All electronics engineers involved in product development should have an appreciation of EMC. Many product designs have hit problems due to an inadequate understanding of this. The most recent example of this was the delay to the launch of the Raspberry Pi in 2012. The product launch was delayed for many months while all the required EMC testing was carried out.
(This is a chapter from the book "From Prototype to Product". For more information or to buy the book, follow the link.)
This European Union directive requires that products must not emit unwanted electromagnetic interference and must be immune to a normal level of interference. The majority of electrical products must comply with these requirements and compliance is usually shown by testing to harmonised standards.
The purpose of this directive is to make sure that electrical products from different sources can work alongside each other with little risk of mutual interference. Without this type of standard no manufacturer could guarantee that their products could work satisfactorily in the field.
There are harmonised standards that are applied throughout Europe to ensure that all manufacturers are working to the same requirements.
There are a number of product specific standards for assessing the EMC performance of equipment. For example there are standards for household appliances, lighting, IT equipment and fire alarms. Where a product standard does not exist there are generic standards that can be applied.
This is carried out for radiated emissions by measuring field strength and conducted emissions along connecting cables.
Radiated emissions are usually tested by using an appropriate antennae and a spectrum analyser. The equipment is powered and put in its normal state of operation while the spectrum analyser is used to sweep through the frequency domain.
Hewlett Packard Spectrum Analyser
The field strength of all the radiated signals from 30MHz up to 1000MHz is noted and compared with the limits in the standard for that type of equipment. Note that for some appliances assessment of radiation above 1000MHz is required.
When testing for unwanted radiated emissions, it is important to be aware of ambient RF signals. An experienced EMC test engineer will be alert to this and account for signals that are not originating from the unit being tested. For this reason, an RF shielded room is often used to carry out much of the EMC testing.
Conducted Emissions are carried out on power and telecom network cables that are connected to the electrical equipment. When measuring this on the mains input, a Line Impedance Stabilisation Network (LISN) is normally used to allow the equipment to function normally and provide a connection point with the right impedance for the spectrum analyser. It also blocks unwanted signals present on the mains network from affecting the measurement. This is used to assess the levels of the unwanted signals from 150kHz up to 30MHz.
Line Impedance Stabilisation Network
This testing includes subjecting the product to radiated and conducted RF signals, fast transient bursts and electro static discharge.
Immunity to radiated RF signals is measured by using a RF signal generator, an RF amplifier and antennae to inject a high RF field around the product and check that it still performs correctly. When testing products for residential, commercial or light industrial environments, frequencies from 80MHz to 1000MHz are used usually at a field strength of 3V/m.
When testing products intended for industrial environments, field strengths of 10V/m is used as well as testing up to 2.7GHz. In this chapter most examples are drawn from the requirements for residential or commercial use. Please refer to the standards referred to in Appendix 4 for the appropriate test requirements.
Immunity to conducted interference is checked over a frequency range from 150kHz to 80MHz and using a level of 3V rms. The signal is injected into all the cables that are connected to the equipment under test under normal operation. A Coupling - Decoupling network (CDN) is used to inject the test signal into the cable.
Coupling - Decoupling Network (CDN)
Testing with Fast Transient Bursts (FTB) is part of the immunity testing. FTBs are generated whenever appliances or circuits within them are switched on and off using mechanical switches. Manufacturers need to ensure that their equipment works as normal in the presence of these interference signals.
Bursts of pulses with a rise time of 5nS and duration of 50nS are capacitively coupled into mains and other cables while they are operating normally. Amplitudes of 0.5kV or 1kV are used depending on the type of port. The harmonised standards also specify the duration and repetition rate of the fast transient bursts.
The bursts of pulses are typically coupled into the cables of the unit under test using a capacitive clamp.
Most people will have had the (unpleasant) experience of electrostatic discharge (ESD) to a metal object such as a filing cabinet or to a body of a car. The designer has to make sure that if this discharge was to a product they have designed or near it that it should continue to work normally. More importantly they need to ensure that there is no permanent damage as a result of a static discharge. For this reason testing for immunity to ESD is an important part of EMC testing.
An ESD gun is used for this testing with the product under test being placed on a large grounded metal sheet. Two types of discharges are used during this testing.
ESD gun with rounded and pointed tips
During contact discharge testing, a pointed tip is fitted to the ESD gun and testing is carried out up to a voltage of 4kV. The tip is bought into contact with the product while it is operating normally and the vacuum relay in the gun is triggered to deliver the current. This is repeated to different parts of the product, including connection ports and also to the metal plane on which the product is resting.
Air discharge testing is carried out up to 8kV using a rounded tip. After charging, the tip is brought close to the product until the voltage discharges across the air gap. This is repeated to different parts of the product being tested.
When carrying out ESD testing it is normal to start with relatively low voltages and slowly step up to the specified voltage. If adverse effects are noted early on, then fixes can be applied before restarting the tests. Most ESD guns allow testing to be carried out with both polarities of voltage.
Electromagnetic Compatibility is an important part of electronic product development. In my experience this subject is not taught well in most university engineering courses. EMC testing requires a lot of experience and is best left to those skilled in this area. However any engineer involved in electronic engineering should understand the requirements of this and take steps to give their products a good chance of passing these tests.