Electromagnetic Compatibility (EMC) is one of the key factors of consideration in the design, construction, manufacture and installation of electrical and electronic equipment and systems. Electrical equipment must be designed not only to meet a functional technical performance specification but due consideration must also be given to the interaction the equipment has with the electromagnetic environment where the system is intended to be installed.
Cable characteristics can have an impact on the EMC performance of a system. Unshielded twisted pair cable is often employed for low and medium special speed data links. In the presence of an incident electromagnetic wave, voltages will be induced in the cable both in common mode and differential mode. Even though the differential voltage is minimized at low frequencies by the twists in the cable, the common mode voltage remains and will be applied to the circuit to which the cable is connected. The balance of the circuit is therefore essential and if the impedances to ground at the circuit are equal, the conversion to unwanted differential mode noise is minimized. However the challenge comes up when parasitic components such as stray capacitance tend to unbalance the circuit at high frequencies and the ability to reject common mode disturbances reduces rapidly with increasing frequency hence causing interference. Therefore additional protection can be provided by the addition of an outer screen typically of copper braid or aluminum foil. At high frequencies, the electromagnetic energy is confined to the inner and outer surface layers of the screen by skin effect, and isolation between the inner conductors carrying the wanted signal and the external environment can be readily achieved through good cable screen design.
Contents
The performance of cable screens depends on their construction. In the following section we look at some common types of cable screens.
Lapped wire screens consist of wires helically wound onto the cable. They are very flexible, but have poor screening effectiveness and are evidently inductive at high frequency, and therefore they are limited to audio use.
Single braid screens consists of wire woven into a braid to provide a metallic frame covering the cable, offering 80-95 % coverage and reasonable high-frequency performance. The braids add significantly to the cable weight and stiffness.
Laminated tape or foil with drain wire provides a full cover but at a rather high resistance and therefore has only moderate screening efficiency. Light weight, flexibility, small diameter and low cost are retained.
Making a proper termination to this type of screen is hard, screen currents will tend to flow mainly in the drain wire, making it unsuitable for magnetic screening, although its capacitive screening is excellent.
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Composite tape and braid combines the advantages of both laminated tape and single braid to optimize coverage and high frequency performance.
Multiple braid screens enhance the performance of single braids by separating the inner and outer current flows, and allowing the screens to be dedicated to different (low and high frequency) purposes.
The prevailing requirement for terminating a cable screen is a connection direct to the metal chassis or enclosure ground which exhibits the lowest possible impedance. This ensures that interference currents on the shield are routed to ground without passing through or coupling other circuits. The recommended connection in this respect is one in which the shield is extended up to and makes a solid 360° connection with ground plane or chassis as illustrated in the figure below:
The above is best achieved with a hard-wired cable termination using a conductive gland and ferrule which clamps over the cable screen. The cable screen must make 360 ° contact, with a screened, conductive backshell which must itself be positively connected to the connector shell.
If the cable screen is made up into a “pigtail” connection, performance at high frequencies will be severely degraded. A pigtail connection is one which the screen is brought down to a single wire and extended through a connector pin to the ground point as illustrated in the figure below:
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Because of its ease of assembly, it is very commonly used for connecting the screens of data cables. Unfortunately, it may be almost as bad as no connection at high frequencies because of the pigtail inductance. At high frequencies, the impedance of the pigtail connection becomes significant and is a point of common coupling between the inner conductors and the external environment, increasing emissions and reducing immunity to external disturbances. Therefore, pigtail screen connections should be avoided.
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