RF Shielding: The Art and Science of Eliminating Interference
Contributed By Electronic Products
In every wireless system, almost every wire becomes an antenna or a receiver, and the spurious signals they inadvertently receive or transmit can degrade the overall performance of the RF subsystem. Good printed-circuit board (PCB) design and shielding techniques can go a long way toward reducing electromagnetic/RF interference (EMI) and delivering reliable systems. This article will examine some of the approaches and off-the-shelf products that can be used to minimize RF interference through the use of shielding and ground planes to prevent spurious signals from interfering with wireless system operation.
EMI is basically disruptive electromagnetic energy that is emitted by one portion of the circuit and reaches other portions of the circuit or the external world via conducted or radiated paths. This energy can adversely affect circuit performance or even, if high enough, cause damage to human brain cells or other organs. Thus the goal in any RF design is to reduce emitted radiation to a level that meets or stays below any mandated industry requirements for operation, safety, and health.
In any of today’s wireless devices, EMI comes from two main sources – the RF front end that transmits the signal, and the internal digital circuits that often switch at Gigahertz and higher clock rates. Today’s designer must create a circuit board that has the lowest possible emitted radiation while operating at top speed to meet its performance goals.
Whether your design is a cell phone, a tablet, a portable computer, or some other portable product, good PCB layout is critical to keeping EMI to a minimum. Good use of ground planes and wiring layout that minimizes parasitic capacitance and ground loops goes a long way toward minimizing EMI.
Emitted radiation, generated by both differential and common-mode currents, propagates along a conducting medium or by radiation through space. Differential-mode currents, flowing synchronously through both signal and power distribution loops, produce time-variant electromagnetic fields and, on simple one- or two-layer PCBs, loops are formed by the digital signals being transferred from one device to another that return by means of the power distribution traces. These loops also act as antennas and pick up the time-variant electromagnetic fields, which create noise in the loops. Additionally, parasitic capacitive coupling occurs between all conductive materials. Even external cables can act like antennas.
Shorter wavelengths can approach the physical dimensions of many EUTs (Equipment Under Test), which leads to possible cavity resonance effects. The resonant frequency is the frequency where integer half-wavelengths correspond to the dimension of the shielding enclosure. A wave is set up inside the enclosure whose nodes (i.e., zero amplitude) lie on the conductive walls of the enclosure. These structures behave as cavity resonators. For example, a 2 in. square by 0.5 in. metallic enclosure resonates at a first order mode of around 12 GHz.
Even weak coupling at these extremely high frequencies can induce strong oscillations which can then couple to any other points in the enclosure or the oscillations can radiate. The danger of a cavity resonance is that if a noise source has a frequency component that corresponds to a resonant point, a large field can be generated at this frequency due to the multiplication or amplification effect by the “Q-factor”. One approach to reduce this phenomenon is to lower the “Q-factor” of the cavity by introducing losses (Q-dampening).
Once the PCB design has been optimized for minimal emissions, the next stage of the design process is to shield critical sections of the board to prevent emissions, or prevent emissions from other portions of the system from affecting sensitive circuits. In addition to metal shields that surround key circuits, additional shielding in the form of gaskets and other materials are often used to effectively shield as much of the entire enclosure as possible. Shielding forms a conductive barrier enveloping an electrical circuit to provide isolation and prevent the EMI from “leaking” out. Although the “ideal” shield would be a sealed conductive container with no openings, such a solution is not practical since whatever is inside the container has inputs and outputs, and requires power and ground connections – all of which will “leak” unless additional measures are taken.
Shielding can be a stand-alone solution, but it is more cost-effective when used in conjunction with other suppression techniques such as grounding, filtering, and, as mentioned earlier, proper circuit-board design to minimize the loop area. Unfortunately, designers often leave shielding as a last option since the shields can be installed once the design is completed. However, by incorporating shielding considerations from the beginning of the design process, designers can deliver a more cost-effective and generally more efficient system design.
In a Technical Note1 Laird Technologies describes how to define the performance of the shield (Shielding Effectiveness, or SE): Take the ratio of the RF energy on one side of the shield to the RF energy on the other side of the shield, and express the result in decibels (dB) as a function of the logarithm of the ratio of incent and exit power densities or of the incident and exit electric, magnetic or plane wave field intensities, as shown in the following:
Pi is the incident power, Pe is exit power, Fi is incident electric magnetic or plane wave field intensity, and Fe is exit electric, magnetic, or plane wave field intensity.
Shields can take on many shapes and sizes, and range from simple metal boxes to conductive foams and metal films. Samples from Laird Technologies, Würth Electronics, FotoFab, TDK, and 3M are shown in Figure 1. In addition to the many off-the-shelf solutions, designers can work with the shielding vendors to create a custom shape to meet their specific system needs.
Figure 1: Designers have a wide choice of EMI shielding products, from simple shield boxes such as offered by Laird Technologies, Würth Electronics, and FotoFab (left to right, top row), conductive films from TDK, and 3M, and conductive foam gaskets such as offered by Laird Technologies (left to right, bottom row).
The ability of the shielding to block or absorb the RF signals is determined by several factors – the conductivity of the material, the permeability of the material, and the material thickness. Additionally, the ability of the material to reflect the signals is controlled by both the permeability and conductivity. Part of the design challenge for shielding must take into account such issues as airflow, mechanical strength, and still other factors. Many shields incorporate apertures (holes) in their structure to allow air to flow through, helping reduce thermal buildup in circuit sections that dissipate significant amounts of power.
Designers have a choice of hundreds of gasket varieties based upon geometry and materials. However, shielding gaskets typically fall into four principle categories: beryllium copper and other metal spring fingers, knitted wire mesh, conductive particle filled elastomers, and conductive fabric-over-foam. Each of these materials has distinct advantages and disadvantages, depending upon the application.
However, regardless of the gasket type, key factors to consider when choosing a gasket are RF impedance (R + jX, where R = resistance, jX = inductive reactance), shielding effectiveness, compressibility, compression forces, compression range, compression set, and environmental sealing. Many other aspects of the system design may also impact the selection decision.
In reference 1 Laird offers the following recommended guidelines for shielded enclosure design:
- Permeable materials should be used for low-impedance magnetic fields.
- Enclosures containing emitters should be designed for maximum absorption loss of the emitted field.
- Enclosures containing receptors should be designed for maximum reflection loss of the received field.
- Discontinuities should be minimized and designed to maintain shielding effectiveness by proper seam treatment, and use of RF gaskets, vent/window screening and shielded/filtered leads. Connectors must be regarded as discontinuities.
This overview of shielding approaches shows that basic shielding theory is really not so basic. There are many factors to consider and designers should have a comprehensive knowledge of circuit design, EMI control, mandatory specifications, environmental issues, and other factors.
- “Shielding Design and Calculation”, Ron Brewer, Laird Technologies
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