EMI
Electromagnetic interference (EMI) is an ever-increasing concern for Data Communication cables due to the use of lower signal voltages applied as well as ever-increasing electrical signals that pollutant the working environment.
Shielding reduces electrical noise and reduces its impact on signals and also lowers electromagnetic radiation, applying good shielding practices on cables and cables assemblies not only protects cable but it can also protects the investment of capital equipment by reducing down time and interference in peoples everyday life.
Typical types of commonly used cable shielding are either braided, spiral design or metal-coated Mylar or foil shield.
The maximum shielding a braided shield can have is app. 95%. Spiral shields can offer 98%, while metal-coated Mylar can deflect 100% of EMI.
Using a thin layer of Aluminum Polyester backed foil covers the gaps in braids and provides the best overall EMI shielding possible
A cable shield consists of a respective amount of strands, depending on the braiding machine (16, 24, and 32). The total amount of strands equals the number of strands in a braiding element times the amount of elements. The strand diameter, the angle of twist and the folding number also determine the density of a shield.
Shielding Braiding
Throughout the last 25 years we have been known by our customers as a manufacturer of high-quality braided wire products in a broad range of flexible cables used in complex and demanding applications in a broad range of industries.
Our fully equipped braiding facilities enables us to offer fine wire braiding using a 16 carrier fine wire braiding machine, as well as standard braiding equipment in 16-24 and 36 carrier configurations.
Science Behind Braid Optimization
LOROM’s development of software models using Dr. Kleys Methods reduces cost and at the same time increases EMI performance by reducing significant sampling and prototyping.
The three basic phenomena that best describe electromagnetic coupling are:
- Diffusion of the E and H fields through the shield material
- Penetration of the fields through the small apertures of the braided shields
- An induction phenomenon due to overlapping of the individual carriers of the shield
Naturally, the last two phenomena only occur for braided shields.
Since the transfer impedance dominates heavily at lower frequencies and for good cables, there has not been much work done on the transfer admittance.
One does know that the admittance increases with increasing frequency. It is however often neglected when discussing characteristics of the shield. For this thesis project, the transfer admittance lies out of scope and will not be treated any further.
Shielding Served Spiral
Poor choices in regards to cable shielding technologies are often the cause of cable failure in the field. The shield is often the root cause of the cable failing to perform in its intended application.
Fundamental cable geometry dictates that the shield is the placed at the greatest distance from the neutral axis of the cable and therefore sees the most stress during bending, torsion and flexing.
We at LOROM understand cable design and very often-served shield are an excellent choice over conventional braided shields. Choosing a correctly deigned double-served wire shield, optimizes flex life and shielding effectiveness even after severe abuse.
In addition, we can apply low friction buffer tapes of non-metallic woven fabrics, which isolate the shield from the conductor bundle or core and the outer jacket. This will reduce internal friction that the cable will see if it is subjected to multiple flexes in its application.
We at LOROM understand these material issues and have several low coefficient of friction options, both static and dynamic, that will protect your investment.
Braiding Cover Capabilities
LOROM’s capabilities in braiding covers:
- Braid – Wire sizes 30-48 AWG single
- Foil Shield -Longitudinal AL/PET foil can be applied in line with braiding bonded or not bonded to dielectric
- Flat Braid – Using flat silver plated wire for micro wave coaxial cables for size reduction and increased EMC/EMI performance
- Shielding Braids – added to assembled cable bundles or wire harnesses.
- Braided Sleeves – added to assembled cable bundles or wire harnesses.
- Flat Ground Straps – braided round conductors flattened to use as ground straps or flat power leads.
- Scope Braids – stainless steel braids used as reinforcement or armoring.
- Braid Reinforced Tubing –Braiding of fiber glass – combined with thin wall extrusion or dispersion of applicable compounds
Model for Braided Shields
Normal woven braided shields are defined by six parameters:
- C – the number of carriers in the shield
- N – the number of filaments (strands) of each carrier
- d – the diameter of the filaments
- d0 – the diameter under the braid
- a – the weave angle
- K – the optical coverage of the braid
There are also other parameters that may be used when discussing braid layouts:
- F – the fill factor of the braid, i.e. how much one layer of the braid covers.
- S – the lay length, i.e. how long distance it takes for one carrier to complete one turn around the cable.
Advantages and Applications
Advantages:
- Superior flexibility
- Increased flex life
- Up to 97% optical coverage
Applications:
- Audio, i.e. headsets
- Robotic and factory automation
- Medical application for flexibility
- Hand held scanning devices
- Spiral and retractable cables
- Pick and place machinery
- Stylus and pens for point of sale equipment
