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
20/12/2009 Copyright Piero Belforte 3

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
20/12/2009 Copyright Piero Belforte 4

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
20/12/2009 Copyright Piero Belforte 5

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
20/12/2009 Copyright Piero Belforte 6

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 .
20/12/2009 Copyright Piero Belforte 7

8. PSEUDO-DIFFERENTIAL SETUP
• TDR connected at port #1
• Remaining ports left open
1
3
4
2
20/12/2009 Copyright Piero Belforte 8

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
20/12/2009 Copyright Piero Belforte 9

10. Full Ideal Setup:Simulation Results
Common
mode
setup
Pseudo-
differential
setup
2xDeltaTD=360ps
20/12/2009 Copyright Piero Belforte 10

11. Ideal Pseudo-diff vs Measure
20/12/2009 Copyright Piero Belforte 11

12. Ideal Common mode vs Measure
(Window 6ns)
20/12/2009 Copyright Piero Belforte 12

13. Common mode TDR
Lossless
Model
Actual
measure
The differences between the actual measure and the lossless model
20/12/2009 Copyright Piero Belforte 13
are outlined in yellow

14. Ideal Near-end Xtalk vs Measure
(Window 5ns)
20/12/2009 Copyright Piero Belforte 14

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
20/12/2009 Copyright Piero Belforte 15

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
20/12/2009 Copyright Piero Belforte 16

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
20/12/2009 Copyright Piero Belforte 17

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

19. Optimized model response vs actual
measure
20/12/2009 Copyright Piero Belforte 19

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)
20/12/2009 Copyright Piero Belforte 20

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.
20/12/2009 Copyright Piero Belforte 21

22. Pseudo Differential TDR: results
3
4
2
Model
Simulation
Actual TDR
1 Measure
Several differences can be still pointed out between this model
20/12/2009 Copyright Piero Belforte 22
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.
20/12/2009 Copyright Piero Belforte 23

24. Coupled pair lossless DWS model using
Bimodal adaptors
20/12/2009 Copyright Piero Belforte 24

25. Adding setup effects and losses(1)
20/12/2009 Copyright Piero Belforte 25

26. Adding setup effects and losses (2)
20/12/2009 Copyright Piero Belforte 26

27. Coupled pair lossy DWS model:
optimized results
20/12/2009 Copyright Piero Belforte 27

28. Checking the near-end crosstalk: setup
20/12/2009 Copyright Piero Belforte 28

29. Checking the Near-end
crosstalk:model vs measure
20/12/2009 Copyright Piero Belforte 29

30. TraceAnalyzer: input configuration
20/12/2009 Copyright Piero Belforte 30

31. TraceAnalyzer: results
20/12/2009 Copyright Piero Belforte 31

32. Results comparison
Z0e (ohms) Z0o (ohms) TDe (ps) Tdo(ps)
Trace Analyzer
From
crossection 217 79.87 755 688
data
DWS meas.
based 190 64 725 675
optimization
20/12/2009 Copyright Piero Belforte 32

33. De-embedding setup and via effects
21/12/2009 Copyright Piero Belforte 33

34. De-embedded model: effect of losses
on reflected edge rise time
21/12/2009 Copyright Piero Belforte 34

35. TLineSim results for a single line: effect
of losses on the reflected edge
21/12/2009 Copyright Piero Belforte 35