Quantcast Twisted Pair

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MIL-HDBK-232A
5.4.1.2.2  Unbalanced signals.  Unbalanced voltage digital signaling depends
upon a voltage relationship of a single line referenced to a signal ground.
This may be implemented by a single signal ground wire in each cable between
two devices, a return link for each signal with all returns terminated on a
common bus at each end, or each device referenced to a station ground system.
A separate return for each signal is preferred. because this arrangement
reduces crosstalk between pairs, and increases the distance which a signal
may be driven.  It further permits single-ended drives to interface balanced
receivers. (See MIL-STD-188-114.) Signals between devices should not he
referenced solely to the station ground system.
5.4.2  Installation.  Cable installation techniques contribute significantly
to reducing RFI/EMI in a facility.  Two cable types are predominant with
variations -- twisted pair and coaxial cable.
5.4.2.1  Twisted pair.  Twisted pair wire is the most common medium used.  It
is normally installed with multipair cables.  Whether to use one overall
shield or individually shielded pairs depends on the application.  Many
applications can be satisfied with one overall shield.  Typically, the use of
individually shielded pairs results in approximately 3 dB additional
attenuation.  Twisted pair may be terminated with crimp on solderless
connectors, soldered to terminal posts, or wrapped around a Post in tight
spirals.  Wire wrap is the best technique for terminations because the post
bites into the wire, creating a more positive bond.  Solder is the least
acceptable method due to the high probability of cold solder joints which
increases the resistance of the bond.
5.4.2.2  Coaxial cable.  Coaxial cable is used in applications where high
frequencies are required.  Coaxial cable, with its center conductor equally
spaced from the outer shield throughout its length, exhibits wide bandwidth
due to reduced skin effect and reduced distributed capacitance.  Coaxial
cable should be terminated only with appropriate connectors.
5.4.2.3  Variations.  Triaxial cable is coaxial cable with an additional
shield.  Its termination should be in accordance with manufacturer's
instructions.  Twinaxial cable is twisted pair, encased in a dielectric foam,
covered with a braided shield.  It may be used where balanced drive at higher
frequencies is required and where a high probability of EMI exists.
Quadraxial cable is twinaxial cable with an additional shield.
5.4.3
Terminations.
5.4.3.1  Facility entrance plate.  Any signal cable entering a facility
should pass through a facility entrance plate.  The facility entrance plate
serves as the point where undesired signals are shunted to ground. where
shields are grounded to dissipate induced currents, and where TPDs are
installed.  Filters on signal lines are circumferentially bonded to the
plate, in order for shunted currents to have a low impedance path to ground.
Some signal lines may not require filtering at the entrance plate if filtered
at the last equipment, provided the plate has not been installed for EMP
purposes.  The facility entrance plate then serves as the point to bond the
shield to a low impedance path to ground.  Fiber optic lines should penetrate
the shield through waveguides-beyond-cutoff.  A waveguide-beyond-cutoff is
sized at a 5 to 1 ratio of length to maximum cross-sectional dimension.
Typically, attenuation of 136 dB can be achieved at frequencies well below
cutoff.  Cutoff frequency is calculated at wavelengths equal to twice the
largest dimension of a rectangular waveguide.  Thus, 0.5-inch conduit has a
cutoff frequency of approximately 1 GHz, while a 1-meter by 2-meter personnel





 


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