Evaulation_Engineering_August_2020 - 24

EMC/EMI TEST

magnetic fields are often hard to control.
Unmatched impedance in an interconnect system can cause an EMI wave that
couples from cable to cable to component
to element, literally killing the performance of the electronics involved. Cable
design and management systems have
adopted a well-defined system of solving many of these problems using cable
shielding.

Cable Shielding
There are many types of shielded cables,
and each has their own advantages and
properties. A cable with an overall foil
shield and unshielded twisted pairs (UTP)
is very similar to the common UTP cable,
the difference being the addition of foil
underneath the main cable jacket. Such
cables are common in 10GBaseT applications. Other cables have an overall braid
shield/screen with UTP. While occasionally referred to as a shielded twisted-pair
(STP) cable, the two do not provide the
same shielding. To identify between them,

and each individual pair is wrapped with
its own foil barrier. The purpose of the additional foil on individual pairs is to limit
the amount of crosstalk between the pairs.
A cable with an overall foil shield
(F) with FTP are commonly used in
10GBaseT applications, similarly to F/
UTP cables. Cable shielding is becoming
more common, even though not all environments require them. At first, shielded
cables were used in spaces with high concentrations of electrical equipment and/
or secure communications applications.
Today shielded cables are commonly used
in many spaces such as government, education, and healthcare buildings.

Cable shielding and wrapping
Wrapping conductor(s) inside the cable
with EMI tape reduces the openings typically found in braided solutions. While its
tradeoff is reduced flexibility, a greater
level of attenuation can be obtained with
the elimination of openings commonly
found in braids. Typically used for high-

Application-specific wire-harnesses are also used in military sensor systems, processors for mine
detection, portable camera and surveillance modules, and unmanned aerial vehicles.
Courtesy of Omnetics

check for an overall barrier and/or individual pairs' shielding.
An SF/UTP cable shielding includes
both an overall braid shield (S) and foil
shield (F) with unshielded twisted pairs
(UTP). This cable is also occasionally referred to as an STP cable. Cables with an
overall braided screen are very effective at
protecting EMI from entering or exiting
the cable. In a cable with an overall braid
shield (S) with foil-shield twisted pairs
(FTP), the shield under the jacket is a braid,

24

EVALUATION ENGINEERING AUGUST 2020

speed applications, materials and wrap
overlap are varied to change the level of
shielding for the application, both internally and externally.
Generally used in high-speed applications where foil wraps or braids are not
present, a conductive shrink-tube is a
great option for shielding, helping control
impedance in high-speed applications, or
to bridge the gap between individual conductors as they exit the group braid and
head to their individual locations within

the connector. Values from 60 to 80db of
attenuation can be obtained with a conductive shrink tube.
Signal type and cable function determines wire type, number of wires,
wrapping, and insulation. Fire-wire, for
example, uses a small wrapped pair of
signal wires next to a single drain wire,
to carry the differential signals and the
ground reference. Placed over the 3-wire
set is typically an aluminum shield-wrap
around the bundle to isolate the signals
from adjacent wiring within the cable.
Adjacent wire pairs can be run within
that cable to provide a power source for
the instrument at the end.
If an RF set of cable is included, additional
and separate wrapped aluminum shielding is needed to separate the coax from
its neighbors inside the cable. Designers
may consider conductive zipper or shrink
sleeves, flat cable with EMI tape, and knitted-mesh shield wrapping to help with
cable flex. Recently-introduced tubular
metalized-fiber braid offers light weight,
excellent flex, and isolation, but at higher
prices with medium levels of isolation.
Shielding within both the connectors
and the cable is critical to preventing EMI
problems. The focus on higher density and
increased functions within instruments,
and the shrinking space available for the
internal wire routing of signals from one
section of the instrument to another, is
getting more critical. Custom designs are
more frequently employed for cabling to
specifically fit within each instrument to
reduce size and weight while increasing
circuit speed, signal routing and impedance matching to other circuits in adjoining modules.
Braided shields can offer the highest isolation, but may need a number of
options or trade-offs when they can be
used. Exterior cable shielding is frequently
employed to ensure that signals are not
affected by exterior signal noise, and to
contain the signals within the cable. The
exterior shield is exceptionally helpful in
protecting from cyber signal threats corrupting the purpose of the cable.
Exterior cable shielding materials can
vary significantly in their ability to attenuate and isolate the signals from interference. Slip-on braid is rated for up to 65 db



Evaulation_Engineering_August_2020

Table of Contents for the Digital Edition of Evaulation_Engineering_August_2020

Editorial
By the Numbers
Industry Report
Signal & Spectrum Analysers
Communications Test
EMC/EMI Test
Industrial Automation
Featured Tech
Tech Focus
Industrial IOT
Evaulation_Engineering_August_2020 - 1
Evaulation_Engineering_August_2020 - 2
Evaulation_Engineering_August_2020 - 3
Evaulation_Engineering_August_2020 - By the Numbers
Evaulation_Engineering_August_2020 - 5
Evaulation_Engineering_August_2020 - Industry Report
Evaulation_Engineering_August_2020 - 7
Evaulation_Engineering_August_2020 - Signal & Spectrum Analysers
Evaulation_Engineering_August_2020 - 9
Evaulation_Engineering_August_2020 - 10
Evaulation_Engineering_August_2020 - 11
Evaulation_Engineering_August_2020 - 12
Evaulation_Engineering_August_2020 - 13
Evaulation_Engineering_August_2020 - 14
Evaulation_Engineering_August_2020 - 15
Evaulation_Engineering_August_2020 - Communications Test
Evaulation_Engineering_August_2020 - 17
Evaulation_Engineering_August_2020 - 18
Evaulation_Engineering_August_2020 - 19
Evaulation_Engineering_August_2020 - 20
Evaulation_Engineering_August_2020 - 21
Evaulation_Engineering_August_2020 - EMC/EMI Test
Evaulation_Engineering_August_2020 - 23
Evaulation_Engineering_August_2020 - 24
Evaulation_Engineering_August_2020 - 25
Evaulation_Engineering_August_2020 - Industrial Automation
Evaulation_Engineering_August_2020 - 27
Evaulation_Engineering_August_2020 - 28
Evaulation_Engineering_August_2020 - 29
Evaulation_Engineering_August_2020 - Featured Tech
Evaulation_Engineering_August_2020 - 31
Evaulation_Engineering_August_2020 - Tech Focus
Evaulation_Engineering_August_2020 - 33
Evaulation_Engineering_August_2020 - Industrial IOT
Evaulation_Engineering_August_2020 - 35
Evaulation_Engineering_August_2020 - 36
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