Copyright © 2014 Wild River Technology LLCSlide 1
Wild River Technology LLCwww.wildrivertech.com
Alfred P. [email protected]
phone 503 679 2429
A VNA Manifesto: A Primer for Practical Mastery
Day 4: Application Topics of S-Parameters
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Day 4
De-embedding with T-matrix approachTRL calibrationPassivity and Causality – practical testsS-parameter work flowRational Compact ModelingTo fix or not to fix a bungled S-parameter
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De-Embedding Approaches
T-matrixCreate S-parameter from scaled T-linePort Extension
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© H. Heck 2008
I1
+
-
V1
I2
V2
I1
I3
+
-
V3
T-matrix, Chaining ABCD matrix
2DCBA
1DCBA
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VNA Approach to De-embedding
Measure a de-embedding structure and get s-parameterModel something and get S-parameterUse VNA firmware to de-embed with file from active measurement using T-matrix de-embeddingMatlab is another option for T-matrix approach
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TRL Calibration, On-Board
Lines 1,2,3THRU
Open
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TRL Calibration is finicky Launch must be decent (low S11, no resonance) – good launch designLaunch Connector Repeatability from SMA to SMA – TDR must be very goodLine lengths accurate – layout, etchImpedance variation across board low – etch, fiber- weave, etc., can wreak havocWe measure the LINE standards group delay, then use those group delays in Cal Kit, also verify impedance
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2 4 6 8 10 12 14 16 180 20
-50
-40
-30
-20
-10
-60
0
freq, GHz
dB(S
(1,1
))dB
(S(2
,1))
THRU - SOLT calibration
2 4 6 8 10 12 14 16 180 20
-60
-40
-20
0
-80
20
freq, GHz
dB(S
(1,1
))dB
(S(2
,1))
TRL calibrated THRU
Establish a Concerted Calibration Verification
Ideal TRL THRU S11=S22=0, S12=S21=1
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2 4 6 8 10 12 14 16 180 20
-1.0
-0.5
0.0
0.5
1.0
1.5
-1.5
2.0
freq, GHz
grou
p_de
lay_p
icose
cond
sav
ggro
upde
lay
Group Delay THRU in psec
Verification of Group Delay of THRU includes noise and moving average, perfect group
delay=0psec
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TRL- tip
Without delving into all the details, I don’t use TRL calibration for general signal integrity work.
There are better calibrations to get reference plane near DUT, especially for multi-port 4,6,.. 12 port structures
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Advanced Cal Verification
• Verification includes– THRU or Insertion Response using insertable
adapter– return loss using wideband terminators– symmetry, S11=S22– Reciprocity, S21=S12– Group Delay
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Again, use validation structure features to your advantage
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Calibration Verification 2-Port using KF-KF Adapter
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Start with Simple Cal Verification: Simple adapter THRU for non-insertable and flush THRU with insertable Cal. KF-KF adapter has approximately 0.1dB insertion loss and 50psec delay.
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Verify Low-Frequency Calibration. Passivity issues!
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Another example using on-board
THRU
Example of Simple THRUfor TRL:
Simple check of obvious Passivity Violations. The check of |S21|>1 is NOT
sufficient however!
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Analysis of Calibration Causality Using Polar Plot,
Causal S-parameters should only rotate
clockwise
Quick Polar view of Insertion locates non-causal behavior
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Cal verification using precision airlinesFirst, examine transmission aberrations, and return loss
I may check this S21 with no calibration
enabled
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Secondly, use Resonant structure like Beatty Standard.Why?
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Simple Matlab routine to analyze for Beatty - Symmetry issues
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For 4 Ports: After Cal Verification, Assess S-parameter of DUT
• Overall return loss• Symmetry• Group Delay Distortion• Reciprocity• Insertion Loss variation• Quick Time Domain Transform
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Rational Compact Modeling of S-parameters
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RCM
Be definition it fixes passivity and causalityIs very accurate, low residual MSE errorAddress DC operating pointNo interpolation issues, continuous functionsWorks in Spice engines directlyRequired intermediate step for Time Domain Simulation
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1st Import S-parameters into Simbeor and evaluate quality, then RCM structure
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Look Closely to S-parameter measured versus RCM simulation, CLOSE FIT!
Stripline Resonator, CMP-28
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S-parameter Work Flow
Validate calibration based on DUT Make measurement of external NIST standard, such as stepped impedance or 50ohm airlineTry to predict what DUT measurement will look like based on length, resonances, pathologies, etc.,Make measurement on on board standard, such as Channel Modeling PlatformAnalyze measurementImport measurement into SI Tool, obtain quality metrics – quality, passivity, causalityRCM model
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What to do with problem S-parameter
Don’t fix S-parameters, the tools on the market don’t work very well.If a tool reports problems, check calibrationSometimes you have to live with and manage issues when using T-matrix, or partial calibration, or fancy de-embedding. See DesignCon2012 tutorial: High-confidence S-parameter Measurement Methodologies for 15-28 Gbps, it is available WRT website.
Copyright © 2014 Wild River Technology LLCSlide 28
Thank You
Questions, discussion?
Alfred P. [email protected] 679 2429