Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -1
www.beTheSignal.com
Data Mining S-Parameters
How to Read S-Parameters
Like a Bookor
Tapping Into Some Of The Information Buried Inside S-
Parameter Black Box Models
Dr. Eric Bogatin, Signal Integrity
Evangelist, Bogatin Enterprises a LeCroy
Company
www.beTheSignal.com
Fall 2011
Bogatin Enterprises and
LeCroy Corp
No Myths Allowed Webinar
Time before start:
Downloaded handouts from www.beTheSignal.com, recent publications: PPT-NMA-855
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -2
www.beTheSignal.com
Data Mining S-Parameters
Overview
• S-parameters as a “black box” behavioral model
• Root cause analysis: a powerful methodology
• Four common patterns and their possible root cause
• A Pop Quiz
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -3
www.beTheSignal.com
Data Mining S-Parameters
For More Information
www.beTheSignal.com
Recent Publications
My Blog: What I learned this month
Class schedule
Published by Prentice Hall, 2009
Boston: Sept 28-29
Hyderabad: Oct 10-11
Bangalore: Oct 13-14
Singapore: Oct 18
Penang: Oct 20
Shah Alam: Oct 21
San Jose: Nov 8-10
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -4
www.beTheSignal.com
Data Mining S-Parameters
Hands on Labs in All Bogatin
Enterprises Classes(simulation software and lab exercises provided)
• Quite Universal Circuit Simulator (QUCS)
Circuit Simulator: transient, frequency domain, S-parameter
• LeCroy’s SI Studio
S-parameter viewer
Simulate eye diagrams from channel S-parameters
Serial data analysis
S21
S11
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -5
www.beTheSignal.com
Data Mining S-Parameters
A Special Thank you for all
Webinar Attendees
• 10% discount to any Boston or San Jose, Bogatin Enterprises class:
SPSI, Sept 28, 2011, Boston
DPD, Sept 29, 2011, Boston
EPSI, Nov 8-9, 2011, San Jose
SPSI, Nov 10, 2011, San Jose
• Use discount code NMA1109A when registering online at www.beTheSignal.com
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -6
www.beTheSignal.com
Data Mining S-Parameters
The Real World:
Interconnects are Not Transparent
Driver BoardPackage Backplane Board Package Receiver
TX RX
1. We use S-parameters to describe the behavior of the interconnects
2. We can answer some “why” questions from the S-parameters
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -7
www.beTheSignal.com
Data Mining S-Parameters
Characterize an Interconnect by How “Precision” Reference Signals Scatter Off a DUT
S11 S21
Incident Wave
Transmitted wave
TDR TDT
tTime Domain
tFrequency Domain
waveform out from a port
waveform in to a portparameter =
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -8
www.beTheSignal.com
Data Mining S-Parameters
What are S-Parameters?
• A collection of scattered responses (at every frequency):
Reflected sine waves
Transmitted sine waves
• Stored in a special format: Touchstone file
.s1p: scattered data from 1 port
.s2p: scattered data from 2 ports
E
1 port into wavesine
1 port from out wavesineS11 =
1 port into wavesine
2 port from out wavesineS21 =
Freq units are MHz
S-parameters
dB and degrees
Reference Z 50 ohm
Magnitude
and Phase or
Real, and
Imaginary
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -9
www.beTheSignal.com
Data Mining S-Parameters
System Simulation with S-Parameter
Behavioral “Black Box” Models
1 2 3 4 5 6 7 8 90 10
-30
-20
-10
-40
0
freq, GHz
Diffe
ren
tial R
esp
on
se
5 Gbps @ RX
U1
CMOS,3.3V,FAST
2
1
V1
TOP
BO...
TL1
83.5 ohms447.547 ps3.000 inStackupTL2
83.5 ohms447.547 ps3.000 inStackup
J2
m1_1234.s4p
Port1 Port2
Port3 Port4
J3
m1_1234.s4p
Port1 Port2
Port3 Port4
U2
CMOS,3.3V,FAST
2
1
TL3
83.5 ohms447.547 ps3.000 inCoupled StackupTL4
83.5 ohms447.547 ps3.000 inStackup
TL5
83.5 ohms447.547 ps3.000 inStackupTL6
83.5 ohms447.547 ps3.000 inStackup
J4
m1_1234.s4p
Port1 Port2
Port3 Port4
1 2 3 4 5 6 7 8 90 10
-30
-20
-10
-40
0
freq, GHz
Diffe
ren
tia
l R
esp
on
se
, d
B
1 2 3 4 5 6 7 8 90 10
-30
-20
-10
-40
0
freq, GHz
Diffe
ren
tia
l R
esp
on
se
, d
B
Turn S-parameter Behavioral Model into a SPICE
compatible model using pole-zero model of S-
parameters which any SPICE can use:
“broad band SPICE”: Simbeor, HyperLynx, ADS,
Sigrity, SiSoft E
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -10
www.beTheSignal.com
Data Mining S-Parameters
Opening the Lid to the Black Box
What treats lay within?
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -11
www.beTheSignal.com
Data Mining S-Parameters
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
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -12
www.beTheSignal.com
Data Mining S-Parameters
Don’t Think Frequency or Time Domain,
Think Frequency AND Time Domain
Single ended S-parameters Differential S-parameters
Differential T-parametersSingle ended T-parameters
Frequency Domain
Time Domain
Circuit simulation
Electromagnetic simulation
Measurement
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -13
www.beTheSignal.com
Data Mining S-Parameters
2 Port S-Parameters
• S11: Return loss
It is the reflected signal
Impedance mismatch from 50 ohms throughout the interconnect
A little about losses
• S21: Insertion loss
It is the transmitted signal
Impedance mismatches throughout the interconnect
Losses
~50Ω Z0 = 50Ω
Vsource
Magnitudeand phaseDetector
DUT
magnitude/phase
detector50Ω
Z0= 50Ω
Transparent interconnect:
S11: large, negative dB
S21: small, negative dB
Applies to single-ended and differential S-parameters
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -14
www.beTheSignal.com
Data Mining S-Parameters
Four Important Patterns in S11, S21
• Ripples in S11, sometimes in S21: reflections
• Monotonic drop in S21: losses
• Broad dips: ¼ wave stub resonances
• Sharp dips (hi Q), coupling to resonances
Insertion loss
return lossInsertion loss
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -15
www.beTheSignal.com
Data Mining S-Parameters
How Reflections Result in Return,
Insertion Loss Ripples
• When Len << ¼ λ
Reflections from front and back, 180 deg out of phase
No net reflection, all transmitted waves in phase and add
S11 large negative dB, S21 nearly 0 db
Z0 < 50 Ohms
When Len << ¼ λ
min S11 max S21
½
0
00
0 0
50 Ω 50 Ω S21
S11
At low frequency
ALL interconnects
are transparent
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -16
www.beTheSignal.com
Data Mining S-Parameters
When Len = ¼ λλλλ, S21 Lowest, S11
Highest
• When Len = (½ x n + ¼ ) λ
Max S11, min S21
Z0 < 50 Ohms
min S21
When Len << ¼ λ
When Len = 1/4 λ
½ ¼ ¼ + 0
½ max S11
½ + 0 ¾
min S11 max S21
½
0
00
0 0
S21
S11
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -17
www.beTheSignal.com
Data Mining S-Parameters
When Z0 ≠ 50 Ohms, Multiple Reflections
From The Terminations Cause Ripples
• When Len = n x ½ λ Reflected waves from front and back subtract
Minimum reflected signal
Transmitted waves all in phase, S21 max
• As frequency increases, insertion, return loss increase, decrease
Longer distance between reflections, shorter the frequency between high and low
Larger the impedance difference, the larger the modulation
Z0 < 50 Ohms
min S21
When Len << ¼ λ
When Len = 1/4 λ
½ ¼
¼ + 0 ½
max S11
½ + 0 ¾
min S11 max S21
½
0
00
0 0
When Len = ½ λ
min S11 max S21
½
1
½ ½ + 0
1 + 0 1½
S21
S11
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -18
www.beTheSignal.com
Data Mining S-Parameters
Attenuation and Insertion Loss
In real interconnects,
amplitude drops off
exponentially with distance
( )dB/len
nepers/lendd 20
out in inV d V e V 10α
−−α= =
d
S21 is attenuation when terminations are matched and there is no
coupling out of the transmission line
out
in
VS21
V=
[ ] [ ]out
in
VS21 in dB 20 x log dB / in x d attenuation
V
= = −α =
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -19
www.beTheSignal.com
Data Mining S-Parameters
Estimating Attenuation per Length
in 50 Ohm Interconnects
dB/inch
w = line width in mils
f in GHz
Dk = dielectric constant
Df = dissipation factor
Len
1atten f 2.3 x f x Df x Dk
w
= − +
w = 5 mils
Dk = 4
Df = 0.02Measured attenuation
from two different
transmission lines
Simple estimate
matches measured
behavior ok
Never sign off on a design
based on a rough estimate
( )Len GHzatten ~ 2.3 x Df x Dk ~ 0.1dB / inch / GHz− −
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -20
www.beTheSignal.com
Data Mining S-Parameters
The ¼ Wave Stub Resonance
TD, Lenstub 2 x TD
When 2 x TD = ½ cycle, minimum received signal
TD = ¼ cycle: the quarter wave resonance
res
1 1TD
4 f=
innsec
LenTD
6=
1.5
Len=res
1 1f
4 TD=
f in GHz Len in inches
Example: Len = 0.5 inch, fres = 3 GHz
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -21
www.beTheSignal.com
Data Mining S-Parameters
¼ Wave Resonance in the
Frequency Domain
Lenstub
1.5
Len=
res
1 1f
4 TD=
Stub 0.5 inches long
fres = 3 GHz
If Nyquist = 3 GHz,
no eye at BR = 6 Gbps
Stub length = 0.5 inches
S21 S11
Where does the energy go?
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -22
www.beTheSignal.com
Data Mining S-Parameters
Measured Insertion Loss with and without
Via Stub in a ¼ inch Thick Backplane
No via stub in the channel
0.2 inch via stub
res
1.5 1.5f 7.5 GHz
Len 0.20= = =
7.5 GHz
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -23
www.beTheSignal.com
Data Mining S-Parameters
Coupling to Hi-Q Resonances
• What are hi-Q resonators?
Any floating metal (copper fill)
Plane cavities
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -24
www.beTheSignal.com
Data Mining S-Parameters
Via to Cavity Coupling in a 4 Layer Board
Plane resonances expected:
Len = 3.25 in fres = 0.92 GHz
Len = 1.187 in fres = 2.5 GHz
Len = 0.8 in fres = 3.75 GHz
Len
GHz3
Lenx2Dk
GHz12fres ==
S21 wo return vias
3.25 inches
1.187 inches
0.8 inches
0.8 inches
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -25
www.beTheSignal.com
Data Mining S-Parameters
Four Important Patterns in S11, S21
• Ripples in S11, sometimes in S21: reflections
• Monotonic drop in S21: losses
• Broad dips: ¼ wave stub resonances
• Sharp dips (hi Q), coupling to resonances
Insertion loss
return lossInsertion loss
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -26
www.beTheSignal.com
Data Mining S-Parameters
Pop Quiz:
What Causes the Higher Loss?
Looks like bottom (blue) line has more SDD21 than top (red) line.
What causes the higher attenuation?
-Conductor loss from surface roughness?
- Poor copper plating
- High Df in one layer
- ???
Same measurement, but up to 20 GHzMeasured SDD21 for 2 differential pairs, up to 10 GHz
What causes the large dip?
- Stub resonance?
- Mode conversion?
- Resonant coupling to other structures?
- Bloch waves and glass weave?
- Dielectric absorption resonance?
Dip at 14 GHz
Data courtesy of Bob Haller, Enterasys
Which S-parameters might have the answers?
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -27
www.beTheSignal.com
Data Mining S-Parameters
Could it Be Mode Conversion?
SDD21 SDD11
SCD11
SCD21
Mode conversion terms:
SCD11, SCD21
SDD11 shows reflected energy
SCD11 shows mode conversion reflected
SCD21 shows mode conversion transmitted
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -28
www.beTheSignal.com
Data Mining S-Parameters
Summary
• Get in the habit of looking inside S-
parameters
“You can observe a lot by watching”, Yogi
Berra
• Look for the four patterns
Ripples
Monotonic drop
Broad dips
Sharp dips
• This has been just a “tip of the
iceberg” look at data mining S-
parameters
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -29
www.beTheSignal.com
Data Mining S-Parameters
Bogatin Enterprises, LLC, a LeCroy Company 2011
Slide -30
www.beTheSignal.com
Data Mining S-Parameters
For More Information
www.beTheSignal.com
Recent Publications
My Blog: What I learned this month
Class schedule
Published by Prentice Hall, 2009
Boston: Sept 28-29
Hyderabad: Oct 10-11
Bangalore: Oct 13-14
Singapore: Oct 18
Penang: Oct 20
Shah Alam: Oct 21
San Jose: Nov 8-10
• Classes with Hands on Labs:
Quite Universal Circuit Simulator (QUCS)
LeCroy’s SI Studio
S21
S11