Shielding is a technique used to control EMI by preventing transmission of noise signals from the source to the receiver. Shields can be located at the source or at the receiver or anywhere in between. In the case of electric fields where it is most effective, it is a function of the shield materials thickness, conductivity and continuity.
The purpose of the shield on a cable is to protect the inner conductor(s) from impinging electric fields and thus lessen the extent of the magnetic field. Remember that when an electric field comes in contact with a conductor, it creates a voltage which appears as noise onto the victim circuitry, this is primarily due to high impedance circuits being more sensitive to the induced noise. As a result, the only way to remove that noise is to filter it out with decoupling capacitors and more complex filtering such as Pi filters. This solution creates a “low pass” filtering effect which allows the transmission of data while effectively blocking the coupled higher frequency noise.
When a grounded shield is placed around the conductor, the electric field energy is usually drained away without affecting the conductors reducing or removing the noise.
NOTE: Utilization of shields is rarely 100% effective, so the residual noise, while reduced, will only be eliminated by employing a filtering solution!
Note: Regardless of the density of the shield, the physics of shielding remains the same.
There are two principle mechanisms present in all shields, and these are reflection and absorption.
When an electromagnetic wave traveling through space encounters a shield two things happen.
First, much of the energy is reflected and then second some of the energy that is not reflected is then absorbed by the shield; only the residual energy emerges from the other side of the shield. These two effects of reflection and absorption are independent from each other, but when they combine together, they produce the overall shields true effectiveness.
A third factor called re-reflection occurs in very thin shields. This re-reflection occurs at the shield boundary on the far side of the shield material.
Most high frequency shielding problems are caused by openings in the shield material, and not by the material itself. Most conductive materials such as aluminum, copper and mild steel provide substantial electric shielding. At frequencies from 30 to 100 MHz, aluminum foil provides at least 85 dB of shielding effectiveness. Unfortunately, aluminum foil is extremely inadequate against low frequency magnetic fields, where thick steel or highly permeable ferrite material provides more adequate shielding. These conditions are just some of the many reasons that shielding has limited effectiveness, creating the requirement for added filtering.
History of the Faraday Cage
Michael Faraday lived from 1791—1867, and was an English scientist. Faraday’s experiments yielded some of the most significant principles and inventions in scientific history. He developed the first dynamo (in the form of a copper disk rotated between the poles of a permanent magnet), the precursor of modern dynamos and generators. In addition to other contributions he did research on Electrolysis, formulating Faraday’s law.
Faradays law: Is a physical law stating that the number of moles of substance produced at an electrode during electrolysis is directly proportional to the number of moles of electrons transferred at that electrode; the law is named for Michael Faraday, who formulated it in 1834. The amount of electric charge carried by one mole of electrons (6.02 x 1023 electrons) is called the faraday and is equal to 96,500 coulombs. The number of faradays required to produce one mole of substance at an electrode depends upon the way in which the substance is oxidized or reduced.
Note: a MOLE = One gram-molecular weight of any molecular substance contains exactly one mole of molecules. The term mole is often used in place of gram-molecular weight; e.g., one speaks of 18 grams of water as one mole of water rather than as one gram-molecular weight of water. The mole is a unit in the International System of Units (SI).
Faraday Cage: The Faraday cage was originally designed to demonstrate the principles of static electricity, and thus allow the user to investigate and manipulate the electrostatic phenomena. Generally, a Faraday cage consists of an Iron mesh or Copper mesh completely surrounding a square wooden housing or cylinder. The Faraday cage originally proved that static electricity can be controlled within an ungrounded cage that is bonded to the structure in a continuous 360 degree manner.
Types of EM Fields to Consider
In analyzing shielding it is helpful to consider the three types of fields that occur. These different field types explain why the same shield can behave differently under different operating conditions.
Plane waves only exist in about 1/6 of a wavelength from their source. In this condition the ratio of the electric field as compared to the magnetic field is constant; and this event is also known as far field radiation.
A good example of this is radio waves, where at 30MHz a wavelength is expressed as 10 meters, and so any transmitter more than about 10/6 or 1.6 m away the source is expressed as the far field.
Electric Fields (E-Fields)
If the energy field is less than 1/6 of a wavelength from a high impedance source, then the wave impedance is known as near field source; and thus a capacitive energy dominates the field during the near field effect, and this is because of the higher wave impedances, thus the EM loss tends to be greater. This is why it is possible to do more effective shielding from these impinging electric fields.
A simpler way of looking at this effect is to understand that the culprit electric fields produce voltages onto the victim circuitry. For instance, if you suspect that a given analog interconnect is producing an EMI event, then try disconnecting the wiring from the circuit driving the line, and then short the signal pair together; any voltage differential should be shorted out and the input should quiet down, confirming that the electric field produced was the culprit.
Magnetic Fields (H Fields)
If you are too close to a low impedance source, or a current source, then a near field energy source is produced; but what differs in this case, is that the inductive energy predominates. Reflection losses are much less effective here due to lower wave impedances and this effect continues as you drop in frequency; So, this is the main reason why shielding becomes less effective against low frequency magnetic fields, and why at this point in your design balanced circuits of twist pair wires are so important.
Another way of looking at this effect is that magnetic fields produce currents over and onto their victim circuits. If you suspect that a given analog interconnect is magnetic field, then try disconnecting the wiring from the circuit driving the line and leaving the signal pair open; any current flowing should be stopped, this will confirm magnetic field coupling. The application of MU-Metal to cover transformer windings will cancel or at least greatly reduce magnetic field disturbances in most cases.
Shielding Application Facts:
- Shields reduce Differential mode coupling
- Shields reduce Electric field coupling onto wires
- Shields Reduce voltage transients
- Shields Reduce lightning transients
- Shields, Supply a Faraday-Shield/Cage Return Reference
- Shields, Control ESD path
- Shields Enhance Filtering, and Isolation techniques
**Important Note: Do not daisy-chain / or attach, more than 6 (shield) wires per Chassis Ground Termination. The use of multiple shielding techniques, by segregating dissimilar signals, within the same cable should be utilized, and this can be done by a well thought out connector pin layout of dissimilar signals.
Equipment Cable Shield Terminations
Electromagnetic cable shields must be circumferentially bonded to connector back-shells in a 360 degree manner, and in turn, through the connectors to the equipment chassis at each connector. Individual internal shields will be co-terminated with the overall cable shield to the equipment chassis ground.
All interfaces should be provided with connectors capable of bonding to double over-braid shielded cables. Connectors must provide electromagnetic shielding and allow 360 degree circumferential bonding from the cable connector body through to the equipment chassis. The maximum mated resistance between the cable connector body and the equipment chassis should be less than 2.5 milliohms. Additionally, high quality bonding of the connector to the interface is standard procedure as it is imperative for maximum performance of filters and filtered connectors.
Why Filter & Shield at the Connector
EMI Solutions’ unique filter inserts and filtered connector designs provide solid ground planes around the filter components to not only provide the lowest impedance path for the filter, but also provide increased shielding throughout the connector area. Additionally, by using filtered connectors you eliminate the EM and RF noise that will be picked up from the contacts/leads from any internal or external source right at the connector’s interface.
Filtered connectors are the best means of eliminating unwanted noise from any electronic system. There are some less expensive ways to filter or shield at a board level, but none of these can outperform interface positioning. The parameters you have to consider when deciding on a filtered connector are as follows:
- Required performance
- Available space
- Component cost
- Shielding availability
In conclusion, while there is certainly some limited benefit for utilizing shielding in electronic circuits to reduce EMI, a filtered solution produces the optimum results and should be a primary consideration in new designs, as well as existing systems where EMI issues are identified.