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TRM usually recommends the use of a good quality screened cable and termination to one end to avoid earth loops. We have found this to be successful in many installations around the world. There are some cases where termination at both ends might be necessary and an explanation of this can be found in the text below.

 

Quoted from the Ofcom (EMC Awareness) website.

Screened cables are used to reduce coupling of the inner cable conductors with the environment they pass through, but it must be appreciated that the screen functions differently at high frequencies and at low frequencies, and also has different effects as between electric field and magnetic field coupling.

How this technique is used

For low frequencies an overall screen, grounded only at one end, provides good shielding from capacitively coupled interference but none at all from the magnetic fields, because this can only occur if current flows in the screen. To shield against a magnetic field, both ends of the screen must be grounded. This allows an induced current (IS) to flow in the screen which will oppose the current induced in the centre conductor. The same principle applies when shielding a conductor to prevent magnetic field emission.

 

To minimize low frequency magnetic field pick-up, one end of the circuit should be isolated from ground, the circuit loop area should be small, and the screen should not form part of the circuit. You can best achieve this by using shielded twisted pair cable with the screen grounded at only one end. The twisting minimizes magnetic coupling, and the screen will reduce external capacitive coupling.

 

Once the cable length approaches a quarter wavelength at the frequency of interest, screen currents due to external fields become unavoidable. An open circuit at one end of the cable becomes transformed into a short circuit a quarter wavelength away, and screen currents flow in a standing wave pattern despite there being no external connection. Even below resonant frequencies, stray capacitance can allow screen currents to flow. For the screen to be successful, it must be connected to ground at both ends to divert these currents away from the internal circuit.

Key issues in employing this technique

Grounding both ends of the shield

A frequent reason given for only grounding the screen at one end is that if there is a significant voltage difference between the connection points at each end of the screen, current will flow in the screen if both ends are connected. This could be enough to damage the cable. Such voltage differentials are not unusual on large sites or between buildings. The preferred solution is to install the cable in a mesh-bonded ground network using parallel earth conductors (PECs), and certainly for new telecomm installations this approach should be insisted on. This type of installation prevents significant ground potential differences.

Quality of the screen

At high frequencies the inner and outer of the screen are isolated by skin effect, which prevents currents on the surface from passing into the bulk of the conductor. Therefore signal currents on the inside of the screen do not couple with interference currents on the outside, and multiple grounding of the screen does not introduce interference voltages on the inside to the same extent as at LF. This desirable effect is compromised by a braided screen due to its incomplete optical coverage and because the strands are continuously woven from inside to out and back again. Foil screens do not exhibit this effect, but introduce other compromises due to their higher resistance; the optimum screen is a combination of foil and braid. The separation is also more seriously compromised by the quality of the screen ground connection at either end.

The screen ground connection

A pigtail connection is one where the screen is brought down to a single wire and extended through a connector pin to the ground point. 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, which appears in series with the cable screen and develops a voltage when interference currents flow down the screen to the ground connection. This voltage then couples readily from the end of the screen to the inner conductors. The emissions coupling process is reciprocal. The impedance of such a connection rises rapidly with increasing frequency and effectively negates the value of a good HF screened cable.

 

The best connection is one in which the screen is extended up to and makes a solid 360° connection with the ground plane or chassis. This is best achieved with a hard-wired cable termination using a conductive gland and ferrule which clamps over the cable screen. Military-style connectors allow for this construction.

 

Of the readily available commercial connectors, only those with a connector shell that is designed to make positive 360° contact with its mate are suitable. Examples are the subminiature D range with dimpled tin-plated shells. The cable screen must make 360° contact with a screened, conductive backshell which must itself be positively connected to the connector shell.

 

If you can’t avoid a pigtail connection then make it as short as possible, and preferably doubled and taken through two pins on opposite ends of the connector so that its inductance is halved. The effective length of the pigtail extends from the end of the cable screen through the connector and up to the point of the ground plane or chassis connection. Cable screens must always be taken to a point at which there is the minimum noise with respect to the system’s ground reference. If there is a conductive enclosure then the screen should be clamped to this, not to a circuit connection on the PCB such as 0V.

Surface transfer impedance

The screening performance of shielded cables is best expressed in terms of surface transfer impedance (STI). This is a measure of the voltage induced per unit length on the inner conductor(s) of the cable by an interference current flowing down the cable outer shield. It can be used to refer to the cable alone, or to the cable/connector assembly.

 

A perfect screen would not allow any voltage to be induced on the inner conductors and would have an STI of zero. Practical screens will couple some energy onto the inner. STI will vary with frequency and is normally expressed in milliohms per metre length. At low frequencies it is equal to the dc resistance of the screen, but at higher frequencies the STI is dominated by the effect of mutual inductive and capacitive coupling between the screen and the inner conductor.

Contact

TRM International Ltd.
86 Moss Road
Southport, PR8 4JQ, UK.

Tel: +44 (0) 1704 563777

sales@trminternational.com

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