1. USC Signal Integrity Lab Course 2 Ansoft High Frequency Structure Simulator (HFSS)
ELCT 762
USC

2. Driven Terminal Problem
Analyze two micro strip lines that pass through via. Analyze the impact of return loss and insertion loss. (S11, S21 )
Use the 4 layer PCB in the following stack up.
Analyze the PCB at a size of 2 x 2 inch

3. Stack up 1 mil = 0.001 inch, Signal trace width = 5 mil1
Copper 1OZ
1.4
Sig
5 mil / 10 mil space
FR4
DK = 4.1 5 2
copper 1 OZ
GND 47 3 4
total thickness ( mil )
62.6
1 mil = inch,
Signal trace width = 5 mil
Signal to signal space = 20 mil

4. Configuration Start HFSS Save file Insert HFSS design
Setup solution type: Driven Terminal
Setup grid to 0.1 inch

5. Structure Design
Draw a BOX at: 0, 0, 0, size: 2, 2, inch. Assign material: FR4_epoxy. Set transparent: 0.7 Rename it: FR4_1
Draw 2nd BOX at 0, 0, 0.005, size: 2, 2, inch. Assign material: Copper. Set transparent: 0.7 Rename it: GND1
Draw 3rd BOX at 0, 0, , size: 2, 2, inch. Assign material: FR4_epoxy. Set transparent: Rename it: FR4_2
Select GND1 by clicking in command history window. Click: Edit-> copy, and Edit -> Paste. Now, GND2 is copied from GND1 and is placed at the exact same location!
Double Click GND2 (create box) at command history window. At property window, put 0, 0, as new origin. This moves GND2 to upper layer

6. Stack up Design
Do the same thing to copy FR4_1 and make it FR4_3. Put new origin as: 0, 0,
Now the stack up is ready. It has two ground layers and 3 FR4 layers.
Grey color is for FR4 and Blue is for GND
Side view of PCB stackup

7. Add signal trace
Draw a BOX at: 0, 1, , size: 1.005, 0.005, inch
Rename it: Trace1, Assign Material: Copper. Change the color to: RED
Copy Trace1, and Paste it. Now you got trace2. Change trace2’s original at 0.995, 1, 0 inch, size: 1.005, 0.005, inch
Now Trace1 and Trace2 are on two different sides of the PCB. They are located at center and overlap each other by 10mil.
In order to see both traces, you need to set transparent property of all drawing to be ~ 0.7

9. What is via and anti-via
What is via & anti-via
What does via do?
What does anti-via do?

10. Add via and anti-via
Click the cylinder icon, draw a cylinder between two traces.
Change the current position to: 1, , 0 inch, radius: and height: inch. Assign the material: copper. Change the color: RED Rename it: via1
Do another cylinder (10mil radius), assign material: Fr4_epoxy, rename it: anti_via1
Two cylinders overlap and center at same location. -- this will cause a problem and confuse the software! Why?

11. Bolean function
The overlap of two different materials will cause design error. This can be corrected by using Boolean function – subtract.
In command history window, hold: Ctrl key and click: FR4_1, FR4_2, FR4_3, GND1, GND2, anti_via1
This selects the 6 items above to perform a Boolean function
Click: 3D Modeler -> Boolean -> Subtract..

12. Bolean Function - subtract
Subtract window pops up.
Move, FR4_1, FR4_2, FR4_3, GND1, GND2 to Blank Parts window
Move anti_via1 to Tool Parts window
Select “Clone tool objects before subtracting” Why?
Click “OK”

13. View - Visibility
After subtraction, graphic verification is hard since all the components overlap each other. (visibility setting can solve this problem)
Click: View -> visibility
Visibility window pops up. Turn off the visibility on all FR4 and anti_via1
It is verified that via passed through two layers of ground without touching it

14. Question. The overlap issue between FR4, GND and anti-via is solved.
There’s another overlap error. Which one?

15. Overlap problem 2 There’s overlap between anti-via and via.
Use Boolean subtraction function, subtract via from the anti-via. Don’t forgot to “clone” the via
Use the visibility command to verify that anti_via1 has a hole in the center.

16. Bolean function - unite
Hold Ctrl key and click: FR4_1, FR4_2, FR4_3, anti_via1
Click: 3D modeler -> Boolean -> unite
All of above items will be united into one item. Name: FR4_1. ( the new name is the first item you selected )
You can only unite the items that touch each other and have same material
Your field solution is more accurate and process is faster after you unite the similar items
If you want to un-do the “unite”, go to command history window and delete the “unite” under FR4_1
Do the same thing to unite: trace1, trace2, via

17. Create 2nd trace In command history window, click and select: trace1
Click: edit -> copy, then click: paste
Now, you have trace3 created at the exact same location of trace1. (they overlap each other)
Click and select: trace3
Click: Edit –> arrange -> move
Use the mouse, drag trace 3 in the positive Y direction 20mils. (note: DY = 0.02 indicator at bottom right)
To verify the move, in the command history window, double click the move command.

18. Create 2nd trace, cont. Now, the sub window pops up.
Make sure the move vector is 0, 0.02, 0
This makes the 2nd trace 20mil away from 1st trace.
What is the air gap size between the traces?

19. 2nd Anti-via
Create an anti-via for the 2nd trace. The center location should be 0, 1.023, 0
Review slide 10 for creating an anti-via
Rename the new item: anti_via2
There’s an overlap issue between anti_via2 and FR4_1, GND1, GND2, trace3. Review slides 12 and 15 to solve the overlap error
Unite the FR4_1 with anti_via2. keep the name: FR4_1 for the united item.

20. Structure with 2 traces

21. Add Air Box Need to add “Air” boxes on top & bottom of PCB. Why?
Draw a box at 0,0, 0.598, size: 2, 2, 1, Material: air, rename it: air1
Draw a box at 0, 0, 0, size 2, 2, -1, Material: air, rename it: air2
Change the color of air to: white with transparent setting of 0.95
Air boxes overlap with trace1 and trace3. Use the Boolean function to do the subtraction

22. Structure Design is completed

23. View at different angle
Click: View -> Modify Attribute -> Orientation
You can set the Top view, front view or side view. This helps a lot when you design with 2D objects.
Now, set the view as: front view. See top picture
Select the FR4_1 item, make the transparent property: 0

25. Port & Terminals Sample

26. Port & terminal Terminals 1 2 3 4 Port is a 2D rectangular areaw h
6h to 10h
10w, w  h or
5w (3h to 4h), w < h
Port is a 2D rectangular area
Terminals are each end of traces
Port contains terminals

27. Port Assignment Set the view at: Front view
Draw a 2D rectangle by clicking: Draw->Rectangle ( Set in plane direction: YZ)
Enter original: 2, 0.988, Y size: 0.05, Z size: -0.05
Set transparency: 0.8
Rename: Port1
Do another 2D rectangle at: 0, 0.988, Y size: 0.05, Z size: 0.05
See picture next slide.

29. Terminals Set the view at: front view Select by clicking: Port1
Click: HFSS -> Excitation -> Assign -> Wave Port
Click: Next , the Wave Port terminal window pops up.
Set: Number of Terminals = 2, Click: update
Change the name of 2nd Terminal line to T3.
At T1 Terminal Line cell, click & select: New Line
Use coordinate system to key in terminal lines

30. Terminal Lines
The terminal lines have to be on the Port1 plane. It has to go from GND1 to Trace1
It is very hard to use the mouse to set the terminal line because it will snap to a point somewhere outside of the Port1 plane.
It is highly recommended to key in the coordinate system below the progress windows ( lower right corner). See picture

31. Terminal lines, cont.
At coordinate system, enter: 2.0, , inch for xyz. Hit: Enter
Enter: 0, 0, for Delta x, y, z, hit Enter
Now you got a terminal line T1 from GND1 points to trace1. It is on the Port1 plane.
Note the point direction, It is point from low voltage (GND) to high voltage (trace1)

32. Terminal lines, cont. Add a second terminal lines for T3
Starting point is: 2, , Delta Z =
Hit: Enter, Enter, Finish.
Now, the Port1 is defined with 2 terminal lines named: T1, T3
Do same thing to define Port2 with terminal lines of T2 and T4. (Note, Port2 is on plane location x=0 )

33. Assign Boundary
Assign radiation boundary on external surface of airbox.
Click: Edit -> Select -> Face
Hold down CTRL key, and select the external face of air1. Hold the ALT key to rotate the object to select faces
Click: HFSS -> Boundary -> Assign -> Radiation. Hit Enter.
In project window, click boundary -> RAD1, the radiation boundary shows on drawing window
Do the similar boundary assignment for air2

34. Boundary Assignment, cont.
Now assign a radiation boundary on all external faces of FR4 material (faces on the outside)
Don’t assign boundary on any metal objects
Don’t assign boundary on internal faces
Sometimes, it is easy to click one face and assign one boundary. ( Click multiple faces and assign one boundary is also OK)

35. Boundary overlap warning message
If you run the validation check, there will be warning messages regarding the boundaries. It is because waveport1 assignment overlaps with radiation boundary. In this case, the port always has higher priority to overwrite the boundary.

36. Solution setup
Click: HFSS-> Analysis Setup -> Add Solution Setup
The Solution setup window will pop up.
We want to solve at 1 Ghz. Change the number of Passes: 5
Change the Max Delta S: 0.01
Click OK to accept default for rest of the settings.

37. Setup another solution
Now, setup a sweep under first solution
Click: HFSS -> Analysis Setup -> Add Sweep. Click OK to pick setup1
Edit Sweep window popup
Change Sweep type: fast
Change Sweep size: 0.5 Ghz
Click OK

38. Simulation! Click: HFSS -> Analyze
Wait 15 min for this small simulation.

39. Simulation Results What simulation results can be found?
S, Y, Z parameters, impedance matrix
Plotting of above matrix
E, H fields inside the structure
Current density of structure
Near & Far field radiations

40. S parameter Click: HFSS -> Results -> Solution Data
Click on tab: Matrix Data
Solution Data window pops up. It shows the S parameter matrix.
To show data for all frequencies, click: All Freqs.
This table gives you data for point analysis. For a general AC analysis, a plot is a better way

41. Plot of S11 Click: HFSS -> Results -> Create Report
Change display type: Smith Chart
Pick Report Type: Terminal S parameters. Press OK
Pick Quantity: first entry, this is S11.
Double Click this entry (trace appears at top)
Click Done

42. S11 on Smith Chart

43. Plot S21 Click: HFSS -> Results -> Create Report
Set Display Type: Rectangular Plot
Click: OK
At Plot window, select quantity: waveport T2, T1. This is the S21
At function sub-window, select: dB. This will plot the magnitude of S21 in dB
Double Click (put entry at top)
Click: Done

44. S21 Magnitude in dB

45. S21, Real & Image Part

46. Homework Plot S31 Magnitude in dB (near end of cross talk)
Find S31, S41 at 1Ghz.
Plot Electric field that generated by via. Hint: Use HFSS -> Field -> Plot Field -> E Mag. Plot the E mag on the face of FR4_1 (center layer face)
Do an animation for above E – field.

47. Project 1
Compare the T-lines pass via with a typical micro strip. ( no via )
Do the same simulation to find the difference on return loss, insertion loss and cross talk
Suggest what is the cause of those differences
How to improve it? Suggestions?
Show the improved result on simulation

48. Project 2 Do the same thing as project 1 for a strip line. Copper 1 OZ
1.4
FR4 
 DK = 4.1   1
Copper 1OZ
Sig
5 mil / 10 mil space
FR4
DK = 4.1 5 2
copper 1 OZ
GND 47 3 4
Copper 1Oz 
1.4