Shielding is a technique used to control EMI by blocking the transmission of Radio Frequency (RF) noise from a source, similar to how lead and concrete are used to shield from nuclear radiation. Shields can be in the circuit at the RF source, the receiver, or anywhere in between. In the case of electric fields where shielding is successfully implemented, the effectivity is a function of the shield material’s thickness, conductivity and continuity. Adding shielding to a cable protects the inner conductor(s) from impinging electric fields, lessening the interactive force of the magnetic field. It is important to remember that when an electric field interacts with a conductor, it creates a voltage which appears as electrical “noise” on circuitry. When a properly grounded shield is placed around the conductor, the electric field energy is usually drained away without affecting the conductors, thereby reducing or negating the “noise”. This basic principle drives the common electrical shield, known as a Faraday cage.
When an electromagnetic wave traveling through space encounters a shield several things happen:
- Much of the energy is reflected and refracted, similar to the Doppler effect on sound waves, which are partially dissipated by anything they travel through.
- Some of the residual energy is then absorbed or shunted to ground by the shield as intended, significantly reducing the magnitude of the electromagnetic wave’s energy, showcasing the effectiveness of basic shielding.
- Additionally, re-reflection of energy within an enclosure must be considered in situations where a system requires thin shielding. Re-reflection occurs at the shield boundary on the far side of the shield material and causes bouncing of the EM wave within the system, leading to a localized spike of unwanted EM energy.
Drawbacks of EMI Shielding with NO EMI Filtering
Most high frequency shielding problems are caused by openings in the shield material, not by failure of the material itself. While shielding is a primary option for most EMI related issues, the following material centers around the issues associated with penetrations through the shielded enclosure, and the Inputs and outputs of the electrical systems. This is the most vulnerable point in a shielded system. No different than the hull of a ship, a hole in the shielding system at any point can easily lead to catastrophic failure. As the inputs and outputs are the weakest points associated with the shielded enclosures, these are the most vulnerable locations, but also the locations where the implementation of EMI filtering is most effective.
In these locations, filtering and transient suppression at the interface of the shielded enclosure is the most efficient way to protect systems from compatibility issues. By using filters and filtered connectors at the input/output interfaces of a system, design teams can eliminate the EM and RF noise from internal or external sources right at the connector interface, shunting the unwanted energy to the grounded shielded enclosure. As a result, this is the optimum location to eliminate higher frequency noise, mitigating any EMI related problems.