EMI challenges in connected vehicles.


This was crafted for the downsizing seminar

In a world of instant connectivity drivers have become accustomed to receiving information about traffic conditions and potential hazards, whether by listening to a traffic report on the radio, viewing a connected GPS map to see if there are slow-downs ahead, or checking a smartphone app that tracks reports from other drivers who have seen accidents, delays, or dangerous situations. To enhance safety advanced driver assistance systems (ADAS), like radar or camera, which actively improve safety by helping to avoid accidents are also increasing in popularity.

In the not-too-distant future, though, drivers of connected cars will have access to even more information, delivered in real time, about everything that’s happening around them. Beyond the capabilities of ADAS, the new communication protocol will enable cars to recognize hazardous situations and obstacles even beyond the driver’s line of sight.

Automotive EMC testing

In this new driving environment, cars will “talk” to other cars, in what is known as car-to-car (C2C) communication, and will interact with intelligent infrastructure such as traffic signals and message boards, using car-to-infrastructure (C2I) communication. Both C2C and C2I are based on an automotive specific version of WiFi, called IEEE802.11p, which is an offshoot of the more familiar consumer versions of IEEE802.11 used to connect laptops and smartphones to a network.

With connected cars relying on interconnected electric/ electronic systems for everything from infotainment to driver safety it’s vital that these vehicles are not affected by electromagnetic interference (EMI), such as aircraft manufacturer Boeing experienced in 2011.

During airline electromagnetic interference certification testing of wireless broadband systems (Wi-Fi) fitted to the Boeing 737NG the Honeywell Phase 3 Display Unit was found to be susceptible to “blanking”. At the time this prompted Boeing to cease fitting in-flight connectivity systems across the full range of aircraft.

Learning from this, it’s clear that placing a large amount of electrical and electronic systems into a very confined space poses the problem of keeping the electromagnetic interference of these systems from interfering with each other through radiated and conducted emissions. If not properly controlled, the interference can cause system-malfunction, and even failure.

EMC coupling

Image Credit: www.learnemc.com

Reducing EMI of video, ethernet and data signals.

In-car display applications are extremely EMI-sensitive. Because Central InformationDisplays, Board Monitors, TFT-CDs in combination instruments, HUDs and rear seat infotainment systems feature a mix of resolutions they may spread out a spectrum with a wide variety of frequencies. At the same time link-distances may vary from 50cm to 10m and may include several connections to allow for fitment.

Under these conditions matching the signal transitions to the cable impedence and its specific transmission characteristics is difficult when trying to achieve long distance data transmission, while at the same time cost effective low EMI becomes very difficult. This EMI includes immunity as well as emissions, which both need to be optimized for acceptable performance.

Flat-panel displays (FPDs) going into automotive applications are typically 15cm and larger, with many not of standard video sizes. For example, a 1280 x 480 FPD having an unconventional “dual VGA” resolution can serve as a central information display. The data rates required to drive FPDs depend on the resolution and blanking period, frame rate, and color depth. To determine the necessary pixel clock to drive the desired resolution, a designer multiplies the total horizontal pixels (including blanking) by the total vertical lines (including blanking) by the frame rate.

Assuming the rate is 60 frames per second, the pixel clock on a 1280 x 720 display is 73 MHz. The total data rate then depends on the color depth, which for current automotive displays is typically 18 bits per pixel (bpp) – 6 bits each for red, green, and blue.

To video-up this display, the interconnect needs to deliver over 1.3 Gigabits per second (Gbps) for an HD visual experience. The data rate for automotive FPDs can range from a hundred Megabits per second (Mbps) to nearly 3 Gbps.

Transferring this wide range of data between head units and display modules requires cables and connecters that carry multi-Gbps data and are economical, lightweight, and flexible. The solution is a single differential pair for the multimedia data stream that is rugged enough for automotive applications whilst minimizing EMI. Therefore, using serialized data streams that scale from a few hundred Mbps to multi-Gbps are the best solution for infotainment displays.

OEMs commonly take advantage of the flexibility and high integration possibilities of technologies such as Field Programmable Gate Array (FPGA) which is used for multiple independent systems such as a GSM phone interface, automotive buses such as MOST, FlexRay, D2B, or IEEE-1394 and sophisticated audio functions. The high levels of integration that FPGAs offer also have the advantage of containing multiple buses, interfaces, and clocks within one device – making efficient EMI design more manageable.

Examples of EMI solutions

National Semiconductor’s FPD-Link II chipset mitigates EMI concerns by using a serializer to take in spread-spectrum and clock data, using this to spread the noise spectrum of the serial data streamed over the cable. In addition, the Serializer uses reduced differential voltage levels and a de-emphasis feature that reduces the signal level but maintains the signal integrity. Furthermore, the integrated terminations, with accessible center tap point, minimize the noise on the serial link. The center tap access allows the designer to build a common-mode filter via a capacitive connection to ground.

The interconnect solution carries these EMI control features all the way through to the display module, whilst the FPD-Link II Deserializer uses an integrated spread spectrum clock generator (SSCG) and very low drive strength to minimize the interference generated by these signals inside the display module.

Inova Semiconductors GmbH take a different approach with the APIX Gbps Pixel Link which uses current mode logic signaling together with an optimized line code to offer a stable transmission and high immunity against external influences and low emissions. The link uses DC-balanced transmission over decoupling capacitors, removing any DC noise “collected” over the line.

Because of the substantial link-distances cables can prove to be particularly challenging when reducing EMI. While unshielded cables provide the benefit of high flexibility and thin diameter of the cable at relatively low cost, the signal is not protected against external noise or interference from other pairs of wires which can have a negative effect on signal quality.

Maximum crosstalk attenuation is necessary to be able to transmit broadband data streams on the two wire pairs independently of one another and without harmful interference between them.

Since there is no additional shield, unshielded cables have low attenuation and therefore offer higher transmission distances compared to shielded cables, however the EMI performance of unshielded cables is low, as any common noise coupling onto the lines (e.g. by local power supplies) is directly radiated which may cause link issues or affect surrounding devices.

In order to fulfill the requirements for EMI shielded twisted pair cables are commonly employed. There are different kinds of shielded cables, commonly utilising shielding of the pairs or outer metal shielding, covering the entire group of cables; or a combination of both.

However twisted pair cabling poses unique challenges when used in Ethernet applications.

Solving EMI in Ethernet applications.

Because system costs are significantly lowered using Ethernet connectivity for multi-camera sensor networks, traditional proprietary methods are making way for open standard Ethernet. Ethernet is already providing an IP-based standard interface for diagnostics and software downloading.

However the next generation Ethernet will form the backbone of automotive multi-media networks, carrying ‘live’ traffic. New standards such as IEEE 802.3AVB (Audi-Video Bridging) ensure real-time performance, with exceptionally low EMI emission.

Ethernet EMI

Image credit: EETimes Automotive

Herein lies the challenge: the use of shielded cables would provide a solution to reducing radiated emissions within the car but complicate grounding strategies, adversely affect reliability and add cost to production. Shielded cables cannot be manufactured in situ using wiring looms during production, but need to be pre-manufactured and purchased. Hence, the ultimate goal would be to operate Standard Ethernet over unshielded cables. This solution dramatically reduces cabling costs, by up to 80%, over shielded counterparts, whilst maintaining interoperability with any other standard Ethernet device. The net result is lowest cost cabling.

After continued investigations into the EMI behavior of 100BASE-TX PHY circuitry, simple techniques have produced solutions that meet automotive manufacturers’ emissions requirements. By adding a low pass filter to the transmission front-end, emissions can be reduced whilst still providing an interoperable standard Ethernet solution.

The result is that no changes to the standard Ethernet PHY are required. The plot below shows the resulting emissions from 100BASE-TX using IC manufacturer Micrel’s latest standard Ethernet PHY technology.

As the Ethernet finds its way into more vehicles, antenna performance for effective C2X communication will be crucial.

Grounding and antenna system integration strategies.

In modern dual power (12/ 48V) vehicles with complex electronic systems, the ground may simultaneously perform two or more functions, and these multiple functions may be in conflict either in terms of operational requirements or in terms of implementation techniques. For example, the ground network may be used as a signal return, provide safety, provide EMI control, and also perform as part of an antenna system.

Within the connected vehicle, antennas which enable 4GLTE cellular connection to mobile devices, infotainment and safety systems offer drivers and passengers a superior in-vehicle experience with the full range of features they have come to expect.

Antennas such as technology company Laird’s, LTE/MIMO provide enhanced cellular connectivity and data throughput, allowing for faster upload and download speeds. Along with superior cellular connection, the antennas also provide satellite radio, Global Navigation Satellite Systems (GNSS) and AM/FM radio in a single antenna solution.

Automotive IQ disclaimer

However with dual 12V/ 48V systems allowing more and higher power electric motors, connected cars with sophisticated communication systems are susceptible to EMI that has previously not troubled engineers. An example of this is the Pulse Width Modulation output from the control systems. Typically, these outputs are sent via long cables to motors that effectively represent an inductive load in electrical terms.

The complex model of a motor amplifier, cable, and motor includes elements of inductance, resistance, capacitance, and current, along with forward and back-EMF voltages. In the majority of cases, this type of system is likely to be affected by the H-field or magnetic input of the PWM signal. Interference can effectively be eliminated by surrounding the cable with a magnetic shield grounded at both the amplifier and motor terminals.

Grounding is one of the least understood EMC subjects, even though an effective grounding strategy will reduce EMI effects resulting from electric field flux coupling, magnetic field flux coupling, and common impedance coupling. With the imminent introduction of higher power equipment which 48V electrification will allow, correct grounding strategies are vital.

Ideally, a ground system should provide a zero-impedance path to all signals for which it serves as a reference. If this is accomplished, signal currents from different circuits that are connected to the ground, can return to their respective sources without creating unwanted coupling between circuits or equipment. The grounding configuration must be weighted with regard to dimensions and frequency, just like any other functional circuit.

An effective grounding system must perform the following functions:
• Analog, low-level, and low-frequency circuits must have noise-free dedicated returns. Due to the low frequencies involved, wires are generally used (more or less dictating a single-point or star ground system).
• Analog high-frequency circuits (radio, video, etc.) must have low-impedance, noise-free return circuits, generally in form of planes or coaxial cables.
• Returns of logic circuits, especially high-speed logic, must have low impedances over the whole bandwidth (dictated by the fastest rise times), since power and signal returns share the same paths.
• Returns of powerful loads (solenoids, motors, lamps, etc.) should be distinct from any of the above, even though they may end up in the same terminal of the power supply regulator.
• Return paths to chassis of cable shields, transformer shields, filters, etc. must not interfere with functional returns.
• When the electrical reference is distinct from the chassis ground, provision and accessibility must exist to connect and disconnect one from the other.
• More generally, for signals that communicate within the equipment or between parts of a system, the grounding scheme must provide a common reference with minimum ground shift (unless these links are balanced, optically isolated, etc.). Minimum ground shift means that the common-mode voltage must stay below the sensitivity threshold of the most susceptible device in the link.

This was crafted for the downsizing seminar

The performance of some EMI control techniques or devices may be significantly influenced by grounding. In particular, cable shields; isolation transformers; EMI filters; ESD, lightning, and EMF protection techniques; and Faraday shields must be properly grounded so as to provide maximum EMI protection.

With C2C and C2X technology reaching maturity the National Highway Traffic Safety Administration (NHSTA) will soon propose rules for vehicle to vehicle (V2V) communications on U.S. roads, it announced in March 2014. The agency is now finalizing a report on a 2012 trial with almost 3000 cars in Ann Arbor, Michigan, and will follow that report with draft rules that would “require V2V devices in new vehicles in a future year.”

In Europe the European Community has created the Intelligent Transport System (ITS) Corridor, which leads from Rotterdam via Frankfurt am Main to Vienna. This will feature smart traffic lights and other intelligent road signs designed to interact with connected vehicles. The system will alert drivers to upcoming traffic jams and road hazards before they come into view.

The expected result is not only a dramatic reduction in road fatalities, but also a reduction in the environmental damage and economic loss caused by traffic jams, which is estimated to have cost Europe approximately €7.4 billion in 2013 alone.

The ultimate success of connected vehicles lies with the safe operation of the vehicles, which in turn will rely heavily on controlling EMI generated by the many electric/ electronic systems.

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