A new modeling method of coupled transmission lines is shown. It is based on a simple one-port TDR characterization of an actual sample of the TL. A BTM (Behavioral Time Domain )S.parameter model is then obtained from this simple measurement. These models are supported by the SWAN/DWS simulation environment: https://www.ischematics.com/
1. MULTIGIGABIT MODELS FOR
COUPLED LINES by Piero Belforte
dec. 2009
Method for quick and accurate
multigigabit model extraction of
coupled line pairs using TDR measures
and DWS simulations
20/12/2009 Copyright Piero Belforte 1
2. TARGETS:
• Mininum experimental setup effort
• Fast measurements using TDR only
• Minumum number of required measurements
• Maximum efficiency of extracted models
• No numerical stability problems
• Possibly Time-Domain only
20/12/2009 Copyright Piero Belforte 2
3. CHOSEN SOLUTIONS
ONE-PORT only TDR measurements
OTHER PORTS LEFT OPEN
MODEL BEHAVIORAL DESCRIPTION
PWL S-PARAMETERS DESCRIPTION
MAXIMUM 3-POINT PWL
DWS for MODEL SIMULATION
2D SOLVER to get STARTING LOSSLESS MODEL
MODEL PARAMETERS FOUND AS OPTIMUM
MATCH WITH MEASURES OF the WHOLE SETUP
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4. TEST VEHICULE
• COUPLED MICROSTRIP PAIR
• FR4 SUBSTRATE H=1.4mm
• Length=136mm W=140um S=394um (Center)
• ACCESS THROUGH VIA HOLES and 60um PADS
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5. VIA HOLES and PADS
• 60 mils (1520 um)square pad
• 50 mils (1270 um) via diameter
• Dielectric (FR4) height H=1400um
• Very large compared to coupled pair
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6. Full Ideal Setup
• No Vias, No TDR parasitics, Ideal lossless lines
• Mode data from 2D Field Solver
• COMMON MODE: TD=891.4 ps
• DIFFERENTIAL MODE: TD=710.9 ps
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7. Ideal (Lossless model) from 2D Field
Solver (DWS description)
Modal
Adaptors Lossless Modal
Transmission
Lines
Voltage
EigenVectors
Matrix (2X2)
This model is inserted as first order approximation in the DWS TDR setup
description .
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8. PSEUDO-DIFFERENTIAL SETUP
• TDR connected at port #1
• Remaining ports left open
1
3
4
2
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9. COMMON MODE SETUP
Ports #1 and #2 connected together
Ports #3 and #4 left open
TDR at ports #1 and #2
1
3
2
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10. Full Ideal Setup:Simulation Results
Common
mode
setup
Pseudo-
differential
setup
2xDeltaTD=360ps
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12. Ideal Common mode vs Measure
(Window 6ns)
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13. Common mode TDR
Lossless
Model
Actual
measure
The differences between the actual measure and the lossless model
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are outlined in yellow
15. Adding Vias and parasitics
• Models of Vias and setup parasitics (Launch
cable tip and common mode connection) are
extracted by means of TDR measures.
• The extracted models are the inserted in the
setup simulation model to evaluate their
effect on overall DUT response
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16. TDR cable tip model
• The effects of TDR semi-rigid launch cable are not negligible and have to be taken
into account
• The unshielded tip of the cable has a major discontinuity contribution on TDR
response
• An accurate model can be derived from an actual TDR measure of a grounded tip
Semi-rigid coax.
1.8mm Outer
Diameter.
Unshielded
PTFE dielectric
Center conductor
(diameter= .3mm)
Via and pad
Microstrip end
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17. Model of TDR Cable tip
• A LCR DWS model of the tip is extracted from the measure of the grounded tip
• The model parameters are optimized to get the best match between measure
and simulation
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18. Pad and via model
• From the TDR response of an open via a
circuital model is extracted by optimization
of the model response
The TDR launch cable tip is also taken into
account
20/12/2009 Copyright Piero Belforte 18
20. Via and pad DWS optimized model
description
• The model is described as DWS subcircuit
(stub TL with radiation losses and RC lumped
load to gnd)
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21. Adding Vias and parasitics
• The previous models of Vias and setup
parasitics (Launch cable tip and common
mode connection) are added to the whole
simulation model including the lossless modal
description of the coupled pair.
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22. Pseudo Differential TDR: results
3
4
2
Model
Simulation
Actual TDR
1 Measure
Several differences can be still pointed out between this model
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and the actual measure
23. Adding losses and adjusting DUT
model parameters
Losses are added to the DWS simulation model of the DUT. A simplified
Behavioral model of both even and odd modes is used instead of lossless
trasmission lines.
A three point PWL description of S21 is used to model losses. The S11 is
assumed ideal. The coordinates of S21 PWL description are optimized to match
the measurement results . The mode impedances and propagation delay times
are also optimized to match the actual measure. The optimization procedure is
applied to both measurement setups (pseudo- differential and common
configurations with all other ports left open).
In this way a true de-embedding of coupled line parameters is performed
because all other setup effects are taken into account in the DWS netlist.
To check the results a near-end crosstalk simulation is performed and compared
to the actual measure.
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24. Coupled pair lossless DWS model using
Bimodal adaptors
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