emi unit 5

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EMC Measurement Techniques: Measurements are performed to meet two requirements. First, measurements to ascertain the emission and susceptibility of equipment are necessary throughout the design phase. The  purpose of these measurements is mainly diagnostic, in that it helps to identify likely problem areas and tests the effectiveness of various remedies. These measurements are under the complete control of the designer and test engineer and hence any number of a range of  techniques may be used according to circumstances. Second, tests on complete equipment are  prescribed by standards and these are mandatory in most cases. The measurement arrangement and receivers and transducers used are normally described in considerable detail. Hence, there are certain aspects of the measurement that are fully speci ed and there is little scope for variation. In many cases, formal tests required by certication authorities must be performed by accredited laboratories. If the equipment under test (EUT) fails to meet the standard, it is brought  back to the design ofce for modi cation and it may thus undergo further diagnostic and standards testing. The purpose of EMC design is to reduce the need for retesting and modication to the  bare minimum. Testing involves, depending on the particular standard, frequencies ranging from a few kilohertz to several gigahertz. It is obvious that testin g on such a scale involves a grasp of many electromagnetic phenomena and it is subject to many uncertainties. It is fair to say that EMC measurements are, in general, less accurate when compared to other high frequency measurements. The reasons for this are complex, as will be explained later. Recognition of this fact does not, however, absolve the experim enter from the responsibil ity of ensurin g the lowest  possible uncertainty in measurements. The purpose of this chapter is to present the main experimental techniques and outline areas where large uncertainties may be introduced. Detailed test arrangements and schedules may be found in the published standards. A fully specied EMC test facility requires a substantial investment in buildings, equipment, and personnel and it is not within the reach of medium- to small-sized companies. However, useful iagnostic work may be done with modest resourc es and indeed it is essential that experience of measurements is part of the range of skills available to the design engineer. The material that follows should therefore be of value to all those involved with EMC. Measurement Tools: Irrespective of the formal standard involved, a number of basic tools are required to do EMC measurements. The issues involved in their selection, characterization, and use are briey described below. Receivers: In EMC testing, interference is measured using receivers of a specied bandwidth and detector function. The most common instruments used for this purpose are measurement receivers and

Transcript of emi unit 5

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EMC Measurement Techniques:

Measurements are performed to meet two requirements. First, measurements to ascertainthe emission and susceptibility of equipment are necessary throughout the design phase. The purpose of these measurements is mainly diagnostic, in that it helps to identify likely problem

areas and tests the effectiveness of various remedies. These measurements are under thecomplete control of the designer and test engineer and hence any number of a range of techniques may be used according to circumstances. Second, tests on complete equipment are prescribed by standards and these are mandatory in most cases. The measurement arrangementand receivers and transducers used are normally described in considerable detail. Hence, thereare certain aspects of the measurement that are fully specified and there is little scope for variation. In many cases, formal tests required by certification authorities must be performed byaccredited laboratories. If the equipment under test (EUT) fails to meet the standard, it is brought back to the design office for modification and it may thus undergo further diagnostic andstandards testing.

The purpose of EMC design is to reduce the need for retesting and modification to the bare minimum. Testing involves, depending on the particular standard, frequencies ranging froma few kilohertz to several gigahertz. It is obvious that testing on such a scale involves a grasp of many electromagnetic phenomena and it is subject to many uncertainties. It is fair to say thatEMC measurements are, in general, less accurate when compared to other high frequencymeasurements. The reasons for this are complex, as will be explained later. Recognition of thisfact does not, however, absolve the experimenter from the responsibility of ensuring the lowest possible uncertainty in measurements. The purpose of this chapter is to present the mainexperimental techniques and outline areas where large uncertainties may be introduced. Detailedtest arrangements and schedules may be found in the published standards.

A fully specified EMC test facility requires a substantial investment in buildings,equipment, and personnel and it is not within the reach of medium- to small-sized companies.However, useful iagnostic work may be done with modest resources and indeed it is essentialthat experience of measurements is part of the range of skills available to the design engineer.The material that follows should therefore be of value to all those involved with EMC.

Measurement Tools:

Irrespective of the formal standard involved, a number of basic tools are required to do EMCmeasurements. The issues involved in their selection, characterization, and use are brieflydescribed below.

Receivers:

In EMC testing, interference is measured using receivers of a specified bandwidth and detector function. The most common instruments used for this purpose are measurement receivers and

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Although such measurements are not specified by the various standards, they are neverthelessuseful in giving an insight into the origin and mode of EM emissions. Several equipmentmanufacturers provide “sniffer” probes that are designed to respond to the near field from printedcircuit tracks and other wiring. These give an indication of relative activity from various parts of a circuit, but cannot and should not be used to predict far-field performance. For the test engineer

who wishes to develop in-house probes, the rudiments of design of small field sensors aredescribed below. Field sensors are small because a detailed mapping of the field may be requiredand also to avoid disturbing the field being measured. A sensor is electrically small if its largest physical dimension is much smaller (say 1/20) of the longest wavelength of interest. In itssimplest form, an electric field sensor is a small piece of wire protruding above a ground plane(small monopole). If this sensor is placed in an electric field E, the open-circuit voltage is V =leE where le is the effective length of the monopole. Normally, le is approximately equal to half the actual length of the monopole. 8 The short-circuit current is similarly equal to I = Cle dE/dt,where C is the capacitance of the sensor. Thus, whether the sensor responds to electric field or its

derivative depends on the impedance of the measuring instrument. If the time constant of theantenna capacitance with the measuring instrument input impedance is longer than the fieldcharacteristic time, then E is measured. Otherwise, the arrangement responds to dE/dt. A smallloop is the dual to the electric field sensor and provides a magnetic field sensor. If a loop of areaA is immersed in a magnetic field H, the open-circuit voltage is V = µ0 A dH/dt and the shortcircuit current is I = µ0 AH/L where L is the inductance of the loop. It is clear, therefore, thateither the magnetic field or its derivatives are measured depending on circumstances. If the timeconstant of the inductance with the input impedance to the measuring instrument is much larger than the characteristic time of the field, then the arrangement measures the magnetic field.Various field sensors have been developed to measure transient fields under hostile conditions

and these may also be used as general instrumentation. Another measuring arrangement that provides for very low distortion of the measured field by the measurement system is one basedon a modulated scatterer. A very short piece of wire is used to probe the field. Incident fields arescattered by this wire and the scattered signal is received by a second antenna placed in thevicinity. This received signal contains complete information about the amplitude and phase of the original incident wave. The only difficulty is that the scattered signal is very weak inamplitude. In order to increase the chances of detection, a photosen sitive component is placed atthe center of the short probing wire and its impedance is modulated by a modulated light signalsupplied through a fiber-optic link. In this way the scattered signal is also modulated and phase-sensitive detection techniques may be used to increase the chance of detection of even very weak signals. The sensitivity of this arrangement is low at low frequencies.

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Antennas:

Antennas are specified almost exclusively as the primary field measuring transducer in moststandards. Understanding their behavior in actual test environments is thus of paramountimportance. The basic theoretical background to the characterization of antennas in terms of their

gain, effective aperture, radiation resistance, etc., was presented in Section 2.3.2. From the practical measurement point of view, the single most important parameter of an antenna is itsantenna factor, defined as

with the parameters defined shown in Figure 14.1a. Normally, the antenna factor is expressed indecibels, i.e.,

Knowledge of the antenna factor and measurement of the receiver voltage permits the calculationof the electric field from Equation 14.2. Similar expressions apply for the antenna factor of antennas sensitive to the magnetic field. The antenna factor may be calculated from first principles. It can be shown that for any antenna the effective aperture and the gain are related

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by the expression Ae = Gλ 2/(4π). The antenna may be replaced by its Thevenin equivalentcircuit as shown in Figure 14.1b. The maximum power available is that supplied to a conjugateload and is therefore

where IVT I is an rms value and RT the real part of the impedance ZT. Assuming that theantenna is oriented for maximum response

where I Ei I is an rms value and Z0 is the intrinsic impedance of the medium surrounding theantenna. Equating the right-hand side terms in Equations 14.3 and 14.4 and substituting for Ae

gives

where is described as the effective length of the antenna.In order to calculate the voltage at the receiver, Figure 14.1b is used to give

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Hence, the antenna factor is

Antennas supplied by manufacturers are usually provided with a generic calibration, i.e., anantenna factor at different frequencies for this type of antenna. The antenna factor of the particular antenna supplied may deviate by several decibels from the generic value. For accuratemeasurements, it is thus necessary to calibrate antennas using either approved test houses or in-house facilities. There are basically three experimental techniques used for antenna calibration.In the standard field method a receiving antenna is calibrated against a calibrated transmitting

antenna establishing a known field at a fixed observation point. Alternatively, an antenna of aknown antenna factor may be used for calibrating other antennas in the standard antenna method.Finally, in the standard site method three uncalibrated antennas may be calibrated by doing threemeasurements between pairs of antennas. An alternative approach to antenna calibration is to usenumerical simulation to establish the AF of an antenna-to-plane-wave illumination. Thisapproach has been successfully applied, using the NEC computer code, to dipole and biconicalantennas. It should always be borne in mind that the proximity of antennas to conductingstructures and their use in screened rooms affects their calibration.

A number of different antennas is in use for EMC measurements depending on the frequencyrange. The are listed as follows.

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Test Environments:

EMC measurements are normally performed in open-area test sites, in screened rooms, or inspecial test cells. Standards specify in some detail the basic requirements for each type of testenvironment and the experimental procedures to be followed. The problems encountered in EMCmeasurements in these different environments are described below.

Open-Area Test Sites

An ideal open-area test site (OATS) consists of a perfectly conducting ground plane of infiniteextent, free from all obstructions, with very low levels of ambient electromagnetic noise. Actual

OATS depart in significant ways from this ideal. They are nevertheless used extensively,especially for commercial EMC measurements. Open site measurement is most direct anduniversally accepted standard approach for measuring radiated emissions from an equipment or the radiation susceptibility of a component or equipment.

In selecting an OATS, a flat ground area is necessary, of sufficient extent to approximatethe properties of an ideal site (infinite plane). CISPR regulations specify that the flat area is atleast an ellipse having major and minor axes equal to 2L and √3L , respectively, where L is thedistance between receiver and transmitter. Typically, this distance is either 3, 10, or 30 m.Ideally, this flat area should also be perfectly conducting. In practice, a somewhat smaller

metallized area constructed out of wire-mesh material is used. The mesh must be made out of material that does not corrode. The mesh must make good electrical contact with the undersoiland surrounding area. The required flatness depends on the frequency of operation and thedistance between transmitter and receiver and it is typically a few centimeters for test frequenciesup to 1 GHz.

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MEASUREMENT OF RS:

• EUT is placed in an electromagnetic field created with the help of suitable radiating antenna.

•The intensity of the electromagnetic field is varied by varying the power delivered to theantenna by the transmitter amplifier

• performance of EUT are then observed under different levels of electromagnetic field intensity.

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Power

EUT

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TEM Cell

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Note: Additional information regarding anechoic chambers and reverberating chambers.

Screened Chambers:

Open-area test sites have several disadvantages and this has led to a search for alternative EMCtest environments. Among these disadvantages are the difficulty of ensuring a clean EMenvironment, dependence on the weather, and land costs. In the case of immunity tests, it is alsodifficult to avoid interfering with other users of the EM spectrum. Most military standards,

immunity, or general diagnostic measurements, are therefore made in screened rooms. Ascreened room is an all-metal structure where all access for personnel, electrical, or mechanicalservices is designed in such a way as to provide a high degree of electromagnetic isolation up tovery high frequencies. A well-designed screened room can be used for emission and immunitymeasurements without any EM interaction with the external environment. Screened rooms mayin turn be distinguished as reverberating and anechoic types. In the former case all the internalsurfaces of the room are unlined and highly conducting and the room is thus an electromagnetic

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cavity. Any EUT or antenna placed in this room interacts with the conducting surfaces in acomplex manner. An empty rectangular cavity exhibits resonances at specific frequenciesobtained from the formula given below:

where a, b, and c are the internal room dimensions in meters and m, n, and p are integers with nomore than one being zero. Typical room dimensions range from a few meters to a few tens of meters. The lowest resonant frequency depends on room dimensions and is generally of the order of a few tens of megahertz. Thus, mode TE101 refers to a mode in which the vertical componentof the electric field has a maximum on a line running from the center of the floor to the center of the ceiling (y-axis). Higher-order modes exhibit a more complex structure. The presence of resonances with pronounced field minima and maxima at particular frequencies make

measurements in reverberating rooms difficult to interpret.

Thus, most rooms, especially at lowfrequencies, can only be described as partially anechoic or highly damped. Each room type isdescribed in more detail below.

Reverberating Rooms — A typical test arrangement specified in military standards is shown inFigure below. The EUT is placed on a conducting bench, which is bonded to the room walls. Aconducting extension to the bench may also be fitted to accommodate a rod antenna. Dependingon the room height, the extension may be offset toward the ground to leave adequate clearancefor the rod antenna. Biconical and log-periodic antennas are generally mounted on a mast. Inmilitary standards, a distance of 1 m is specified between the EUT and the measurement antenna.

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The presence of the bench, extension, EUT, and antenna disturb the resonant pattern inside theroom. Small shifts from the resonant frequency values obtained from above Equation areobserved and the presence of the bench introduces new resonances. At low frequencies,understanding of the measurement may be achieved by resorting to lumped-circuit concepts. Atfrequencies for which the largest room dimension is smaller than λ/10, quasistatic concepts may

be applied, whereby the coupling between EUT, antenna, and the room is essentially capacitiveor inductive. At higher frequencies and below the first resonant frequency of the room, coupling between the EUT and antenna is more complex. Apart from the direct coupling (capacitive or inductive) between the two, a further coupling TEM mode is established. This propagates on acoaxial line formed by the bench extension (inner conductor) and the room side walls (outer conductor). This line is terminated by a short circuit (back wall) and an open circuit (end of extension).

At even higher frequencies, the presence of cavity resonances makes the couplingmechanisms difficult to follow in detail and progress can only be made by resorting to

sophisticated numerical modeling techniques. It is generally anticipated that measurementuncertainties in this complex environment can amount to up to ±40 dB. As a result there is littlecorrelation between the same measurements taken inside different screened rooms or in anOATS.

Anechoic chambers — Improvements to the uncertainty of measurements in a reverberatingchamber can be made by lining the walls and ceiling with absorbing material (RAM). Two typesof material are used for this purpose, namely, carbon-loaded polyurethane foam in the shape of pyramidal cones and ferrite tiles. Several manufacturers provide such materials with lowreflectivity, typically in the range –20 to –40 db. For the pyramidal RAM to be effective, its

thickness must be a substantial fraction of a wavelength. At low frequencies this becomesimpractical and uneconomical. In small rooms the reduction in working volume is unacceptableand in large rooms costs are excessive. Thus, below approximately 100 MHz, rooms cannot beregarded as anechoic and it must be accepted that a substantial amount of reflection will be present. An alternative is to use ferrite tiles, which are particularly effective at low frequencies.In broad terms, conventional RAM is associated with electric field losses whereas tiles areassociated with magnetic field losses. In practice, a combination of ferrite tiles and pyramidalRAM is the most effective arrangement for a broadband anechoic room, but costs are verysubstantial.

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Special EMC Test Cells:

arrangement used for EMC testing is the gigahertz transverse electromagnetic (GTEM) cell,shown schematically in Figure below. It consists of a tapered transmission line that isterminated by a distributed 50-Ω load and RAM. Careful design ensures the setting up of a

spherical wave without exciting higher-order modes up to very high frequencies. The maximumworking size of the EUT is again limited to a third of the septum height. The septum is offset, asshown, to increase the working volume. The GTEM cell can be used in a similar way as theTEM cell.

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Measurement Precautions:

1) Electro magnetic environment :

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According to American national standards describes that is conducted and radiatedambient radio noise and signal levels measured at the test site with the EUT deenergized, be at least 6 db below the allowable limit of the applicable specification or standard.

2) Electro magnetic scatters:

One method fro avoiding interference from underground scatters is to use a metallicground plain to eliminate stror reflections from under ground sources such as buriedmetallic objects.

3) Power and cable connections:

The power needs used to energize the EUT, receiver and transmitter should also passthrough filters to eliminate the conducted interferences carried by power lines.