Wire and cable can often cause EMC problems that need to be considered early in the design phase. Here, Ron Brewer describes four fundamental design requirements to be followed to avoid EMC problems in external wire and cable.
Wire is not ordinarily thought of as an electromagnetic compatibility (EMC) problem. It is just assumed that this passive device will provide the needed conductor path for the electrons to move from one place to another. This also happens with connectors, with grounds and even with power supplies. But wire does not just sit there. It is a unique component that can be a conductor, a resistor, an inductor, part of a capacitor and even an antenna, depending on the wire material, its continuity, the surroundings and the operating frequencies.
Unwanted electromagnetic effects result primarily from the inductive, capacitive and antenna effects. The worst case is all three of these wire characteristics acting simultaneously. Bundling several wires together to form a cable dramatically complicates the situation. Two wires have 63 possible combinations of the three wire characteristics, three wires have 511, four have 4095 and so on - and this does not even include the connectors!
Connector issues
Ideally, a wire should be continuous from one end to the other, with the ends metallurgically bonded at the circuit termination points. Manufacturing and configuration requirements that allow easy separation of the wire generate the need for a connector.
Each time a connector is introduced into a wire, three additional series junctions are added to the conductor path, and one of these junctions is a switch. It was not planned that way, but the connector construction consists of spring-loaded contacts pressing against a round or flat mating surface, just like a switch. In addition to the arcing that occurs during the occasional hot insertion or removal, mating surfaces made with high-resistance contact materials or inadequate contact pressure permit arcing during shock and vibration, which may be a continuing problem, and allow the penetration of electrolyte between the mating surfaces. Arcing directly produces broadband conducted and radiated interference along with intermittent functional operation of the system. Corrosion by-products produce nonlinear behaviour.
Connector manufacturers have overcome many of these problems by using sliding contacts, precious metal plating and high localised contact pressures that create a gastight seal. But, it is still up to the system designer to select, install and utilise the wire, cable and connectors in the final application.
If the wire or cable leaves the confines of the system, the unwanted radio frequency (RF) current in the wire represents a conducted emissions problem. If the wire is directly connected to an external source of unwanted RF current, then there is a potential conducted susceptibility problem. In addition, wire makes good antennas (especially in the near field) that work well for both radiated emission and radiated susceptibility coupling. Although there are a number of things that can be done with external wire and cable to meet both inter- and intra-system EMC, four fundamental design requirements must be faithfully followed:
1. Short wire and cable: Keep wire and cable as short as possible and always pair the outgoing and return leads. Each wire in the cable is a conductor associated with a circuit loop, and high-frequency loop areas outside of the system enclosure must be minimised because both radiated emission and susceptibility are direct functions of the loop areas. Twisted-pair and coaxial cables keep their outgoing and return leads closely coupled as a result of their mechanical configuration.
By reducing the coupling area, both inductive and capacitive crosstalk within the cable is also reduced. For radiation coupling of differential mode currents and magnetic field reduction, twisted-pair line has a distinct advantage over coaxial line because each half-twist creates a small coupling loop that is out of phase with its neighbour and, thus, provides some field cancellation. Over the past 10 years, twisted-pair construction/balance has been improved to the point that it is now being used for high-speed differential signalling at frequencies over 10 GHz.
2. Segregating wire: Segregate wiring based on signal-level classification to reduce inductive and capacitive crosstalk within the cable. Five classifications are suggested, but Murphy's influence still prevails: high-power RF, alternating current/direct current (AC/DC) and pulse; low-power RF and AC/DC; high-speed digital, video output and sync pulse; audio, telecommunications and video input; and highly sensitive circuits.
Sometimes, the segregation can take place within a single but costly cable, but often it requires a separate cable for each classification. Furthermore, each cable generally has two connectors. Occasionally, the end-to-end connection may have four connectors just to terminate the shield on a cable. In the interest of reducing cost and improving signal integrity, unnecessary connectors should be eliminated. Figure 1 shows methods of terminating the cable shield without using connectors. These methods work well at the lower frequencies, and often provide 6 to 8 dB at frequencies in the 600 to 800 MHz range.
3. Minimise circuit bandwidth: Sometimes, minimising circuit bandwidth is a major point of contention because reducing system speed is not a popular marketing option. However, many external wires and cables are operating at high speeds or are being driven by high-speed logic devices when it is not necessary. Because radiated susceptibility increases with frequency and radiated emission increases as a function of frequency squared, reducing the bandwidth will desensitise the circuits to external susceptibility fields and will dramatically reduce emissions. This is especially true with those circuits connecting to long external wire and cable.
Also, because both inductive and capacitive crosstalk increases with frequency, this helps reduce crosstalk problems within the cable. Although expensive, one way of reducing cable bandwidth is to use filter pin connectors. Another is to use ferrites.
4. Shield problem circuits: This noninvasive suppression technique reduces both the radiated emission and susceptibility of a circuit. Because shielding is not inserted into the circuit, it does not affect high-speed operation or signal integrity. In fact, it is the only suppression technique that can be used to attenuate unwanted RF energy on wire and cable within the operational passband of the cable.
Cable shielding differs from enclosure shielding because, unless the terminating circuits at both ends of the cable are also shielded, the outgoing and return leads pass completely through the shield forming a coaxial configuration. Care must be taken to assure that radiated coupling loops are not formed within the enclosure by the cable shield termination method. And the shield should definitely not be terminated directly to the printed circuit board (PCB). If PCB termination of the outer conductor of a coaxial line is required for functional reasons, then the outer conductor is not a shield, it is the return for the centre conductor. Coaxial lines are not shielded wires! Shielded coaxial lines are called triaxial cables.
Integrity of the shield
If the cable is being used with a shielded enclosure, its design must protect the integrity of the shield. There are two ways to do this: filter the wire or shield the cable. Filtering the wire is not as easy as it sounds because to protect the shield, the filter must be inserted into the shell of the enclosure. This can best be done by:
* Constructing the filter within a small shielded metal box (called a 'doghouse') within the overall enclosure located behind and shielding the backside of the connector.
* Using a filter pin connector.
The advantage of the filter-in-a-box approach is that the filter can be constructed with large-value capacitors and inductors. This permits a filter design with low cutoff frequencies. On the other hand, the advantage of filter pin connectors is primarily one of convenience. Unfortunately, filter pin connectors are expensive, and because of the small size of the filter components, the cutoff frequency is quite high. Regardless of the approach, the filter insertion loss should be approximately the same as the shield attenuation in the critical frequency ranges.
Shielding the cable is not easy either. The best-shielded cable generally fails as a result of the interconnection of the shield with the connector. The primary problem is the connection of the cable shield to the connector, followed closely by the connection of the backshell to the connector and finally, the connector to the enclosure junction. These leakage points and their solutions are illustrated in Figures 2 and 3. Note the cable shield has been brought through a pin in the connector and then routed to ground within the enclosure. This is one of the worst possible leakage points!
Other approaches can be used to improve the connector shielding. For example, many military connectors have been specifically designed with round bodies to enable uniform compression forces against the mating halves and against the enclosure surface to provide a low-impedance continuation of the cable shield. Others have special backshells available to do the same thing. DIN and RJ connectors were not designed to be used in shielded applications - but they are. To accomplish this, special RF gasketing is added to allow them to be used in such applications.
Conclusion
Wire and cable are often EMC problems that need to be considered early in the design phase. They cannot be taken for granted, and the connectors must be considered as an extension of the cable. Because the unwanted electromagnetic effects result primarily from the inductive, capacitive and antenna effects, system designers must design, select and install cable using techniques that minimise corrosion, crosstalk, insertion loss and RF emission. All of this must be accomplished while protecting the overall enclosure shielding.
For further information contact Connecta, 011 463 2240.
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