Most important step in solving a problem: find the root cause
5.
Apply the Youngman Principle to optimize designs
6.
Sometimes an OK answer now! is better than a good answer late
7.
Evaluate “bang for the buck” with virtual prototypes
8.
Watch out for mink holes
9.
Never perform a measurement or simulation without first anticipating
anticipating what you expect
to see
10. There are two kinds of designers: those who have signal integrity problems and those who will
14
Rule #9: Never do a measurement or simulation without first
anticipating what you expect to see: what is reasonable?
If you are wrong, there is a reason- either the set up is wrong or your intuition is
wrong. Either way, by exploring the difference, you will learn something
If you are right, you get a nice warm feeling that you understand what is going on.
Corollary to rule #9: You can never do too many consistency tests
Is your interpretation of the root cause of the
behavior “consistent”
consistent” with the observations?
Slide 15
15
Data Mining S-Parameters: 8 Important Consistency Tests
“You can observe a lot by looking”- Yogi Berra
1.
Impedance profile: diff and single-ended
2.
End to end symmetry of the interconnect
3.
Extract characteristic impedance and time delay
4.
Estimating channel losses and bandwidth
5.
Possible resonances
6.
Coupling between the two lines in a diff pair
7.
Channel to channel cross talk and its source
8.
Common to differential cross talk between two channels
16
Differential Impedance Profile
Time domain
TDD11
SDD11
Whenever spatial information is important
consider a time domain reflected response:
- SDD11, SDD31, SCD11
Impedance Profile
Reflection coefficient (mRho)
Z diff ~ 100 Ω
1 + TDD11
1 − TDD11
(First order estimates only!)
17
End to End Symmetry of the interconnect
TDD11
Compare TDD11 with TDD22
Is the slope a real impedance variation?
Are the connectors really different?
Some asymmetry
Slope up is real- very lossy traces
18
Impedance Profile Doesn’t Mean Much for
Electrically Small Structures
Impedance
Courtesy of Wild
River Technologies
• Impedance of uniform lines
does not change with rise
time
1 Ohm/div
Launch
Launch
150 psec
100 psec
75 psec
50 psec
• Displayed impedance of via
depends on rise time
• What is the characteristic
impedance of the via?
• How can we use the Sparameters to model a via?
19
Quick and Simple Way of Interpreting S-Parameters of a Via
#1: Matched to a Uniform Transmission Line
Z0_diff = 72 Ohms
Len = 39 mils
Dk = 21
Df = 0.02
20
If an S-parameter behavioral model can be more accurate than a transmission line
model, why not always use the S-parameter model?
The value of a transmission line model?
“Sometimes and OK answer NOW! is better
than a good answer late”- use worst case
parameters as a starting place
As a simpler model, it can often provide
valuable insight and get you to an acceptable
answer, faster
Can be incorporated in ALL circuit simulators
It is scalable- can be used to establish limits on
“how low an impedance”, “how long a stub
length”
“I’ve tried to make it as realistic as possible”
possible”
Transmission line model can sometimes be
“lower cost”: time, $$, expertise, resources
Important caveat about transmission line
models
If it is not accurate enough, even if it is free, it is
worthless
21
Who Killed the SDD21 Signal?: Forensic Analysis
How do you find the root cause?
Capt. Renault: “Major Strasser has been shot. [pause] Round up the
usual suspects.”, in Casablanca (1942)
The “usual suspects” who may have killed the signal
Attenuation: estimate SDD21/inch/GHz
Reflections: ripples in S21, S11
Dips: coupling to ¼ wave stub resonances (S11)
Dips: mode conversion (SCD21, SCD11)
Dips: coupling to adjacent lines (SDD31, SDD41)
Sharp dips: coupling to hi Q resonators
22
What Causes the Higher Loss?
Measured SDD21 for 2 differential pairs, up to 10
GHz
Same measurement, but up to 20 GHz
Dip at 14 GHz
Looks like bottom (blue) line has more
SDD21 than top (red) line.
What causes the higher attenuation?
What causes the large dip?
- Mode conversion?
- Stub resonance
- Resonant coupling to other
structures?
- Bloch waves and glass weave?
- Dielectric absorption resonance?
- Conductor loss from surface roughness?
- Poor copper plating
- High Df in one layer
- ???
Data courtesy of Bob Haller, Enterasys
Which S-parameters might have the answers?
23
Could it Be Mode Conversion?
SDD21
SDD11
Mode conversion terms:
SCD11, SCD21
SCD11
SCD21
SDD11 shows reflected energy
SCD11 shows mode conversion reflected
SCD21 shows mode conversion transmitted
24
Possible Root Cause of Bandwidth Limitations
Look at other lines without
vias but same material
Estimate via stub length- is it
reasonable?
Suggests, via stub sets a
fundamental limit on
maximum channel data rate
Possible fixes:
Backdrill
Micro vias
Restrict layer transitions
Via stub termination
Influence of via stub resonances
extends to much lower freq
Microstrip- no via
Stripline- residual
stub ~ 75 mils
long, with NFPs
fres =
1.5
1.5
=
= 20 GHz
Len 0.075
why is frequency so low?
fres = 14 GHz
25
A Puzzle: 20 inch Microstrip
S21
S41
S21
What could cause this dip?
How could you test the root cause?
Is this a problem?
26
Differential and Single-ended Response
(has nothing to do with Mode Conversion)
Single-ended response
Differential response
SDD21, 20 inches long
S21, 20 inches long
S21, 80 inches long
SDD21, 80 inches long
Just evaluating single-ended response is NOT an indication of differential response
27
Channel to Channel Cross Talk
SDD21
SDD11
SDD31
SDD51
1
3
5
2
4
6
SDD41
Where is the
SDD31 cross talk
coming from?
SDD61
How could we tell?
28
Two Backplane Examples
Where is the Coupling Coming From?
TDD11
TDD11
1
3
5
2
4
6
TDD51
TDD51
TDD31
TDD31
Backplane 1
Backplane 2
29
Take advantage of all the information!
Single ended S-parameters
Frequency Domain
Differential S-parameters
Measurement
Circuit simulation
Electromagnetic simulation
Single ended T-parameters
Time Domain
Differential T-parameters
30
31
For More Information
www.beTheSignal.com
SI Library
Webinars, feature articles, presentations, hands
on labs
Class schedules
Blog: www.beTheSignal.com/blog
Published by Prentice Hall, 2009
32