8.2. Signal Transport 479
A.2 TwistedPairCable.......................
The coaxial cable is a well shielded cable but unfortunately never provides 100%
shielding specially against radio frequency background. An alternative to this is
the twisted pair cable, which is used to transport the so calleddifferential signals.
Before we discuss the geometry of the twisted pair cable, we should first understand
the difference between single and differential outputs.
Most of the general purpose commercial preamplifiers produce the so calledsingle
output. Such a signal is highly susceptible to external noise sources such as back-
ground radio-frequency emitters that are generally present in environments contain-
ing different electronics devices. The main mechanism is the pickup of energy by
the signal cable through electromagnetic interactions.
The shielding of the signal cable (such as in coaxial cable) is the most widely used
method because of its simplicity. However, as we stated earlier, this does not always
yield perfect results as most of the commercially available shielded signal cables are
not 100% effective.
The other method is to use differential output. Such an output is carried through
two cables instead of one such that the information is contained in the difference
of potentials on the cables instead of the absolute value of the potential as in the
case of single output. The noise in this case is picked up equally by both cables,
keeping the difference unchanged. However, since the electromagnetic interference
is proximity dependent, the cables must be twisted together in the shape of a spiral
to ensure equal pickup at each point. Such cables are generally referred to astwisted
pair cablesand are available in unshielded form.
To further process this signal, for example by an amplifier/shaper, the subsequent
circuitry must be able to accept differential input. It should however be noted
that the notion ofexactly samepickup by the two cables is somewhat idealized.
The pickup is actually dependent on the gains of each input and therefore modern
amplifiers are built with adifferential gain balancingcircuitry to match the two
gains for proper pickup noise cancellation.
A.3 FlatRibbonCable
Flat ribbon cables are extensively used to transport digital and analog signals. Such
a cable is in the form of a ribbon with several individually isolated wires bonded
together. Ribbon cables come mostly in unshielded form but their shielded versions
are also available.
Ribbon cables should not be used to transport detector signals because even in
their shielded versions the cables are not individually shielded making the inter-cable
cross talk a possibility.
Such cables are generally used to transport power and digital signals. Digital
signals are basically logic signals and therefore a small attenuation in the signal does
not cause any problems. However if an unshielded ribbon cable is used to transport
high frequency (such as few hundredMhz) signals, the same cable becomes a source
of radio frequency emissions. Consequently the environment becomes hostile for
analog signal cables and even good coaxial cables may pick up substantial noise. The
result of this pickup is generally appearance of high frequency oscillatory components
in the data.