EMC Measurements/Standards

Measurements

___ Measurement of conducted emissions--Ground plane

Emissions:

Conducted emission tests comprise measurements of voltage across a defined network which simulates the RF impedance of a typical mains supply. These Line Impedance Stabilizing Networks (LISNs) also provide filtering of the supply to the Equipment Under Test (EUT) and are also known as artificial mains networks or isolating networks. The EUT is connected to the LISN in a manner which is representative of its installation and use in its intended environment. ___ the general arrangement.

The EUT is con figured in a typical manner with peripherals and inputs/outputs attached, and operated in a representative way which maximizes emissions.

Radiated emissions are made by measuring the field strength produced by the EUT at a defined distance, usually 3m or 10m. The measurements are made on an open area test site which comprises a metallic ground plane, over a flat surface with no reflecting objects and within a defined ellipse.

The ground plane should cover a larger area than the test range, for example a 6 x 9 m area would be ideal for a 3m range, and a 10 x 20m area for a 10m range.

The EUT is situated 1 m above the ground plane on an insulating support (unless it’s floorstanding equipment) and a calibrated antenna is placed at the required test distance from the EUT. At any emission frequency, such as the harmonic of the clock oscillator in a PC, the receiver is tuned to the frequency and the antenna height is raised between 1 and 4 meters in order to observe the maximum field strength radiated by the product. (The net field strength is the sum of the direct and ground- reflected waves and it varies with height.) The EUT is also rotated about a vertical axis in order to measure the maximum radiation in the horizontal plane.

Measuring instruments for both conducted and radiated emission measurements comprise spectrum analyzers or dedicated measuring receivers. The spectrum analyzer usually has to be modified to have a stage for pre-selection which prevents overload and damage in the presence of impulsive noise, and it may require additional external pulse-limiting protection when performing conducted emissions measurements with a LISN. Both instruments usually have facilities of computer control by the IEEE bus, avoiding the necessity for manual operation. When using spectrum analyzers it’s important to check for overload or spurious emissions by ensuring that the observed indication on the display reduces by 10 dB when an additional 10 dB RF attenuation is introduced at the front end of the analyzer.

EMC measuring receivers are designed to meet the stringent requirements of CISPR (Committee International Special Perturbations RadioElectrique), a subdivision of the IEC. This sets out specifications for input impedance, sensitivity, bandwidth, detector function and meter response, such that the reproducibility of tests can be guaranteed.

Immunity:

ESD tests are made with an ESD 'gun', set to the desired voltage which is equivalent to the human charge potential, and having well-defined charge and discharge characteristics. The ESD discharge is applied to all user-accessible parts of the EUT. The operation of the equipment is thus observed for any malfunction. Immunity to the ESD event is improved by minimizing the ESD energy that can enter the enclosure containing the electronics. The ideal solution is either a good shielded enclosure with small apertures and good bonding between sections, or a totally non-conducting surface. Generally it’s difficult to design a product which completely satisfies either solution, but designers should attempt to steer towards one or the other.

Measurements of immunity to RF fields are made in a shielded enclosure, the modern types being lined with absorbing materials, such as ferrite tiles on at least five of the six inner surfaces. The EUT is subjected to radiation from an antenna situated in the near vicinity. The field is pre-calibrated to the required level of field strength specified in the appropriate standard, prior to the introduction of the EUT into the chamber. The RF frequency is swept slowly from 80-1000 MHz and any equipment mis-operations noted, the performance level of the EUT having been defined prior to the start of the test.

___ RF field immunity-test arrangement: Screened Ferrite tiles

Transients, surges, dips and interruptions tests are performed with dedicated test instrumentation which fully satisfies the requirements of the relevant standards.

Generally the tests are much quicker to perform than the RF field test, and information on the EMC performance of the EUT can be gathered rapidly.

For ESD tests and fast transient tests the equipment should carry on working after the application of the disturbance without any loss of data. For the RF field test there should be no loss of performance outside that specified by the manufacturer at any time during the test. For dips and surges etc., provided the equipment works satisfactorily, after a manual reset, it will be deemed to have passed the test.

Standards

The three types of EMC standards in current use in Europe are basic standards, generic standards and product-specific standards. Basic standards contain the test methods and test levels at limits but don’t specify a product type. Product standards contain comprehensive details on how the product should be con figured and operated during the test and what parameters should be observed. Generic standards apply in the absence of a product standard and are relevant to all products which may be operated within a defined environment. Both product standards and generic standards may refer to basic standards for their test methods. The important standards in current use in Europe are listed below.

Generic standards:

EN 50 08 1- 1 : Emissions; Residential, Commercial and Light Industrial Environment EN 50 08 1-2 : Emissions; Industry Environment EN 50 081-1 : Immunity; Residential, Commercial and Light Industrial Environment EN 50 081-3 : Immunity; Industrial Environment --7.2 Important product standards EN 50011 : Emissions, Industrial, Scientific and Medical Equipment EN 50013 : Emissions, Radio and TV Equipment EN 50014-1 : Emissions, Household Appliances and Portable Tools EN 50015 : Emissions, Lighting Equipment EN 50022 : Emissions, Information Technology Equipment EN 55014-2 : Immunity, Household Appliances EN 55020 : Immunity, Radio and TV Equipment EN 55024 : Immunity, Information Technology Equipment --7.3 Basic standards

EN 61000-4-2 : Immunity: ESD EN 61000-4-3 : Immunity: RF fields EN 61000-4-4 : Immunity: fast transients EN 61000-4-5 : Immunity: surges EN 61000-4-6 : Immunity: conducted RF voltages EN 61000-4-8 : Immunity: magnetic fields EN 61000-4-11 : Immunity: dips and interruptions

Other product standards are emerging at a high rate which will mean that reliance on generic standards will diminish significantly in the future.

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