Electromagnetic (EM) shielding in today’s high-end communication technologies is critical. The trend in electronic devices is focused on building faster, yet smaller products. Serious electromagnetic interference (EMI) problems can occur when dealing with high frequencies and crowded PCBs. The unwanted energy can come from either internal parts or external sources. It couples with a sensitive part of the circuit, such as a low-power signal trace or a cable, which then acts as an antenna.

To increase efficiency and reduce costs, shielding must now be part of the initial design rather than a last option solution

[1]. A good PCB layout design must be optimized for minimal unwanted emission/reception prior to the shielding design process. In this post, we will first discuss the basics of electromagnetic waves as related to shielding and then take a quick look at the radio performance of the Nutaq Radio420X ‘s shield.

Electromagnetic waves consist of two in-phase time-varying fields, perpendicular to each other and to the direction of propagation. One is the electric field (in V/m) and the other is the magnetic field (in A/m), as illustrated in Figure 1. Any barrier (i.e. a change in medium impendence) placed in the path of an incident EM wave will attenuate its power. A part of the wave will be reflected back while the remaining attenuated wave will make its way through.

Electromagnetic Wave

Figure 1 – Electromagnetic wave [4]
 The amount of attenuation depends on the material’s proprieties, which are used to define its attenuation constant α (in Neper per meter). For a good conductor, α is given by the following relation [2]:
Attenuation Constant α
The wave amplitude will be damped by a ratio called the attenuation factor which is simply e-αz, where z is in the propagation direction. For copper at 900 MHz, the attenuation constant will be:
Attenuation Constant for Copper at 900MHz
Since the attenuation factor is e-αz, the amplitude of the wave will be attenuated by a factor of e-1(0.368) when it travels a distance of δ=1/α=3.90 × 10-6 m, or 0.00390 mm. Quite a small distance! This shows that an electromagnetic wave is attenuated very rapidly as it propagates in a good conductor. This distance δ=1/α is known as the skin depth, and it is a good reference when choosing a shielding material. Figure 2 shows the resulting skin effect. From Equation 1-1, we see that the skin depth for a given material is inversely proportional to the square root of the EM wave frequency. Therefore, one will need a thicker shield to block low frequencies. For example, with copper at 60Hz, δ=8.53mm [1].
Figure 2 – Attenuation of an EM wave when crossing a conductive material
Figure 2 – Attenuation of an EM wave when crossing a conductive material
Other factors such as shield dimensions, aperture sizes, seam and gasket types, cooling rate, and mechanical strength must also be considered. For instance, a good conducting shield that entirely surrounds a given part (known as a Faraday cage) will block any external electrical field from getting inside. The external field will cause the material charges to reorganize and effectively cancel the field inside. However, the enclosure can also act as a resonator for specific EM emissions if the dimensions are not properly chosen. For example, a 2 inch square by 0.5 inch metallic enclosure resonates at a first order mode of around 12 GHz [3].
Let’s now have a look on the Radio420X shield performance. The following comparative snapshots clearly show the contribution of shielding in rejecting unwanted emissions.
Figure 3 – Shield performance in receiver (left) and transmitter (right)
Figure 3 – Shield performance in receiver (left) and transmitter (right)
These two tests are representative of the shield’s performance over the entire Radio420X’s frequency range, with shielding performance slightly increasing at higher frequencies. Without shielding, several spurs appear near the transmitted and received tones and could become problematic when dealing with weaker signals.
In this post we have seen how unwanted electromagnetic waves can be attenuated by proper shielding design. A shield’s material proprieties such as permeability µ, conductivity σ, and thickness are critical parameters that must be carefully chosen. Skin depth δ for good conductors is derived from those proprieties and gives a good idea on how fast the EM attenuation is achieved for each material, as a function of frequency. Finally, a reliable RF transceiver such as the Radio420X must take advantage of a good shielding design to perform well in today’s high-end communication technologies.

References

[1] Tecknit. Darcoid. [Online]. http://www.darcoid.com/images/uploads/pdfs/EMI%20Shielding%20Design.pdf[2] David K. Cheng, Field and Wave Electromagnetics.: Addison Wesley, 1989.[3] Dave Bursky. (2013) Digi-Key TechZone. [Online]. http://www.digikey.com/ushttp://www.nutaq.com/techzone/wireless/resources/articles/rf-shielding-eliminating-interference.html[4] Tim Murphy. Magnet Lab. [Online]. http://www.magnet.fsu.edu/education/tutorials/tools/faradaycage.html