Saturday, January 29, 2011

Cable Shielding

Cable shielding is used to help shield the signal line or lines in a cable from outside interference as well as prevent the signal from radiating noise to the outside world. The shield is usually grounded at one or both sides of the cable. There are those who say you should always ground one side of a cable shield only, at both sides every time, at one side or both sides depending on the frequency of the cable's signal, or at one side with a capacitor, resistor, capacitor and resistor, or some other scheme.

This can make choosing the best shield termination scheme for a particular application difficult. It seems there are more data sources dictating to you the best action to take (or at least the author's opinion of it) without informing you to why, which is really no help at all. While there are no be all end all rules when it comes to shielding termination, understanding a few common points regarding cable shielding can help you make an educated decision for you application.

One Side or Both Sides, What's the Difference?

The main difference between grounding one or both sides of a cable shield is the type of radiation protection offered. Grounding one side of a cable's shield provides electric field protection. Holding the shield at a constant potential makes all electric field noise outside the cable invisible to the internal conductor or conductors. This shielding effect is exactly what we think of when we hear the word “shield” (think of the Star Trek enterprise shielding itself from a Klingon attack).

Single sided shield grounding, unfortunately, provides no protection to magnetic field radiation, which simply plows through our measly single sided shield. Magnetic radiation, in fact, is extremely difficult to shield in a 'blocking' manner without the use of exotic shielding techniques involving very thick shields made of magnetic material.

The best way to deal with magnetic radiation is to use its own force against itself (just as a smart Ninja would do to a larger attacking enemy). This is done by grounding both sides of the shield. Doing so allows magnetic field radiation to push current through the shield, which in turn creates its own radiation focused on the internal conductor or conductors (but in the opposite direction of the original radiation) to cancel out the impact of the noise on the signal. With only a single side grounded, however, there can be no current flow in the shield, and therefor no “shielding” effect to magnetic field radiation.

It is important to note that grounding both ends of a cable has potentially negative consequences. If there is a difference in ground potential at either side of the cable, ground loops can form. The current flowing through the shield due to the ground differential creates radiation which is injected into the cable's signal conductor or conductors, actually adding noise to the signal. So, while grounding a cable at both ends can improve things, it can also make things worse.

It is also important to note that this form of magnetic shielding only works at high frequency (typically above audio frequencies). This is why some recommend grounding one side of a cable for slow signals and both sides for high speed signals. The thinking is that as magnetic shielding is only effective at higher frequencies, there is no point trying to use it for lower frequency signals (and risk the possible negative consequences of ground loop noise). Shielding works both to protect radiation from the signal lines and to protect the signal lines from external radiation, however, so magnetic shielding may still be beneficial for low frequencies signals in certain applications.

One more note on electric field and magnetic field radiation, such radiation is typically only a “near field” (within approximately one wavelength of the emission source) concern. As either electric or magnetic radiation moves further away from the source it turns into electromagnetic radiation, which is effectively dealt with using electric field shielding techniques. This means that by routing a cable away from magnetic radiation sources the need for magnetic shielding can be reduced. Magnetic field noise radiating from the cable however, will still be an EMC concern.
Termination is Everything

When it comes to shielding, the actual connection (or termination) of the shield to ground is extremely important. The connection should be a 360 degree connection (all the way around the cable) and as low impedance as possible. Sometimes, the shield is rolled into a “flying lead” wire at the end of a cable and inserted into a terminal block type connection or tied down with a screw. This is undesirable for multiple reasons. For one, the inductance of the flying lead increases its impedance at high frequencies, killing its shielding effectiveness where it matters most. Secondly, the current flowing through the cable shield tends to bunch up towards the side of the shield that the lead is coming off near the end of the cable, effectively making the exposed or unshielded portion of the cable look bigger from a shielding perspective. Finally, the flying lead connections at the end of the cable leaves part of the signal wire or wires completely exposed without shielding. The best connection is a 360 degree connection directly to the outside of the enclosure. This ensures the entire signal wire or wires are protected and that there is no room for radiation to be carried into or out of the enclosure.

It should also be noted that as a low impedance connection is important, any scheme involving a resistor is a lost cause. Some may, for example, recommend terminating both sides of a cable, but with one side through a resistor (such as 100 Ohms) to help limit ground loops. In order for magnetic protection to work, we need to allow current flow through the shield. A resistor would limit that current flow significantly and in the process limit the shielding effect significantly.

You might be thinking, why not use a capacitor at one end to allow high frequency magnetic shielding while blocking DC ground loop currents. This is a good idea with a difficult implementation. Using a capacitor typically means using a flying lead single point connection between the shield and capacitor, the inductance of which kills the low impedance, high frequency response we were striving for. There are, however, some special (read expensive) cables out there designed with 360 degree capacitive shielding termination at one end.

If you have the money, there are also some more exotic shielding solutions as well, such as double shielded cables (featuring a shield inside a shield). In addition to shielding, there are alternate noise fighting techniques, such as using a balanced signal over a twisted pair of wires, perhaps a subject for a future entry. For now, I hope to have provided at least some insight into the basic physics behind cable shielding.

No comments:

Post a Comment