### Differential Pair

```ELCT 361 - EM
Overview of EM Simulation Project (Differential Pair)
Purpose:
Learn how Maxwell’s equations solve the EM fields associated with the operation of an electronic
structure.
Learn Computational EM techniques using EM simulation software tools.
Tools:
Ansys/Ansoft HFSS (High Frequency Structure Simulator - version 14).
Ansys/Ansoft Designer (Circuit Simulator)
Operation:
Divides up a three-dimensional electronic structure into thousands of tiny cells.
Then solves the Electric and Magnetic Fields within each cell as they interact with all other cells.
Files Provided:
1. Instructions.
2. Report template (in ppt format).
3. HFSS and Designer templates.
Objectives:
1. Draw Structure.
PCB (Printed Circuit Board) Stack-up (Ground, Dielectric, Trace, Air Box).
Transmission Line (Differential Pair, Ports for excitation).
2. Set up Analysis.
Frequency Domain (Terminal Solution Type)
Time Domain (Transient Solution Type)
3. Results from Simulation.
Channel Losses (from Frequency Domain Analysis).
Signal Attenuation and Delay (from Time Domain Analysis).
Eye Diagram (Defines allowable voltage and time variation of a received signal).
HFSS Windows
Tool Bar
Project Mgr
Model
Boundaries
Report
Air Box
Excitations
Graphs
Ground
Analysis
Optimetrics
Traces
Results
Markers
Dielectric
Properties
Ports
Independent
Variables
Dependent
Variables
Model
Tree
Message
Progress
1.Structure Overview. Differential Pair
T3
Air Box
3-D View
Port 2
Differential Pair
Dielectric
Port 1
T1
T2
Front View- Stack up
Port
Dielectric Layer
Ground
T4
Copper Traces
1.Draw Structure. Detail of Ground, Dielectric, Ports
z
-(wPort/2),LT,hG
y
wPort=10*wT+xD
Board
-(wPort/2),0,hG
-(xD/2+Board),0,0
xD
Origin
Location of coordinates:
Ground
Dielectric
Ports
hPort=8*hD
x
LT
1.Draw Structure. Detail of Differential Pair
(The “Trapezoidal” shape is typical of manufactured traces.)
Location of coordinates to form
“Trapezoidal” shape.
-(gT/2+wT-xT),0,(hG+hD+hT)
-(gT/2+xT),0,(hG+hD+hT)
xT
Trace 1
Copper
gT
Dielectric
FR4
z
x
Origin
hT
wT
-(gT/2+wT),0,(hG+hD)
-(gT/2),0,(hG+hD)
Trace 2
Ground
Copper
hD
hG
2.Analysis Set-ups (must do).
Windows Explorer
HFSS Options
2. Then add these 2 files
and open
1. Create this folder under
“Temp” in the C: drive
3. Set these options under:
>Tools
<Tools
>Options
>HFSS Options
Design Properties (Variables)
Excitations
Frequency Sweep
Differential Pairs
Solution Setup
Report
Parametric Sweep
3.Results. Frequency Plots.
Differential
0.00
X
m1 20.0000
1cm
5cm
m2 20.0000
10cm
m3 Curve
20.0000
Info
20cm
m4dB(St(Diff1,Diff2))
20.0000
Y1
dB
-6.00
10GHz : Sw eep
dB(St(Diff2,Diff1))
Imported
-12.00
0.00
5.00
Name
m2
m3
-0.1738
-0.7252
-1.4576
-2.9208
X
ANSOFT
Y
m1 0.0761 998.1549
m2 0.6867 760.8279
800.00
m4
Curve Info
Input(Trace_T1)
Setup1 : Transient
Output(Trace_T2,Trace_T1)
Setup1 : Transient
m2
600.00
400.00
Name
Delta(X)
Delta(Y)
Slope(Y)
InvSlope(...
d( m1,m2)
0.6106
-237.3270
-388.6803
-0.0026
200.00
10.00
Freq
[GHz]
Freq (GHz)
15.00
X
1cm
m1 20.0000
5cm
m2 20.0000
10cm
m3Curve20.0000
Info
20cm
m4dB(St(T1,T3))
20.0000
01_1cm
Y1
dB
-4.00
-6.00
10GHz : Sw eep
Y
-0.2380
-1.3329
-3.4996
-12.1328
0.00
1cm
10cm
0.25
Time [ns]
Time
(us)
0.50
0.75
1.00
ANSOFT
m1
m2
m3
V
Name
20.00
0.00
5cm
-2.00
Attenuation, Delay
S21-Insertion Loss
Single Ended
0.00
-10.00
10cm
20cm
P1 P2 active
1000.00
dB(St(Diff1,Diff2))_1
Imported
dB(St(Diff1,Diff2))_2
Imported
-8.00
Y
XY Plot 1
m1
ANSOFT
m1
Y1 [mV]
mV
Name
-4.00
-10.00
1cm
5cm
-2.00
-8.00
01_1cm
3.Results. Transient Plots.
20cm
dB(St(T1,T3))_1
Imported
dB(St(T1,T3))_2
Imported
dB(St(T1,T3))_3
Imported
-12.00
0.00
5.00
Eye Diagram
S21-Insertion Loss
10.00
Freq
Freq (GHz)
[GHz]
15.00
20.00
Time (us)
1.25
EYE DIAGRAM. Time Domain.
Eye Diagram:
Superimpose consequetive bits in a data stream
1 0 0 1 1 0 1
Statistical Analysis
EYE DIAGRAM. Time Domain.
Eye Diagram:
Superimpose consequetive bits in a data stream
Statistical Analysis
Bit
1
0
0
1
1
0
1
S-Parameters. Frequency Domain.
Po
S11 (Return Loss)
S21 (Insertion Loss)
Pi
2-Port
Network
Port 2
2-Port
Network
Port 2
Port 1
Po
Port 1
Pi
Po
Pi
Prefer to have least amount of power
reflected (eg: Po/Pi = 1/10)
example
 Po 
10log10  dB
 Pi 
 1 
10log10 
dB
10


10log10 0.1dB
10-1dB
-10dB
0
S11
Prefer large
negative dB
dB
-10
Prefer to have more power pass
through to load (eg: Po/Pi = 5/10)
example
 Po 
10log10  dB
 Pi 
 5 
10log10 
dB
10


10log10 0.5dB
10- 0.3dB
freq
- 3dB
0
S21
dB
Prefer near to
zero dB
-10
freq
37
Frequency Plot. 1cm
S-Parameters
XY Plot 2
m3
m1
0.00
Name
S21
-20.00
S11
ANSOFT
Y
m1 7.8500 -0.4088
m2 7.8500 -38.8654
m3 16.2000 -0.9989
m4 16.2000 -45.5763
dB
Y1
-10.00
X
P1 P2 active
-30.00
m2
-40.00
Curve Info
dB(St(Trace_T1,Trace_T1))
Setup1 : Spectral
m4
dB(St(Trace_T1,Trace_T2))
Setup1 : Spectral
-50.00
0.00
2.50
5.00
7.50
10.00
Freq [GHz]
12.50
Frequency
15.00
17.50
20.00
Designer Circuit
Circuit Components
HFSS
Model
Probes
Report
Elct 361. EM Simulation Project Report.
Fall 2013.
Name:
Date:
Specification:
A. Ansoft HFSS (Structure Simulator).
1. Draw structure in Ansoft HFSS (Transient).
2. Set up Attributes, Excitation , and Boundary Conditions.
3. Simulate the design.
4. Results:
a. Time Domain (Transient).
b. Frequency Domain (Spectral).
B. Ansoft Designer (Circuit Simulator).
1. Set up Signal Generator Circuit in Ansoft Designer.
2. Import HFSS Model.
3. Simulate the Circuit (Vary “Data Rate” to get EYE opening).
4. Results.
a. EYE Diagram.
HFSS Operation
1. HFSS Solution Process.
(C:\Program Files (x86)\Ansoft\HFSS14.0\Win64\Help\ hfss_onlinehelp
Ch 18 p 2-5.)
FEM. (eg: HFSS uses FEM (Finite Element Method) to mesh the Computational Domain. FEM divides this space into
thousands of smaller elements or cells called tetrahedrals (three-dimensional triangles).)
Matrix Equations.
Basis Functions.
HFSS Solution Process.
2. Solution of Maxwell’s equations.
(C:\Program Files (x86)\Ansoft\HFSS14.0\Win64\Help\ hfss_onlinehelp Ch 18 p 36-39.)
“Maxwell’s Equations” Ch 7. p. 205-206.


  

Maxwell’s Equations.
  H  J   D t
  E   B t
DGTD Solver. (eg: each mesh element advances in time using its own time step in a synchronous manner. This
results in a significant speed-up.)
Parallelism.
HFSS Model
Air Box
3D View
Trace
Dielectric
Port
Ground
Identify figure (eg: This is a figure of a Microstrip line drawn in “HFSS” for analyzing the transient and spectral
characteristics.)
Describe components.
(eg: The Air Box confines the computational domain to within ¼ wavelength from conducting surfaces in order to
model the radiation of EM waves in space while also reducing the simulation time.)
Transient Plot
Voltage In (5cm)
Voltage Out (5cm)
Identify plot (eg: This is a plot of a broadband pulse in the time domain over the interval …ps)
Compare time delay and attenuation due to the trace length (from delta markers on plot).
Compare results for time delay from plot with empirical formulation (from formula below).
e 
r 1 r 1
2

2
1
,
12Dz
1
Tx
vp 
c (speedlight)
e
,
timedelay 
lengthof line
s 
vp
Frequency Plot (1) Differential Pair
Differential Pair S21 5cm
Differential Pair S21 10cm
Differential Pair S21 20cm
Identify plot (eg: This is a plot of the insertion losses in the frequency domain for a Differential Pair of
Traces over the interval …GHz)
Compare insertion losses (S21) due to the trace lengths and frequency (from maximum markers on plot).
[The interval between resonant troughs for the S11 graph (not shown. Change step size to 0.1GHz and rerun.
Takes 5x longer.) can be approximated by the formulation given below.]
e 
r 1 r 1
2

2
1
,
12Dz
1
Tx
vp 
c (speedlight)
e
, resonant roughs
t

vp
2T y
Hz
Frequency Plot (2) Single-Ended Trace
Single-Ended S21 5cm
Single-Ended S21 10cm
Single-Ended S21 20cm
Identify plot (eg: This is a plot of the insertion losses in the frequency domain for a Single-Ended Trace over
the interval …GHz)
Compare insertion losses (S21) due to the trace lengths and frequency (from maximum markers on plot).
Frequency Plot (3) Compare Diff and S-E
Differential Pair S21 5cm
Single-Ended S21 5cm
Identify plot (eg: This is a plot in the frequency domain comparing the insertion losses for a Differential Pair
and a Single-Ended Trace over the interval …GHz)
Compare insertion losses (S21) due to the type of transmission line (from maximum markers on plot).
Designer Circuit
HFSS
Model
Pseudo Random
Binary Source
Binary to
M-ary Coder
Symbol
Repeater
Root Raised
Cosine Filter
Bit Error
Rate Probe
Signal
Probe
Identify figure (eg: This is a Signal Generator circuit drawn in “Designer” for analyzing the EYE Diagram and BER
(Bit Error Rate))
Describe components.
(eg: The PSRB assigns ___ random bits at a rate of ___ GHz .)
Voltage level
5cm
Acceptable Time duration of signal
Identify plots. (eg: This figure is the EYE Diagram in the time domain for the EYE opening of a 5 cm line at the data
rate of ___ over the interval 1/rate = ___.)
Analyze plots. (eg: Notice that the Voltage and Time differential fall within the given constraints of the EYE Mask
for the 5 cm line.)
Define eye diagram. An eye diagram is a graph that measures the voltage and time variations in a signal caused
by the signal channel. It is constructed by superimposing numerous consecutive bits in a data stream.
Conclusion
HFSS tool:
(C:\Program Files\Ansoft\HFSS14.0\Win64\Help\hfss_onlinehelp p1-1)
HFSS simulation results:
Designer tool :
(C:\Program Files\Ansoft\Designer7.0\Windows\Help\getstart p 4-1, 5-1, nexxim p 1-10, 1-12).
Designer simulation results:
```