A matter of choice for EMC chambers

Cranage EMC and Safety explains the differences between available chambers for EMC testing

Any electronic or electrical device can generate an electromagnetic field unintentionally and therefore interfere with other products operating in the same environment. It is therefore necessary to measure such devices in isolation of any electromagnetic disturbance or influence which may cause an undesired response.

EMC test chambers are three-dimensional spaces with fully enclosed conducting surfaces used to evaluate such products in a simulated low-noise electromagnetic environment. They enable emissions from products to be measured accurately at any time, without influence from either internal or external electromagnetic perturbations.

EMC test chambers are isolated from external electromagnetic fields and electrical supply disturbances by shielding and filtering. Mains supplies, signal and control cables for equipment under test (EUT) are filtered to maintain the shielding effectiveness of the chamber and prevent external interference from entering or leaving the testing environment.

Chamber Types

EMC test chambers are categorised according to their internal wave reflections and echoic or anechoic characteristics:-

Semi anechoic test chamber (SAC)

The semi anechoic test chamber (SAC) has radio absorbing material affixed to all internal conducting surfaces except the floor or ground plane. The ground plane is used to reflect radio waves emitted downwards from the EUT and requires the antenna to be swept in height from 1m to 4m to capture and summate direct and indirect radiation so that measured results at each frequency of interest can be compared against product standard limits for radiated emissions (dBuV/m) specified for equipment. Radio waves propagated by products are directional, not uniformly spherical, so the EUT is required to be physically rotated to direct emissions towards the antenna. Rotation is performed in sync with antenna height adjustments to maximise all the radiated signals and noise coming from the product so that amplitudes and frequencies can be compared against the test specification limits.

Semi anechoic chambers have traditionally been used for radiated emissions compliance evaluation because test results correlate well with open area test site methods, and flat conducting ground planes have general uses for all EMC compliance tests. For radiated emissions compliance testing, SAC’s are calibrated against CISPR-16 and ANSI C63.4 to meet the requirements of normalised site attenuation (NSA). This calibration requirement is factored into all product standards for radiated emissions testing. A different calibration procedure is required for the SAC if it is going to be used to evaluate radio frequency immunity to electromagnetic fields.

Fully anechoic test chamber (FAC)

To perform this test, the SAC needs to be more anechoic, ideally fully anechoic, to minimise wave reflections from the ground plane in order to achieve the required field uniformity for testing. Unlike the SAC, a fully anechoic chamber (FAC) will meet the field uniformity requirements for testing immunity to EN61000-4-3 without needing to be modified.

The Cranage EMC & Safety SAC has a large air-conditioned control room attached for customer and support equipment with the capacity to test EUT up to 1 tonne in weight. It is a multi-functional testing facility for commercial, industrial and military projects. A semi-automated biconilog antenna is used for measurements up to 3GHz and a double ridged horn from 1GHz to 18GHz. Measuring receivers are time domain and stepped frequency scanning. Emissions testing in the SAC is UKAS accredited up to 6GHz. It is predominantly set-up and used for measuring emissions.

Examples of emissions standards that are used by Cranage in the SAC are shown in the following table:-

The fully anechoic test chamber (FAC) has radio absorbing material affixed to all conducting surfaces including the floor. The ground plane is non-reflective and therefore does not require the antenna to be swept in height. Measured results at each frequency of interest can be compared against product standard limits for radiated emissions (dBuV/m) which specify equipment to be measured in a FAC.

For radiated emissions compliance testing purposes, the FAC is also calibrated against CISPR-16 and ANSI C63.4 to meet NSA requirements which is a requirement of all product standards when testing for radiated emissions. Additionally, the FAC readily meets the field uniformity requirements for testing products for immunity. Cranage has realised the benefits of the FAC for immunity testing and has constructed one in the shape of a 5m Cube which fully complies with field uniformity requirements of EN 61000-4-3. It is a permanent set-up frequently used up to 6GHz but with a range in development up to 18GHz.

ESD Simulation and Testing

Electrostatic discharge testing, or ESD, is a common form of immunity testing that is applicable to anything electrical or electronic, and is used to check whether static charges, which are usually imported by operators, can cause damage to semiconductors or materials or interruptions to the functioning of a product whilst it is running. The simulation of multiple electrostatic discharges at different test points on a product provides the design engineer with confidence that it will operate as intended when put into use.

ESD testing on products is usually assessed against IEC/EN 61000-4-2 methodology, in association with the product specific standard requirements, and tests must be carried out under controlled climatic and electromagnetic conditions to ensure that the test environment does not influence the test result.

The electrostatic discharge simulation test is applied to all parts and surfaces that are accessible to a human touch using a contact or air discharge electrode attached to the ESD generator. The contact discharge test is applied to conductive parts, for example outer shells of connectors, control panels, toggle switches, handles, fasteners, apertures and seams, and momentarily transfers a stored electric charge in less than 1ns from the ESD generator to the equipment under test (EUT).

Tests are usually conducted at an applied voltage of 4kV and up to 8kV. The air discharge test is applied to insulated parts, eg. membrane switches, dials, LCD/LED indicators, exposed connectors (but not pins), cables and exposed surfaces which are insulated for safety reasons. When conducting the air discharge test, the electrode tip is charged and moved closer to the ESD target until a visible voltage breakdown occurs across to the device.

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Jonathan Newell

Jonathan Newell is a graduate of Loughborough University and has three decades of experience in engineering as well as broadcast and technical journalism.

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