Electro-Optics - Applied Optics Fundamentals and Device Applications - page 41

lines for point to point connections. Interspersal of signal lines between
ground and power planes reduces crosstalk. Additional “shield” layers may
be introduced while clock distribution on the boards can be accomplished
with equal phase length-terminated branches. Various practical empirical
rules have been determined for similar particular semiconductor technolo-
gies such as TTL compatible CMOS [56].
2.5.3 Backplane Interconnects
Backplane connections are proportionately long and the mutual inductive
and capacitive coupling between conductors produce unwanted and intol-
erable crosstalk between circuits carrying fast rising signal waveforms.
Backplane electrical design centers around minimizing and controlling
these couplings. It is important that the couplings remain the same in suc-
cessive copies of systems and not change during normal operation or ser-
vicing. To this end, orderly arrangement of printed circuit panels, ribbon
cables, twisted pairs, and coaxial cables are used for these connections.
Special interfacing line drivers are required for high-speed lines between
chip modules.
Normally, a backplane structure is passive, serving only to supply inter-
connection and mechanical support. In some cases, active circuitry has been
incorporated in the backplane presumably to reduce the lengths of some con-
nections or to equalize lengths. This arrangement can create serious trouble-
shooting and maintenance problems however; and some systems made in
this way are being retrofitted with passive backplanes.
2.5.4 Power Distribution
In multilayer modules, power distribution is accomplished by adding an
additional layer to act as a power bus. By employing wide traces with mini-
mum lengths, series inductance is minimized. In this manner, power busses
can be considered as a line with low characteristic impedance.
A phenomenon that occurs in RF circuits and is detrimental in very fast
digital circuits is the creation of relatively high Q resonators when the length
dimension of the lines approaches a quarter or half wavelength of signal in
the dielectric substrate. In some cases, the Q can be “spoiled” by introducing
resistance across the bus at the ends.
Power distribution is a key determinant of a backplane's performance
and reliability. It is usually accomplished by heavy vertical bus bars. Before
choosing a backplane configuration, the system's current flows should be
understood. Proper placement of inputs and returns ensures current densi-
ties will be uniform and voltage drops low enough to permit reliable opera-
tion. Paralleling connector pins at power entry points requires a significant