Download Long Range Connector Via Coupling Effects for High Speed

Long Range Connector Via Coupling
Effects for High Speed Signals
Dr. Roland Frech2), Dr. Thomas Winkel2),
Dr. Thomas Gneiting1)
1)AdMOS GmbH, 2)IBM Entwicklungs GmbH
[email protected]
Abstract
• The interface between a multi pin connector and the printed circuit board
is a critical part of the signal path in high speed digital systems like
mainframe computers.
• Long range via coupling effects are analysed for connectors with a huge
signal count and high speed signals using CST Microwave Studio
simulations. A realistic cross section of the printed circuit board with a
mixture of ground and voltage layers have been taken into account. It
was possible to understand the reason for this behaviour by analyzing
the field distributions during the signal propagation inside the printed
circuit board.
• A further important result was the calculation of the magnitude of the
distortions to judge their impact on the signal integrity of a complete
system. Finally, the long range crosstalk effect was verified with time
domain measurements on a specially designed test card system.
High Speed Connector Test Board
The PCB has a
realistic thickness
of 5mm.
SMA connectors to
feed in test signals
High speed
connector
Distance between
nearest wafers: 2mm
Long range distance
between 50 wafers:
100mm
Result of XTALK Measurements
The measured near end crosstalk at
a distance of 100mm between
agressor and victim is still around
1mV at a signal amplitude of 500mV
and 8V/ns slew rate.
The measured near end
crosstalk between 2
neighboured signals in the
same wafer is 10mV at a signal
amplitude of 500mV and 8V/ns
Noise adds up
with several
hundred signals
in this distance
range !
Details of vias and lines
signal vias
lines
voltage planes
ground planes
ground vias
voltage vias with discontinuity
for HF signal return
Details of vias and lines
Reference case: only GND planes
Real case: Voltage and GND planes
GND
Voltage
GND
This cross section shows the reference case, where all This configuration shows a case, where two planes
planes belong to GND and are connected through belong to GND and the plane in the middle belongs
the ground vias.
to Voltage. The voltage plane has a void area around
the ground vias to avoid a short. The transmission
line is located between the Voltage plane and the
GND plane.
Transmission lines in PCB
Agressor line
Agressor line
Victim line
Victim lines
Reference case with 2 lines in
a distance of 10mm or 100mm
Realistic case with a set of 10
lines, 2mm spacing
Principle of signal transmission
GND via
signal
via
GND via
ground layer
signal layers
voltage layer
HF signal current
ground layer
EM field
HF signal return current
Via clearances
in voltage layer – HF return current discontinuity
Evaluation E-field between GND / VCC plane
Signal feed into
aggressor via
with adjacent line
Measurement
point: victim via
with adjacent line
Traveling wavefront
of distortion signal in
the planes
Traveling
wavefront of main
signal in the line
E-field in the
layer of the
signal lines
Snapshots: E-field in signal line layer
Wavefront after
200ps
Wavefront after
700ps
Visualization
Snapshots: E-field in cross section
Parasitic wavefront
travelling between
GND and VCC plane.
“Main” E-field of
signal in the line,
coming out of
drawing plane
Parasitic
wavefront after
hitting the
victim lines
Visualization
Multi line cross talk
1
2
10
The final simulation setup shows the
distortions on many parallel signal lines.
The input signal has an amplitude of
500mV and a risetime of tr=35ps, 20-80%
Signal near
aggressor
Decreasing max. amplitude
with larger distance to
aggressor
11
20
The amplitude seems to decrease with 1/d
(d=distance between agressor – victim).
Conclusion
•
•
•
•
The described effect of long range cross talk was first detected by
measurements in the time domain.
It was not clear, whether this cross talk is induced by the high speed
connector or the printed circuit board.
The simulation with CST Microwave Studio gave the required insight into
the components. With a detailed analysis of the electrical and magnetic
fields it was possible to understand the signal distribution mechanism or
the “parasitic” waves.
Based on those results, design rules could be established to minimize the
described effects.
Literature
(1) Long Range Connector Via Coupling Effects for High Speed Signals,
Thomas-Michael Winkel, Roland Frech, Thomas Gneiting, IEEE EPEP
2007, 16th Topical Meeting on Electrical Performance of Electronic
Packaging
(2) Effects of Partially Broken HF Signal Return on Different Packaging
Levels , Thomas-Michael Winkel, Roland Frech, Thomas Gneiting, IEEE
EPEP 2008, 17th Topical Meeting on Electrical Performance of Electronic
Packaging