IMPEDANCE MISMATCHING
Considering just board-to-board interconnects for a moment leads us to address
the problem of impedance mismatching.
In a practical computer/router system, the electronic boards are plugged into a
backplane which connects the many boards to each other. A signal generated on one board,
for example, must pass through the ball grid array on the transmitter chip onto the printed circuit board (PCB),
travel along a metal trace on the board to the edge, cross over to the backplane through
a connector, along the
backplane, through another connector, along another
board, and through another ball grid array contact in order to arrive at the receiver circuit. At each interface, two traces come together in
a way that is likely to
contain some discontinuities in
the size and shape of the joint
between them.
Electrical signals passing through such discontinuities generate reflections due
to the impedance mismatch
at the interface [2, 7]. It
is essential that these
reflections be minimized because they can cause intersymbol interference (ISI). ISI
means the information in one bit is
corrupted somewhat by the energy or information in
some
other bit(s) in the data stream.
ISI, in this case,
is essentially an ‘echo’ – the first
part of a signal tries to pass these interfaces, but some of the energy is reflected and when it comes back in the same direction a split second later, this ‘echo’ gets superimposed
with the signal that is currently trying to pass the interface for the first time.
Clearly, the ability of the receiver to properly distinguish the original information in the signal will be
compromised somewhat by this echo, just as it is difficult
to understand the speech of someone
who is standing in a strongly echoing
environment, like a shower room. (Note that
reflections are not the only cause of ISI. Other channel imperfections, such as
dispersion, can lead to ISI as the energy from one
bit corrupts the adjacent bits in time. Regardless of its source, ISI should be minimized and it is significantly more problematic in electronic channels than in optical channels, as we
will see.)
CROSS-TALK
As the highest frequency in an electrical signal approaches 5-10 GHz, a wire with
an oscillating electric field
at that frequency will be emitting radiation that will be picked
up by nearby wires. Thus, a signal that is supposed
to be confined to one wire will actually be contributing to the energy or signal carried on another wire.
This “cross-talk”
is obviously
noise on the receiving signal that degrades
the ability of the receiver circuit to properly distinguish the
digital levels. The large amount of data
that must pass through a chip’s I/O necessitates a dense array of interconnects. If an electrical
interconnect scheme is utilized, this provides so many opportunities for cross-talk that each line must be well protected.
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