3M has decades of electromagnetic interference (EMI) experience developing products to help improve the performance of electronic devices. These products can be broadly categorized as shielding, absorbing, and grounding materials.
Electronic devices all around us emit signals at multiple frequencies, intended or not. EMI (electromagnetic interference) is an electronic emission that interferes with components, RF systems, and many other electronic devices, degrading performance or even causing a complete malfunction. EMI may also be referred to as RFI, or radio frequency interference.
Shannon’s Equation in Digital Communication
A consistent trend in information devices is toward higher frequency operation to more rapidly process or transfer data. This enables greater functionality within higher-density, more compact packages.
In transferring data, the limits on data rates imposed by EMI are formalized in Shannon’s Equation. This equation shows that increased data channel bandwidth and/or increased signal to noise ratio are the available strategies to increase data transfer rates; viz.
In Shannon’s Equation BW is the bandwidth of the data channel in Hertz and SNR is the ratio of the signal strength to the noise level. Increased BW drives the need for spectrum availability at higher frequency bands, such as C-Band and mmWave, while increased SNR requires reduced EMI noise levels and therefore improved shielding, EMI absorption and (increasingly) reduction of EMI arising from previously tolerable Harmonics and Passive Intermodulation (Hx and PIM in what follows).
Why is Improving Signal to Noise Ratio Important?
To understand the need for improved SNR, consider the following representation of a single transmitted symbol as data density in increased (left to right). In this figure each dot represents a value for the symbol being transmitted. Each such value must be unambiguously distinguishable from its nearest neighbors to ensure accurate interpretation of the symbol. For the lowest density symbols on the left this can be done even in the presence of significant noise, as in the bottom row. However, an acceptable level of noise for a 1-bit symbol (on the lower left) is no longer acceptable for a 4-bit symbol (16 values), and the maximum noise allowable at 4 bits in incompatible with communications of 8-bit symbols on the right (256 values). It is interesting to note that Symbols containing up to 1024 values (1024 Quadrature Amplitude Modulation, or 1024 QAM) are included in the current 5G standard.
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Market Segments Affected by EMI
- Consumer electronics
- 5G Infrastructure
- Other: Auto, Medical, Defense, Industrial
Customer/Market Segment Need
- Improved signal integrity
- Higher signal to noise
- Low PIM (4G/5G)
- Increase in network coverage
- Increase in network capacity
- Increase in data throughput
- Reduction in dropped calls
Methods of EMI Control
Shielding is accomplished by isolating a Culprit device (the source of EMI) and/or a sensitive Victim device by means of conducive enclosures. However, shielding can be an unattractive solution at lower frequencies, where it loses effectiveness due to the existence of a “Skin Depth” for RF energy in conductors. The skin depth scales with , becoming larger as frequency is reduced. When the skin depth becomes a significant fraction of the thickness of the metal shield, the shield will lose effectiveness. As a result, shielding is generally most effective at frequencies above 10 MHz or so, below which shield thickness and weight can become excessive for many applications. The other downside of shielding is that EMI is reflected from a shield with little if any attenuation. That reflected energy can cause unanticipated problems for components aside from the initial Victim of concern.
Effective, low impedance grounding is critically important in preventing the generation of biases within a device. This is shown schematically in the figure below. More recently, in addition to intra-device bias, the generation of Hx and PIM at problematic levels has been noted when grounding currents pass through insufficiently linear conducive interfaces. These interfaces can occur at connections to conductive tapes, gaskets or even pins and screws. It is important to select materials and material combinations that are engineered for low PIM and Hx in many high-performance devices.
Hx in MHH Devices results when the current induced by a voltage is not linearly proportional to that voltage. This occurs at “non-linear” electrical junctions. The Hx themselves can create EMI issues when, for example, the third harmonic of a transmitted signal at ~850 MHz arrives at a C-band receiving antenna at ~3.8 GHz. In addition, since transmission bands must have significant bandwidth (see Shannon’s Equation above) Hx of signals being transmitted from two different frequencies, f1 and f2, within that band may intermodulate in the presence of a non-linear junction. This gives rise to PIM noise at nf1 ± mf2 and at nf2 ± mf1, where m and n are positive integers. As it happens, these intermodulation products often fall within the receive bands of Frequency Domain Duplexed (FDD) signals, creating significant levels of noise. A description of the theory underlying PIM is not within the scope of this document; the main point is that the grounding tapes used in a MHH device need to be engineered to provide the maximum possible I/V linearity when applied to the substrates being grounded. 3M has extensive experience in engineering and helping customers select solutions for minimum PIM and Hx.
3M Solutions for EMI
3M has developed extremely linear shielding and grounding products through a deep understanding of the underlying causes of conductive interface nonlinearities. Innovative shielding and grounding products from 3M include conductive gaskets, conductive pressure-sensitive adhesives (CPSAs) and tapes, and absorbers that can help device manufacturers reduce noise in Mobile Handheld Devices. These and similar products are now also being deployed for 5G antennas, base band units, mobile edge computer hardware and more (see below). Check out our grounding and shielding products:
Absorbing of EMI is accomplished using either magnetically or dielectrically lossy materials. In general, for lower frequencies (below 1 GHz or so) magnetic solutions dominate while at higher frequencies (greater than 6 GHz or so) the electrical ones do. Since absorbers convert EMI energy to heat there is less chance of that energy causing unexpected problems, as sometimes happens when shielding. Absorbers are often combined with shields for situations where neither on its own can protect all the sensitive components. 3M has a broad line of absorbers intended to address this need for customers, from the kilohertz range up to 80 GHz.
3M taps into a storied history of expertise in controlling EMI in electronics to bring products that can help you reduce noise throughout your 5G infrastructure, so you can deliver a better connection to customers.