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Testing
Technical Information
Introduction
EMC is about the ability of different items of electrical equipment to work together without suffering the effects of interference. Equipment should also operate without interfering with broadcast and communications signals and to be immune to normal levels of such signals. EMC implies that a system will not generate unacceptable levels of conducted or radiated signals which could cause interference to other well designed products. Systems should also be designed in such a way that normal ambient levels of electrical noise will not cause degradation of performance - they must have an adequate level of immunity.
Information
Interference
Any electromagnetic activity which disturbs the normal operation of an electrical system can be called interference. The effects may range from increased background noise in a communication channel or corruption of digital data, to the destruction of electronic circuits. Interference sources may arise wherever there is an electrical current. This may cause direct coupling to other circuits, or radiated fields which then couple unwanted signals into other circuits. Sources are characterized by their magnitude, frequency and bandwidth. Where the source is identified, protection may be possible on a selective basis. Where it is not known or is intermittent, a more general approach to protection is required.
The higher the frequency of the interference, the more easily it will radiate. In most EMC standards, energy is assumed to be conducted from one system to another at frequencies below 30MHz. Above 30MHz, the transfer mechanism is assumed to be radiation. The problem of electrical interference is becoming worse with the trend towards smaller devices operating at higher frequencies. Higher speed switching logic increases emissions while low operating voltages and currents, with circuits packaged more closely together, decreases immunity.
Bandwidth and magnitude
In general, unintentional radiating sources are characterized by wide bandwidth and have, in some cases, extremely large magnitudes. In addition, equipment may be affected by the proliferation of narrow band deliberate radiation sources, such as cellular radio and hand held transmitters. Conducted sources can be of a similar nature to radiated ones (sometimes a radiated signal induces conducted interference on cables) and the bandwidth can be extremely wide. High power transients can be generated, for instance when switches are closed.
Electrostatic discharge
When one insulator slides over another, an electrical charge accumulates causing high voltages. In dry and well-insulated conditions, a centrally heated office having a nylon carpet, for instance, a person wearing rubber soled shoes may retain the charge for some time. Rapid discharge occurs when the person touches a conductive surface. If that conductive surface is part of electrical equipment, it will be exposed to a fast electrical pulse which is capable of delivering a destructive current to integrated circuits. This also creates a wide spectrum of radiated interference.
Transients
Lightning is another source of transient interference. A lightning strike induces a transient waveform on cables and can be conducted for some distance. Electromechanical switches produce interference through the combined interactive process of arcing, bouncing and load circuit oscillations. In inductive circuits, interruptions can lead to high induced voltages, transients and arcing. Extreme cases can produce dielectric breakdown. Changes in power loading, both local and remote, can cause variations in the supply voltage. This is known as sag and surge, and can result in transients at the time of switching as well as long term voltage variations. Drop-outs occur when alternative power sources are switched in and out. If the drop-out duration is short, machinery may not be affected, but there may be a critical effect on data processing equipment. Transients of many times the supply voltage can occur on both public and private supplies, creating a wide spectrum of interference.
Narrow band sources
Single frequency signals can be described as a narrow band. A sine wave is a pure tone and has one frequency only. A square wave, such as that produced by a digital switching circuit, contains more than one frequency, comprising a fundamental and harmonics. Each harmonic represents a narrow band source. In digital switching circuitry, the harmonic frequencies can be very high. A fast digital rise time results in high magnitudes of frequencies many times the fundamental. If the rise and fall times of a digital signal are increased (in other words, the harmonics are reduced), the effects of high frequency interference sources can be minimized. The amplitude of radiated emissions from electronic systems increases with frequency. Thus to reduce such emissions narrow pulses and fast switching should be avoided if possible (easier said than done, perhaps).
Electromagnetic waves
Radiated emissions are in the form of electromagnetic waves which can be thought of as an electric field vector (E) and magnetic field vector (H) which are at right angles to each other and to the direction of propagation of the wave. If an imaginary plane surface in space is placed perpendicular to the direction of propagation, there will be a flow of power through this surface. This is a vector quantity and is known as the Poynting vector. The instantaneous power through an area is given by E x H (Ohms per square meter)
Small loop radiation
For a small loop carrying a current, it is easy for a current to flow but difficult for positive and negative electric charges to accumulate in different positions so as to provide an electric field between them. Thus the H field is dominant. The space close to the loop is dominated by fields which decay at different rates, the H field decaying faster than the E field. The E/H ratio increases with distance (r) through a ‘transition’ when r=wavelength/2xPi, the ratio approaches 377Ohms, the impedance of free space.
Monopole radiation
A monopole antenna acts as a high impedance source, so that the E field predominates. Close to the source (in relation to wavelength), this field decays inversely in proportion to distance cubed while the magnetic field (H) decays in proportion to distance squared. In the region when r=wavelength/2xPi, the ratio E/H approaches 377Ohms as in the case of small loop radiation.
Differential and common mode
Differential mode interference is caused by currents flowing in loops and its magnitude is proportional to the current, loop area and the frequency squared. Emissions are maximum in the plane of the loop. Differential mode emissions can therefore be controlled by reducing frequency, loop area and current flow, and by using different orientations within circuit layouts.
Common mode interference is caused by current flow in a monopole like conductor (a long circuit board track or cable, for instance). Its magnitude is governed by current level, monopole length and frequency. Reduced emissions can be achieved by minimizing currents, track and cable lengths and frequency. Common mode currents are often caused by poor grounding and cross coupling.
Ground impedance
The voltage developed between equipment grounds is a function of the ground plane current and its ground plane impedance. Clearly the ground plane impedance must be as low as possible to minimize the noise voltage and hence the resultant noise injected into the electronic circuit.
Limits in the standards
The mechanisms for radiation from equipment are extremely complex due to the number, nature and interaction of interference sources. The field equations for elemental sources show that radiation is dominant beyond a distance of wavelength/2xPi from the source. Today’s standards therefore assume that any true measurements of emissions must be made with the antenna at least this distance from the equipment under test. The magnitude of any specification limits must therefore reflect the nature and type of the equipment covered. The limits are also devised to achieve a good balance between the cost of achieving EMC and the technical complexity involved.