Simple Steps to increase Performance of Gigabit Backplane Systems
Title: Simple Steps to increase Performance of Gigabit Backplane Systems
Authors: Nitin Jain, Jitender Kumar Misra
Nitin Jain
Nitin is currently a design engineer in Board Design group at Wipro Technologies. His
activities include the design & analysis of high speed next generation products. His
other function includes Signal Integrity Analysis.
Jitender Kumar Misra
Jitender is currently Technical Manager at Wipro Technologies. During his 15+ year
of industry experience he has been involved in numerous high speed board design
projects.
Very Special thanks to Sandeep Kumar (Group Head, Wipro Technologies) for his
infinite help with all aspects of this paper.
Abstract:
There is a growing demand for moving data over copper backplane systems at rates
approaching 5 Gb/s with 10 Gb/s as a goal. This paper aims to present simple but
very practical approach to enhance the performance of backplanes without adding
any further cost to designs. The points discussed here can be applied to any
backplane design running in Gigahertz range.
This paper will examine the pin assignment of differential pairs in a connector to
avoid crosstalk. It would be very helpful for those designs which have very few pairs
high speed links (in GHz range) and using today’s modern connector of high speed
differential links could be a waste of money. It will be shown how the NFP removal
specially ground pads in signal layers can improve the performance of design. Also it
will be presented how connector stub effect and skew can be minimized.
1 Introduction
As signal speeds increase, system performance is limited by long lengths, impedance
mismatches and various noise in the system. This paper brings out various methods
that can be applied to backplane designs to enhance the performance without adding
any substantial cost to designs.
Simulation results (using Specctraquest & Hspice) have been presented to show how
the ground shield around differential pairs helps in minimizing crosstalk. It has been
shown how nearby ground pads in signal layer affect the impedance of trace.
Analysis has been done on how NFP (Non functional pad) removal in signal layer can
aid in routing through connector (which will help in minimizing number of signal
layers required) & minimizing crosstalk. Removal of ground pads in signal layer can
help in avoiding the impedance discontinuity when passing through connector.
Methods of minimizing connector stub effect and skew effect have been discussed in
detail.
Simple Steps to increase Performance of Gigabit Backplane Systems
2 Crosstalk
Many connectors are available in market that are specifically designed for gigabit
differential pairs. They have ground shield around each pair of connector pins to
avoid crosstalk between two adjacent differential pairs. But they come with an added
cost. A design that has very few gigabit differential pairs running on backplane will
add a cost to the design by choosing such connectors. A better approach could be to
assign differential pairs effectively in traditional connectors (of course with controlled
impedance pins) that don’t have ground shield around every pair of pins. But if the
number of gigabit signals is large then going with today’s modern connectors (such
as VHDM etc) will be better and cost effective solution. The below section describes
how a differential pair should be assigned so as to minimize the crosstalk effect. Also
this will show the benefit of ground shield around every pin pair in shielded
differential connector.
The objective of effective connector pin assignment is to minimize the crosstalk
between differential pairs running at gigahertz range. Analysis done below shows the
effect of different pin out on crosstalk.
25 pins connector (2mm Hard Metric), right angle connector, is used in simulation.
Table 1 shows connector available for simulation. Please note that pins in same row
have the same length and row ‘a’ has the shortest pins of connector.
1
2
3
4
5
a
6
7
8
9
10
b
11
12
13
14
15
c
16
17
18
19
20
d
21
22
23
24
25
e
1
2
3
4
5
Columns/Rows
Table 1: 2mm Connector
The differential pair running at 3.125 GHz is used as aggressor nets. Net connected
to 5V is used as victim net. The effect of differential pair (aggressor net) is analyzed
on victim net.
2.1
Crosstalk Analysis I
Simulation Steps:
 Aggressor net is connected at pin A1 & A2 and all the pins are grounded
except pins B1, B2, C1 & C2 (5V signal) on which effect of crosstalk will be
observed (Victim nets). The connector looks like as shown in Table 2.
Tx+
Tx-
G
G
G
a
S
S
G
G
G
b
S
S
G
G
G
c
G
G
G
G
G
d
G
G
G
G
G
e
1
2
3
4
5
Columns/Rows
Table 2
Simple Steps to increase Performance of Gigabit Backplane Systems
Please see waveforms in Figure 1. The waveform is observed at pin B1 (shown by
red color) & C1 (shown by purple color).

Now Pin B1 & B2 are grounded too and connector looks like as shown in Table
3. The waveform is observed at pin C1 (shown by green color).
Tx+
Tx-
G
G
G
a
G
G
G
G
G
b
S
S
G
G
G
c
G
G
G
G
G
d
G
G
G
G
G
e
1
2
3
4
5
Columns/Rows
Table 3
Figure 1: Crosstalk Analysis I
Result:
Total Crosstalk at pin C1 when B1 & B2 pins are not grounded: 60.99mV
Crosstalk at pin C1 when B1 & B2 pins are grounded: 7.63mV
Thus crosstalk at pin 11 is less if the pins B1 & B2 are grounded.
2.2
Crosstalk Analysis II
Simulation Steps:
 The trace is connected at pins C2 & C3 and all other pins are grounded except
pin B1, B4, D1 & D4 (5V Signal). The connector looks like as shown in Table
4.
Simple Steps to increase Performance of Gigabit Backplane Systems
G
G
G
G
G
a
S
G
G
S
G
b
G
Tx+
Tx-
G
G
c
d
S
G
G
S
G
G
G
G
G
G
e
1
2
3
4
5
Columns/Rows
Table 4
The waveform is observed at pins B1 & B4. The waveform observed is shown in
Figure 2.
Figure 2: Crosstalk Analysis II
Result: Crosstalk of around 16mV is present on pin B1 & B4.
2.3
Crosstalk Analysis III
Simulation Steps:
 Trace is connected at pin C3 & C4 and all pins are grounded except
A1,A2,A3,A4,A5,B1,C1,D1,E1,E2,E3,E4,E5 (5V signal).The connector looks
like as shown in Table 5.
Rows/Columns
A
B
C
D
E
1
S
S
S
S
S
2
S
G
G
G
S
3
S
G
Tx+
G
S
4
S
G
TxG
S
5
S
G
G
G
S
Table 5
The waveform is shown in Figure 3. The waveform shows the signal at pin E4
(highest crosstalk at this pin among all other 5V signal pins).
Simple Steps to increase Performance of Gigabit Backplane Systems
Figure 3: Crosstalk Analysis III
Result: Crosstalk of only 5mV is present on victim nets.
The High Speed differential pins of connector should be surrounded by
ground pins to minimize crosstalk.
3 NFP Removal
Connector pins in a multilayered backplane board have connections to traces in
different layers. Thus there are many pins in a layer that have no connections in that
layer. Removing pads (Non Functional Pads) on these pins can enhance performance
of backplane by more than one ways.
3.1
Ease of Routing
The removal of NFPs can ease the routing of signals through the connector. To see
this more clearly, let us analyze this by taking an example of 2mm HM connector.
Following are the parameters of connector pins:
Drill Size: 28mils
Pad Size: 48mils
Pin to Pin Spacing (Centre): 78mils
Assuming for differential pair:
Trace width: 5mils
‘P’ & ‘N’ spacing in differential pair (for tightly coupled traces): 6mils
Trace to Differential pair spacing: ≥3W (15mils), where W is the width of trace.
Drill to trace separation: 12mils
Based on above assumptions, 46mils of cross section width is required to route a
differential pair as shown in Figure 4.
Simple Steps to increase Performance of Gigabit Backplane Systems
Figure 4: Differential Pair Routing Requirement
The analysis has been done for following 3 different combinations of connector pins:
1. No connector pin has a pad
2. Only one connector pin has a pad
3. Both connector pins have pads
Configuration
Without Pads
Spacing
Required
46mils
Pad on one
connector pin
46mils
Pads on both
pins
46mils
Spacing available
78 (Centre to Centre
Spacing between two
pins) – 14 (Drill width
of first pin) – 14 (Drill
width of
second pin) = 50mils
78 (Centre to Centre
Spacing between two
pins) – 14 (Drill width of
pin without pad) – 24
(Drill width of pin with
pad) = 40mils
78 (Centre to Centre
Spacing between two
pins) – 24 (Drill width of
pin without pad) – 24
(Drill width of pin with
pad) = 30mils
Remarks
Can be routed easily.
The available spacing of 44mils can be routed
without much concern as the 15mils spacing
between a trace and differential pair is
required from crosstalk point of view and the
violation of 6mils (46mils – 40mils) for a very
small coupling distance is negligible.
This configuration is not recommended.
It is clear from the above analysis that differential pair can be easily routed through
two adjacent connector pins either when both pins do not have pads or only one pin
has got the pad.
3.2
Ground Pads removal on signal layers
NFPs that are grounded can decrease the impedance of a nearby signal by adding
the parasitic capacitance. The result is impedance discontinuity that certainly affects
signal quality.
The below given Figure 5 shows gigabit differential traces passing through two
connectors.
Simple Steps to increase Performance of Gigabit Backplane Systems
Figure 5: Differential Trace Routing through Connector
Pins connected to ground have been shown by green color. TDR plot of the trace
shown by blue color in Figure 5 was captured (shown in Figure 6). The impedance of
trace drops to 41 ohms two times (through each connector) because of the NFPs that
are grounded and add parasitic capacitance due to proximity. Between two
connectors the impedance is around 51 ohms.
51 ohm
41 ohm
Figure 6: TDR Plot
3.3
Reduced Crosstalk
The removal of NFP also reduces the possibility of coupling between trace and
adjacent connector pin (due to pads) that leads to improved performance.
Simple Steps to increase Performance of Gigabit Backplane Systems
4 Connector Stub
The thru hole (press-fit) backplane connector add another source of impedance
discontinuity- stub effect. Stubbing occurs when the signal does not traverse the
complete via length and a “stub” remains. The stub represents an un-terminated net
that will allow for reflections and/or ISI of the higher order wavelengths as a function
of the stub length.
Thus to counter this effect, it should be ensured that the gigabit differential signals
are routed in layers with the least amount of stub (in bottom layers). If the number
of gigabit signals is more then the signals traveling more distance on backplane
should be placed in lower layer and signals traveling less distance can be placed in
layer above that. In other words, high speed signal traveling more distance should
be given highest priority.
5 Skew
Differential skew is caused by differences in signal transmission times between
differential pairs. The high-speed edge rates of differential signals will require tight
etch length matching. The eye-pattern will close with additional skew.
There are two sources of skew on backplane:
 Skew caused by the right angle female connector
Normally two adjacent pins of a column in right angle connector are of different
lengths and the difference in length remains constant for any two adjacent pins in a
column. To avoid any skew due to this, in a differential pair running through
two connectors on backplane, the signal connected on one connector with
short/long pin should be connected to long/short pin of other connector.
The below given figure shows the common mode voltage (which is indicator of skew)
waveforms obtained after simulations for the following two cases:
1. The P & N trace was connected to two adjacent pins in same row (no length
mismatch) on both connectors.
2. The P/N trace connected to short/long pin of one connector & connected to
long/short pin on other connector.
Figure 7: Common Mode Voltage
Simple Steps to increase Performance of Gigabit Backplane Systems
The above waveforms clearly show that the effect of pin length mismatch has been
compensated effectively.
 Skew caused due to trace bends
Normally differential pair gets routed with many bends and this introduces skew
between ‘p’ & ‘n’ trace. The skew matching (if required) should be performed
at the connector pins only. This is required because impedance changes when
traces are separated and then brought close in order to maintain the skew. Below
given figure shows how skew matching is done at connectors pin by adding extra
length of trace at connector pin.
Skew matching
Figure 8: Skew Matching
6 Other Considerations
It is recommended to assign the differential pairs on same wafer. The reason
is pins of different wafer can cause impedance mismatch due to process variation
etc.
Signal conditioning techniques like Preemphasis can be used to improve eye
diagrams on backplane. Pre-emphasis is a unique signal improving technique that
opens the eye pattern at the far end of the cable for point-to-point applications. Preemphasis add additional output current during the transition time of the bit. This
tends to speed up the edge rate and also provides a bit of over-shoot to the signal at
the driver output. This modified waveshape is still loaded by interconnect, but the
end effect is now much different and improved.
Below waveform shows the preemphasis effect on a PCI express interface simulated
with 16 inches of trace on backplane.
Simple Steps to increase Performance of Gigabit Backplane Systems
Figure 9: PCI Express Receiver Waveform (10% Preemphasis)
Figure 10: PCI Express Receiver Waveform (20% Preemphasis)
Figure 10 shows how the Preemphasis (20%) has improved the receiver waveform
for the same topology used in Figure 9.
Simple Steps to increase Performance of Gigabit Backplane Systems
7 Conclusion
In the design of a backplane inter-connect system; a myriad of options is available
for PC board materials, trace topologies, and connectors. Fortunately there are some
simple steps that can be taken to minimize detrimental effects. Additionally, at these
speeds, old design rules or assumptions may need to be reviewed analytically to
achieve an optimized low crosstalk, low reflection design.