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Shielding Cables


The connecting cables between devices and their connectors are the preferred route by which electromagnetic interference propagates in the external environment and by which, from the outside, it couples to the electrical conductors that exchange signals between the devices of the system, thus creating situations of susceptibility.

It should be remembered that cabling is an area of uncertain responsability; it is never clear who should choose the cables, their route, or their location inside the system. Therefore it may happen that an apparatus is designed perfectly from the EMC point of view, but that the choice of the connecting cables between the apparatus itself and the outside world is not made with equal care. Thus, the apparatus-cable whole may not pass the tests of EMC. There are essentially three mechanisms by which phenomena of emission and susceptibility manifest themselves in electrical connections:
  • radiated emission from cables, which act as transmitting antennae;
  • radiation susceptibility of the cables, which act as receiving antennae;
  • electric and/or magnetic coupling between cables.


In the control of interfering emission the principle of reciprocity rules, in the sense that what is done to a cable to reduce radiated emission is also valid for increasing its resistance to radiation susceptibility.

As far as cable screens are concerned, it must be remembered that their effectiveness depends on the value of the SE of the cable braid and on the way in which the screens are terminated. From the termination of cable screens point of view, connectors come into play; these are an indispensable means of connection between cables and devices.

Only recently has the great difflculty of seeing cables and connectors as two separate entities been overcome and have they begun to be considered as one. In the literature there exist many studies aimed at verifying and quantifying the effect of the connection of a cable screen to the chassis ground of an apparatus (the so-called pigtail); everyone agrees on the criticality of this element with regard to screening efficiency and radiation.

In any case the influence of the type of connector starts to make itself felt at a few MHz, or the frequency at which the length of the cable is equal to a tenth of a wavelength.
Overlooking the cable contribution to the system EMC performance can ruin the effect of the best system shielding. In fact, cable shielding very often is the weakest link in the overall system shielding.

For a given frequency, the key contributors to the overall performance of a shield placed upon a cable are :
  • shield material
  • termination method (clamp, solder, pigtail, type of connector)
  • geometry of the installation (cable lenght, cable height, orientation vs. the field pointing vector, type of conductive ground)

In the final analysis, each of these contributors behaves differently, depending on the nature of the field : near E-field, near H-field or far field.

SHIELD MATERIAL AND THICKNESS VS. THE TYPE OF EMI

A cable shield can be made of a thin aluminum film flashed over a paper or mylar substrate, a copper braid, a thinly wrapped metal foil, a corrugated pipe. All these material have basic shielding properties which depend on two mechanisms reflection and absorption.
Reflection is a radiated phenomenon and is the result of the mismatch between the impinging wave impedance and the shield barrier impedance.
Absorption depends on the skin effect. At frequencies high enough for the skin effect to occur, absorption becomes significant. If the shield is a solid tube, this absorption will increase exponentially. If the shield is a braid, all the minuscule rhombic aperture due to the weave of copper wires will spoil the absorption effect by making the shield more and more trasparent as frequency increases. Depending on the optical coverage, this effect can show up as low as 1 to 10 MHz, ruining the Shielding Effectiveness (SE) of the braid.

SHIELD TERMINATION

Where and how the shield is terminated can radically affect the performance of a shield. Once a cable shield is mounted, its termination may be the weakest link in the chain, especially at higher frequencies. As a first approximation, the quality of a shield can be associated to its DC sheath resistence, altough this statement becomes invalid above the HF region. Since it is very difficult to have the shield terminated by a connector clamp or pigtail whose impedance is much less than the shield material impedance, the termination hardware is always the limiting factor of the in situ performance.

For example, a braid shield resistance of a 0.75 m piece of shielded cable, with a braid resistance of 3 mohm/m is equal to 2.25 mohm. A typical bonding resistance of the braid to the serrated clamp is 0.5 mohm. An ordinary hand-tightened connector shall exhibits a contact resistance of about 3 mohm to the receptacle. Finally, the contact resistance of the connector socket flange to the rack wall, assuming it is normally tool tightened using four screws, is again 0.5 mohm.
Since there are two ends to this cable, the total termination resistance is :

Rterm = 2 (0.5 + 3 + 0.5) = 8 mohm > 2.25 mohm of the braid shield resistance

Il contributo delle terminazioni sulla resistenza complessiva dello schermo è maggiore di più di tre volte quella della calza considerata da sola.


The termination contribution to the whole shield resistance will be more than three times that of the braid alone. Not only can an improper shield termination spoil the cable shield efficiency, it can be the source of secondary mechanism which can cause more EMI than if there were no shield at all.

The key for preventing or solving such problems is always the same : identify any existing EMI current paths.

OVERALL GEOMETRY OF THE INSTALLED CABLE

Lo schermo di un cavo può essere visto come la congiunzione di due linee di trasmissione, con lo schermo che è il mezzo di trasferimento tra loro. I due modelli circuitali sono :
  • Il circuito interno 2 ha parametri ben controllati come le impedenze di sorgente Zg e di carico Zl, e impedenza caratteristica Z02. Questi parametri definiscono i valori di corrente e tensione in ogni punto nel circuito, includendo gli effetti di riflessione e onde stazionarie.
  • Il circuito esterno 1 ha parametri molto poco controllabili, in quanto ognuna delle impedenze di terminazione Zg1 e Zg2 possono variare da 0 a infinito, a seconda delle condizioni di grounding, e la sua impedenza caratteristica Z01 dipende dal rapporto h/D, ovvero l'altezza del cavo sopra il piano di massa rispetto al diametro del cavo.
L'efficienza di uno schermo è infatti la misura della percentuale di energia trasferita dal circuito 2 al circuito 1 nel caso di emissione EMI, oppure trasferita dal circuito 1 al 2 nel caso di suscettibilità.

L'impedenza caratteristica del circuito1 ha un notevole effetto su questo trasferimento di energia, specialmente a valori multipli di landa/4.

Perciò, per un dato schermo installato, con certe terminazioni ci saranno comportamenti diversi, a seconda della sua lunghezza e della sua altezza sopra il relativo piano di massa.


UNBALANCED VS. BALANCED SHIELDED CABLES

La definizione di base dell'efficienza di schermatura (SE), è:

SE(dB) = 20 log [Vindotta senza schermo / Vindotta con lo schermo ]

Il metodo di terminazione dello schermo influenza fortemente tale risultato e può nascondere completamente i parametri da misurare.

A causa di questi problemi, una misura preferibile per valutare la qualità dello schermo, è quella che sfrutta la grandezza denominata impedenza superficiale di trasferimento Zt. Questa grandezza fornisce in pratica una figura di merito come parametro assoluto, piuttosto che relativo come la SE definita sopra. Inoltre, la misura di Zt è relativamente facile da effettuare.

DEFINITION OF SE FOR CABLE SHIELD

The basic definition of SE is :
SE(dB) = 20 log [Vinduced without shield / Vinduced with shield ]
The shield termination method strongly influences the result and can even totally obscure the parameter being measured, i.e., the quality of the shield alone.
Because of these problems, another measure of the shield quality, the shield surface transfer impedance ZT is preferred. It provides a figure of merit which is an absolute parameter, instead of being a relative term like SE. Also, the measurement of ZT is relatively easy to perform.

TRANSFER IMPEDANCE OF COAXIAL CABLE SHIELDS

To define the quality of a cable in the EMC sense, the concept of surface transfer impedance has been introduced. Consider figure, where we show the screen of a cable in which a surface current Is flows and let DV be the voltage drop that is generated inside the cable over a length ,DX.

Then the surface transfer impedance Zt is defined as: < Zt = ΔV / (Is· Δx) (ohm / m) Below about 100 KHz, Zt is practically equal to the shield DC resistance.
The transfer impedance will consist of two components :
  1. penetration component representing the energy diffusion through the metal of the screen
  2. coupling component representing the H field diffusion through the rhombus-shaped holes
The complete Zt for the braided wire shield can be written :
Zt = RDC + jwM12
At very low frequency, the STI is equal to the direct current resistance of the braid.
With increasing frequency, two effects will occur :
  • due to the skin effect the STI will drop
  • due to the H field coupling through the braid structure there will be a rise of the STI proportional to the frequency

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