Measurements
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. Figure 14.14 shows the general arrangement.
The EUT is configured 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 3 m or 10 m. 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 × 9m area would be ideal for a 3 m range, and a 10 × 20 m area for a 10 m range. The EUT is situated 1 m above the ground plane on an insulating support (unless it is floor- standing 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 m and 4 m 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 analysers or dedicated measuring receivers. The spectrum analyser
usually has to be modified to have a stage for preselection 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 analysers, it is 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 analyser.
The EMC measuring receivers are designed to meet the stringent requirements of Publication 16 of CISPR (Committee International Special Perturbations Radioélectrique), a subdivision of the IEC. This sets out specifications for input impedance, sensitivity, bandwidth, detector function and meter response, such that the reproducibility of the tests can be guaranteed.
Immunity
Electrostatic discharge 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 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 is 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 as shown in Fig. 14.15. 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 is swept slowly from 80–1000 MHz and any equipment misoperations are noted, the performance level of the EUT having been defined prior to the start of the test.
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.