Signal Integrity

1. High Speed Printed
Circuit Board Design
Ground effect on PCB
A short presentation at FermiLab
Hamid Kiabi

2. Case-1
This brief presentation shows
an analysis of a two-layer PCB
fabricated with 5 mil thick
layers of FR4 and the effects of
cutting a hole in the ground
plane.

3. Ground Effects
The EM analysis consists of three
parallel traces on the top layer of a 3-
layer PCB, with a cutout in the middle
ground plane, and no metal on the
bottom. The dielectric above and
below the PCB is air, and the top and
bottom of the enclosure are
approximate open circuits

4. 2D view without GND Plane under
Traces
3 6
1 4
2 5
63
41
52

5. 3D View3D View

6. Result for the Reflection Coefficients
The "Loss and Match" graph shows
the match and the loss for the
traces setup with the input signal.

7. Loss and Match
2000 2200 2400 2600 2800 3000
Frequency
Loss and Match
-20
-15
-10
-5
0
-1.2
-1.05
-0.9
-0.75
-0.6
DB(|S(1,1)|) (L)
DB(|S(4,1)|) (R)

8. Cross Talk Analysis
Circuit analysis is used to pass a
2.4GHz clock with 50ps rise and fall
times through the 6 port structure.
The "Xtalk Test" schematic applies
a square wave to port 1, and the
"Input Side" and "Output Side"
graphs show the voltage
waveforms at the six ports.

9. Xtalk Schematic
W1
W2
W3
1
2
3
4
5
6
SUBCKT
ID=S1
NET="Ground_FX"
PORT_SQR
P=1
Z=50 Ohm
AMP=2 V
TR=50 ps
TF=50 ps
TD=0 ps
Offset=0 V
DCVal=0 V
PORT
P=2
Z=50 Ohm
PORT
P=3
Z=50 Ohm
PORT
P=4
Z=50 Ohm
PORT
P=5
Z=50 Ohm
PORT
P=6
Z=50 Ohm

10. Input Side (Response)
0 300 600 833
Time (ps)
Input Side
-2
-1
0
1
2
Port 1 (V)
Port 2 (V)
Port 3 (V)

11. Output Side (Cross Talk)
0 300 600 833
Time (ps)
Output Side
-1
-0.5
0
0.5
1
Port 4 (V)
Port 5 (V)
Port 6 (V)

12. Stray Coupling result:
The “Stray Coupling” graph shows
the magnitude of coupling between
traces around the fundamental
frequency of the input signal,
2.4GHz. (See Cross Talk Analysis,
below).

13. Coupling Effect
2000 2200 2400 2600 2800 3000
Frequency
Stray Coupling
-30
-25
-20
-15
-10
-5
0
DB(|S(2,1)|)
DB(|S(3,1)|)
DB(|S(5,1)|)
DB(|S(6,1)|)

15. Case-2
What happens when a
GND plane is added
under the Traces?

16. 3D view with GND plane added

17. Input Side with GND Plane
0 300 600 833
Time (ps)
Input Side
-2
-1
0
1
2
Port 1 (V)
Port 2 (V)
Port 3 (V)

18. Output Side with GND Plane
0 300 600 833
Time (ps)
Output Side
-2
-1
0
1
2
Port 4 (V)
Port 5 (V)
Port 6 (V)

19. Loss & Match with GND Plane
2000 2200 2400 2600 2800 3000
Frequency
Loss and Match
-50
-40
-30
-20
-10
0
-0.022
-0.02
-0.018
-0.016
-0.014
-0.012
DB(|S(1,1)|) (L)
DB(|S(4,1)|) (R)
S11 >-40db compared to
-12db without GND plane
S11 >-40db compared to
-12db without GND plane
S14 is now negligible compared to
-1.2db without GND plane
S14 is now negligible compared to
-1.2db without GND plane

20. Stray Coupling with GND Plane
2000 2200 2400 2600 2800 3000
Frequency
Stray Coupling
-45
-40
-35
-30
-25
DB(|S(2,1)|)
DB(|S(3,1)|)
DB(|S(5,1)|)
DB(|S(6,1)|)

22. Enclosure
STACKUP
Name=SUB
ENCLOSURE
ID=ENCL
X_Dim=500 mil
Y_Dim=500mil
Grid_X=10mil
Grid_Y=10 mil
PORT
P=1
Z=50 Ohm
PORT
P=2
Z=50 Ohm
PORT
P=3
Z=50 Ohm
PORT
P=4
Z=50 Ohm
PORT
P=5
Z=50 Ohm
PORT
P=6
Z=50 Ohm

23. Thank you