Long Range Connector Via Coupling Effects for High Speed Signals

Transcription

1 Long Range Connector Via Coupling Effects for High Speed Signals Dr. Roland Frech 2), Dr. Thomas Winkel 2), Dr. Thomas Gneiting 1) 1) AdMOS GmbH, 2) IBM Entwicklungs GmbH

2 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.

3 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

4 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!

5 Details of vias and lines signal vias lines voltage planes ground planes ground vias voltage vias with discontinuity for HF signal return

6 Details of vias and lines Reference case: only GND planes Real case: Voltage and GND planes GND This cross section shows the reference case, where all planes belong to GND and are connected through the ground vias. GND Voltage This configuration shows a case, where two planes belong to GND and the plane in the middle belongs 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.

7 Transmission lines in PCB Agressor line Agressor line Victim line Reference case with 2 lines in a distance of 10mm or 100mm Victim lines Realistic case with a set of 10 lines, 2mm spacing

8 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

9 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

10 Snapshots: E-field in signal line layer Wavefront after 200ps Wavefront after 700ps

11 Visualization

12 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

13 Visualization

14 Multi line cross talk 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 The amplitude seems to decrease with 1/d (d=distance between agressor victim).

15 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.

16 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