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is directly traced to microdefects
in the PCBs and connectors used
to manufacture very large, high
performance systems, removing
these signals from those PCBs and
backplanes can solve the problem.
This is not a new idea. If one
looks back to several of the high
performance computers designed by
Cray Research and other companies
in that market, all of the very high
speed signals were moved from PCB
to PCB over shielded twisted pairs or,
in some cases, unshielded twisted
pairs. This latter technique is how the
Ethernet has been able to operate
over long distances using ordinary
phone wiring at data rates as high as
1 Gb/s.
The first advantage of the cable
method is the opportunity for crosstalk
between signals to be eliminated.
A second advantage the cable
method has is the backplane can now
be manufactured from standard PCB
laminate material as its only task is to
carry power to the modules plugged
into it and to hold all of the connectors
in a rigid structure.
What about the problem caused by
those plated through holes that are
necessary to hold the connector pins
in place as well as the plated through
holes required to connect component
pins to traces in the daughter cards?
What has been demonstrated by
simulations as well as by laboratory
measurement is that when a signal
travels the length of the plated
through hole or via, the parasitic
capacitance of the hole is distributed
along the length of the hole, rendering
it virtually invisible.
This leaves the task of tackling
skew. When the differential pairs
are connected with shielded pairs,
such as twinax, the two sides of
the differential pair travel in a very
uniform dielectric that is common to
both sides of the pair. The result is
that skew or difference in travel time
between the two sides of a pair can
be made virtually zero.
Figure 5 is an example of a design
that uses this twinax method to
implement a very complex high
performance switch/router. Using this
method, performance as high as 56
Gb/s can be achieved using ordinary
PCB materials. This avoids the
problem of designing very complex
PCBs and then managing the supply
chain process to insure that all of the
complex manufacturing problems are
kept under control.
Figure 6 shows the loss vs. frequency
of several 2.6m - 3m differential paths.
The measured paths include two
daughter cards connected through
twinax cable as shown in Figure 5.
The insertion loss is near ideal up to
about 25 GHz. This demonstrates that
this assembly is potentially capable of
50 Gb/s.
A further advantage of implementing
all of the high performance signals
in twinax cable is that it is possible
to make wiring changes at the
backplane level by reconfiguring
the twinax cables to implement a
new function not available when the
original backplane design is done.
This helps eliminate the “fork lift”
upgrades often required with hard
wired backplanes.
Conclusion
Advances
in
semiconductor
technology are making it possible
to connect components in products
such as switches and routers at rates
as high as 56 Gb/s. As these higher
speeds are achieved, micro-scale
variations in the materials used to
fabricate PCBs and backplanes can
significantly degrade signals. Among
the problems encountered are loss,
skew, crosstalk, and degradation
due to the parasitic capacitance of
the plated-though holes required
to mount the connectors to the
backplanes and daughter cards.
By using twinax cables to make these
connections instead of implementing
them in PCBs and backplanes with
traditional traces, skew, crosstalk, and
degradation from the plated-though
holes can be virtually eliminated. Due
to the ultra-low loss of the twinax
cables, path lengths can be longer,
or the frequency of operation can
extend much higher than is possible
with the laminate systems currently
available.
Authors Ritchey & Knack are with
Speeding Edge, McMorrow with
Samtec division Teraspeed Consulting.
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