EMI Cuts A Wide Swath

By Ann Steffora Mutschler
Electromagnetic interference (EMI) cuts across all application segments, whether it’s aerospace and defense and its various tangents, or in a handset, virtually touching a large majority of engineering teams today.

The reason this issue affects so many engineering groups is because as modulation schemes become ever more complex they become even more sensitive to signal-to-noise ratios. Signals that are significantly lower and weaker to receive are much more susceptible to interference either intentional or unintentional.

“If you look at CDMA technology which really uses old military spread-spectrum technology, essentially you’re trying to pick a very weak signal out from the noise floor using a very elaborate algorithm to do that,” said Erick Olsen, director of marketing for mil/aero at NXP. “On the RF side that signal is still very, very small and so even if you know exactly where to look for it if there is unintentional jamming, which comes in the form of a handset that has wireless LAN. There are other signals roaming around (Zigbee, Bluetooth, etc.), and those signals can unintentionally interfere with that very, very low signal level.”

In the military, that jamming can be intentional. In the consumer world, that jamming is unintentional and sometimes even self-induced.

The aerospace industry, meanwhile, is constantly striving for more efficient “green” aircraft that can travel further using less fuel. “One way to achieve this is by reducing the weight of the aircraft,” said David Johns, vice president of engineering at Computer Simulation Technology. “AG Composite materials are increasingly used instead of aluminum sheet. Such materials typically do not provide the same levels of electromagnetic shielding because their effective electrical conductivity is lower, making the aircraft more susceptible to interference.”

A modern aircraft contains literally miles of cabling, and this can contribute significantly to the overall weight. That weight can be reduced by removing layers of shielding in the cable, or implementing shielding over reduced sections of the cable, but there is inevitably a tradeoff between weight and EMC/EMI shielding performance. The electromagnetic “threats” to aircraft systems include severe currents and fields associated with lightning strikes, electromagnetic pulses (EMP), high-intensity radiated fields (HIRF) from external transmitters and EM noise generated by internal systems such as wireless devices, he said.

In defense applications, there is a requirement for very good control of the spurious, Olsen noted, referring to any signal that is not the intended signal.
“If I’m operating on a UHF communications link between the ground and the air on an aircraft or a long-range radar for tracking inbound attacks, by design I’m only allowed to transmit one specific frequency or within a band of frequencies, and that is very tightly controlled,” he said. “Any energy that is outside of that band is considered spurious or unwanted radiation, so if you put a lot of energy into that unwanted frequency before it actually transmits you are actually wasting a lot of energy. By creating unwanted energy you then have to find a way to filter that out so you are expending energy in the first place in an unwanted RF spectrum, and then you have to add cost to the system to filter it out. So it has an effect on efficiency. If it’s a big ship-borne radar maybe it’s not a big deal, but if you are in an aircraft-borne or in a remote-sensing-type application, every little bit of current or power consumption is a big deal. Then of course adding additional filtering adds weight and complexity so therefore adds cost which is not wanted either,” he continued.

Another impact of EMI is received desensitization. If the RF transmitter is putting out a bunch of unwanted energy, even if it can be filtered out before it gets to the antenna that unwanted energy might be still around on the transmitter. “If your transmitter and receiver are well isolated or protected from this extra energy, that energy bleeds into the RF on the receive side and essentially decreases the signal-to-noise ratio. It makes it harder to pick out that very low signal level from the noise floor as the noise floor now comes up. So you get desensitized, making it harder to make that communication link on a radio or on radar, which makes it harder to pick up a target at a longer distance,” Olsen explained.

Advanced algorithms are used on a lot of multiple tracking radars these days with the advanced phased arrays and Active Electronically Scanned Array radars to detect both the target as well as its characteristics, such as its speed, size and whether it’s a plane or a missile. If the signal is too weak to be fully detected that can pose a significant problem.

One brute force way to deal with this kind of EMI is to shield parts of the system with metal enclosures, adding ferrite beads and other filters to the system. Another option is to transmit at higher power levels. If it is a radar application, an aircraft-borne radar or a land- or sea-based radar, detecting a that is farther away may mean hitting it with more energy so that if there is any jamming the signal is strong enough to overcome that.

Another technique used often in both military and commercial applications, as well as cell phones, is called frequency hopping. As the phone transmits voice or data coming it constantly changes the frequency. This uses very precise timing and a lot of it is GPS-based, so having an accurate GPS timing signal coming from the satellite is a big deal.

EMI in the car
With the ever-increasing amount of electronic content in automobiles today, EMI is a significant concern. Aveek Sarkar, vice president of product engineering and support at Apache Design, observed that too much EMI can cause coupling between subsystem PCBs within the vehicle. “Let’s say you have a printed circuit board, which has the chips that control the airbag, and then there is another printed circuit board that controls the GPS. Let’s say the GPS one is transmitting too much EMI and that couples onto the wires on the board that controls the airbag. That leads to noise and it can prevent the airbag board from operating properly. For that you have to ensure that the compatibility of the devices in the presence of EMI is sufficiently high.”

The question is how to prevent it. There are certain things that can be done in the design to suppress the noise, but there is only so much de-capacitance that can be put on a package or board to suppress the noise coming out of the chip. “You need to design the chip in a proper manner so that the current is not as leaky, there is enough decap and the noise is not generated,” Sarkar said.

For aerospace applications, engineers are dealing with these issues by designing EMC/EMI protection into the airframe and avionics systems, noted CST’s Johns. “At the airframe level, conducting strips may be applied to composite panels in order to provide a deliberate path for RF current flow, diverting high currents and field strengths away from critical or sensitive equipment. Electromagnetic field simulation is used earlier in design, and more extensively, to determine optimal locations for EMC/EMI protection. Engineers may simulate the EMC performance of the airframe for different structural designs. Broadband shielding effectiveness can be calculated and important effects captured, such as electromagnetic resonances generated by EM waves bouncing across airframe structures or inside compartments/cavities. Ultimately the goal of simulation is to reduce the number of physical tests required to certify or qualify the aircraft for EMC, which is both a time-consuming and costly process.”

Further, simulation can be used to model various “threats” such as lightning strikes and visualize the resulting current distribution induced in the airframe, and can also be used to predict voltages and currents coupled into internal cabling, enabling the impact of removing layers of shielding to be assessed. At the avionics level, filtering may be applied to suppress interference over certain frequency bands and non-linear transient protection devices employed to clamp voltages/currents to acceptable levels. A tradeoff between filtering/transient protection and shielding could be an important design scenario to consider, he pointed out.

“Electromagnetic analysis may be applied in the design of avionics enclosures, printed circuit boards and components, ensuring that they are less susceptible to electromagnetic interference. Simulation models can be parameterized, so that trends in performance for different design options can be easily extracted. Electromagnetic simulation codes can be run on high performance computing systems to speed up the analysis and solve highly complex models. A large model can be decomposed into many smaller pieces and distributed over a computing cluster. The different pieces are solved in parallel and recombined at their boundaries to yield the overall system response,” Johns added.

The bottom line
As with all power-related issues, setting goals at the very beginning about what the intended signal should look like and how to reduce the unintentional energy is critical.
“Understanding the spectral requirements is a starting point for a good, solid design,” NXP’s Olsen concluded.