VXS
Impedance Measurements
At
higher clock speeds, the PCB requires cleaner signal transmission
without compromising the stability of the system. Signal integrity
issues such as reflections, cross talk, frequency dependent
transmission line loss and dispersion can significantly lead
to poorer system performance propagating through the interconnect.
As VXS is a relatively new specification, we are first measuring
the impedance of the PCB.
Depending
on the configuration, routing a VXS backplane with superior
performance can be challenging. In the higher slot sizes, the
number and length of the traces can have an effect on the signal
integrity. Particularly with larger backplane the number of
traces and lack of physical space, it takes creative and intelligent
routing schemes from an experienced designer. Avoiding undesirable
stubs for upper layer backplane traces presents some tough choices.
One option would be to have these worse case vias back-drilled
-- a costly fabrication process which removes the unused portion
of the plated via structure below the layer at which the signal
is terminated. Another possibility is to minimize the length
of via stubs by choosing a laminate with a lower dielectric
constant as Bustronic did with its 12-slot Dual Star Backplane.
To provide
an illustration, let's look at two VXS backplanes - the 5-slot
switchless Mesh (where the Mesh spans 3 of the slots) and the
12-slot Dual Star. The smaller backplane with traces that spanned
3 slots was done using FR-4. The larger backplane with traces
spanning 8 slots was built using Nelco4000-13SI - a high-grade
material.
To ensure
a clean signal, it is necessary to understand and control impedance
in the transmission environment through which the signals travel.
Impedance mismatches (due to vias and connectors) and variations
can cause reflections that decrease signal quality as a whole.
Time Domain Reflectometry (TDR) measures the reflections that
result from a signal traveling through a transmission environment
like-a circuit board trace, cable, connector. The impedance
values of typical transmission lines as a function of the trace
geometry and the dielectric constant of the surrounding environment.
The VXS
backplanes being analyzed had the following features. The 12-slot
Dual Star VXS has an 18-layer controlled impedance stripline
design. To ensure the highest possible results in early development,
the initial version was fabricated using Nelco 4000-13SI material,
a laminate with a lower dielectric constant. The material also
has a significantly lower loss tangent value than FR-4. Therefore,
the backplane has superior signal integrity and stronger overall
performance, but would not be necessary for all designs and
requirements. Elma Bustronic SI engineers are looking at using
FR-4 for new 12-slot designs and are confident that with intelligent
routing strategies and HSPICE simulation studies, the backplane
would still have more than adequate performance while keeping
the layer count low. But, this is a topic for a future discussion.
The 5-slot
Switchless backplane has a 10-layer controlled impedance stripline
design in standard FR-4. Both the 5-slot and 12-slot were designed
in a 7U height to allow extra power bugs below the card cage
for high current options and for easy cabling.
The connector
used is the Multi-Gig Rt-2 7 row from Tyco Electronics. The
company gives an approximate capability of 6.4 Gpbs. With a
unique wafer design, the connector does not have a typical pin
and socket interconnection.
Signal
Impedance (VXS J0 connector) 12 Slot
Only the
longest trace connection paths of the 12-slot Dual Star were
tested. (Based on experience, we are assuming that in this design
these will be the worst-case paths. However, we will verify
this with our simulation tools in the near future.) The expected
value of differential trace line is 65+/-10% ohm.
Diagram
3
Diagram
3: (Layer_INT04_Slot06_J1_G10 Impedance waveform). The measured
average value of differential trace line for the 12-slot VXS
(6380.2 mil) is 68.7 ohm.
Diagram 4
Diagram
4: (Layer_INT03_Slot07_J0_A15_ Impedance waveform) The expected
value of differential trace line is 100+/-10% ohm. The measured
average value of differential trace line is (5232.5 mil) 106 ohm
Diagram 5
Diagram
5: (Layer2_Signal_Slot03_J0_CD12_ Impedance waveform) The expected
value of differential trace line is 100+/-10% ohm. The measured
average value of differential trace line for the 5-slot VXS
is (2892.6 mil) 99.8 ohm
Measuring
the impedance is just the first step. For both the 5-slot backplane
with shorter traces and the 12-slot with longer traces, the
impedance measurements were very strong. In the near future,
Elma Bustronic will be posting our signal integrity studies
on VXS backplanes. This includes model extraction, HSPICE simulation,
and in depth backplane characterization.
The signal
integrity is just one element of VXS design. Another important
issue in the success of VXS will be making prototyping and development
easy and cost-effective. One facet that is needed is a backplane
that is convenient in size and configuration to perform this
task.
Mahamud Khandokar
Signal Integrity Engineer
Elma Bustronic
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