Electromagnetic Compatibility (EMC)

Overview

Many refer to EMC as "Black Magic", mainly due to the complex nature of solving these types of problems. The biggest problem to overcome is that a single-point failure results in complete failure. If just one emission is greater than the imposed limit, then the product cannot be given a "pass". As products incorporate more higher speed electronics, the problem is exacerbated. Also, even if both radiated and conducted emissions pass and allow product shipments to take place in countries that have only emissions requirements, to then ship into the European Union (EU) requires additional immunity testing. If compliance to these standards are not initially designed into the product, then changes will have to be introduced while the product is in manufacturing, requiring a logistical plan to implement.

It is certainly best to consider all the possibilities in the original design in order to allow the product to move unabated into additional markets as a company's sales and marketing groups work to expand their revenues. A brief understanding of the requirements, and how they are tested for, can help design engineers and product managers plan development and to be prepared for accessing the markets they intend to sell into. EMC requirements are broken into two major categories: emissions and immunity.

Emissions

Emissions are those unintentional "radio frequencies" that any product emits. Some are radiated through the air and some are conducted via power and signal connections. Most standards provide radiated emissions limits for unintentional radiation from 30 MHz to 1 GHz. As the product clocks go higher in frequency, then proving compliance at frequencies greater than 1 GHz becomes necessary. Some standards (for some products) require radiated emissions testing below 1 GHz but greater than 9 kHz. It's important to identify which standard applies to your company's product so that it will be known what frequency range you must ensure that the radiated emissions are below the limits.

For any product that connects to the AC Mains, conducted emissions limits in the frequency range of 150 kHz to 30 MHz must be in compliance, with some standards requiring testing down to 9 or 10 kHz. Conducted emissions limits apply to all connections to the AC Mains, even if your company is buying an off-the-shelf power supply (internal or external to the product). This is a frequently misunderstood requirement and can cause major delays in product release if not planned for. There are Class A limits for non-residential products and lower, Class B limits, for products intended to be used in a residence.

To complicate things further, some standards have limits for DC input and output ports, and some signal lines. If this is not identified as part of the design specification then the product just might be designed without the necessary filtering or properly shielded cables, resulting in further delays in the product release. Therefore, for both radiated and conducted emissions, the design requirements must be identified up front.

Immunity

Some countries require products to be immune from various static or transient environments, meaning that a product needs to meet certain performance criteria when subjected to these phenomena. In other words, a product must not be susceptible to these environments. Most standards reference seven (7) different phenomena with varying levels of immunity.

Electrostatic Discharge (ESD)

Everyone has probably experienced this phenomena when touching something like a light switch with relatively low humidity. It usually exhibits itself as a visible flash when you touch an object, and can sometimes be felt! In the case of sensitive electronics, this discharge can cause equipment to malfunction or even to be physically damaged. Standards such as IEC or EN 61000-4-2 provide the test methods for ESD but do not, unfortunately, describe how to mitigate a discharge in the design of a product.

Radiated Disturbances

Cell phones, base stations, radios, television, and other devices use the radio frequency spectrum to service a need for us. This also infers that products will be used in an environment where these intentional radio frequencies exist, so that products must be able to tolerate their existence and continue to function without issues. IEC or EN 61000-4-3 is one standard that provides a test methodology to determine if a product can function can operate in this environment. Depending on the type of product, this may not be an area of great concern; but for others, it requires more engineering effort than other phenomena.

Electrically Fast Transients (EFT)

For longer cables that may be placed near the building wiring supplying AC power, capacitive coupling frequently occurs to these product cables and causes interference to the product. The product's AC Mains power cord and many types of signal cables (e.g. telephone, ethernet) will be required to operate in this environment and EFT is one of tests imposed to determine immunity to this phenomena. The standard, IEC or EN 61000-4-4 provides a standard test method of coupling short bursts of very fast transient pulses to the cables of a product. All the pulses can have a magnitude of a few thousand volts, the energy contained in these pulses is fairly low. The design difficulty comes from the fact these pulses have very fast rise and fall times, allowing them to couple quite easily to a product's signals.

Voltage Surge

IEC or EN 61000-4-5 calls for a pulse of a few kilovolts with a 1.2 ┬Ásec rise time and a 50% duration of 50 ┬Ásec, providing substantial energy delivered to the product. The protection devices for this type of transient pulse can be relatively large compared to the components they are protecting and if not designed in up front, can definitely cause a fairly major re-design effort late in the project.

Conducted Disturbances

Any metallic connection your product makes to the AC Mains of other signals are likely subjected to the requirements of IEC or EN 61000-4-6. An amplitude modulated signal in the frequency range of 150 kHz to 80 MHz will be coupled directly onto the conductors of your product with levels of 3 or 10 volts. This simulates the ability of longer wires acting like an antenna and lower frequency radio signals being directly injected into your product. Once again, this phenomena must be considered in the initial design to prevent product introduction delays.

Power-Frequency Magnetic Field

Although the world's power frequencies of either 50 Hz or 60 Hz, are hard to consider as radio waves to their extremely low frequency, they can produce magnetic fields that can reek havoc in some electronic components. Any transducer that converts some physical parameter (i.e.speed, temperature, etc.) into an electrical signal should be carefully investigated to ensure that those types of components and related sub-assemblies continue the product to properly function in this type of environment.

Voltage Dips and Interruptions

Commonly referred to as "brown-outs" or "black-outs", IEC or EN 61000-4-11 describes the standard test methods for the environments a product must function in. The voltage of the AC Mains is either lowered by some percentage or eliminated completely for a specific amount of time. The amount of time can be as short as a half-cycle (10 msec or 16 msec) to as many as 200 cycles of the power frequency. Depending on the product specific standard, the product will either have to continuously function through short (half or full cycle) interruptions or longer periods of time (many cycles). These requirements will dictate the power supply design or purchasing specification.

Conclusion

The engineering needed during the conceptual stages of product development must specify the electromagnetic environment. This is a crucial undertaking in order to reduce any delays in the deployment of a product. The design engineers need to know what their design rules are and the project managers have to ensure it is part of the project plan. The product managers must report to the company when revenue can be expected. And, of course, there is never time to repeat any part of the cycle! It is always cheaper to implement any regulatory requirements earlier in the process and it is always more expensive to implement them later in the process.