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PCB Channel Characteristics (48 in. Trace)
Channel losses are dominated by skin effect and dielectric loss
Channel noise is dominated by crosstalk in connectors, packages, and PCB
SNR near 1-2 GHz deteriorates at 40 dB/decade
OK, now let's consider the world of PCB connections. What's happened here is that we are still
in the "dumb link" mode of usage for these connections.
The natural bandwidth of a short PCB trace being several Gigahertz, we haven't needed anything
more, at least not yet. But thinking about the future, and thinking about LONG connections
(like backplanes) it's obvious what will happen next:
More sophisticated use of the backplane traces; more sophistication in the form of adaptive
equalization, and multi-level signaling.
This chart illustrates several important points relevant to signaling over a long PCB trace.
The assumption here is that we are talking about a BIG backplane, something for a BIG piece of
network hardware.
I'll make the leap of assuming we are talking about a multitude of serial point-to-point
connections on this backplane, a fully-connected non-blocking configuration of some sort.
The traces (including the backplane plus stubs on each card) are about 48 inches long (that's
pretty long). The traces are 7-mils wide, implemented as FR-4 striplines. Now, we could debate
all day long about the exact assumptions used, and the exact amount of loss, but that's not
what I want to get across in this figure.
What I want to get across is that at high speeds the loss is dominated by dielectric losses
in the FR-4 material. In this region the loss (in dB) increases proportionally to the
operating speed at a slope of 20dB or more per decade.
At the same time the noise from connectors, packages, and so forth (mostly crosstalk) increases
at a rate of +20 dB/decade. The difference between these two effects, the SNR, is deteriorating
at -40 dB/decade as we go up above a gigahertz. This is a very steep drop-off.
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