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Electromagnetic compatibility 2719080Mains conducted limitsClass A QPClass A AvgeClass B QPClass B AvgedB V706050400.1 1 MHz 10 100Figure 8.4 Conducted emission limits50Radiated emissions limits45dBuV/m at 10 m4035302520EN/CISPR Class AEN/CISPR Class BFCC Class AFCC Class B10 100 MHz1000Figure 8.5 Radiated emission limitsNB consult current specifications to confirm limit values
272 The Circuit Designer’s CompanionImmunityCISPR publication 20 lays down requirements for the immunity of radio and TVreceivers and associated equipment to RFI and transients. Originally it was aimedprimarily at immunity from citizens band transmitters, although it now covers a widerfrequency range. CISPR 14-2 and CISPR 24 cover immunity of those classes ofproduct, household appliances and information technology equipment respectively, forwhich related CISPR emissions standards already existed.Virtually all other immunity standards are based on IEC documents, including boththe generic and other product specific standards. As well as referencing basic teststandards for the methods of test, these have to define acceptable performance criteriato evaluate the test results, which because of the diversity of equipment they must coverproves difficult. These criteria are based on loss of function or degradation inperformance, but have to distinguish between temporary, operator-recoverable andpermanent failure.8.3 Interference coupling mechanismsAs we have already discussed, interference can be coupled into or out of equipmentover a number of routes (Figure 8.6). Chapter 1 (section 1.3) noted that electronicinteractions follow the laws of electromagnetic field theory rather than the moreconvenient rules of circuit theory, and nowhere is this more evident than in EMCpractice. At low frequencies the predominant modes of coupling are directly along thecircuit wires or by magnetic induction, but at high frequencies each conductor,including the equipment housing if it is metallic, acts as an aerial in its own right andwill contribute to coupling.8.3.1 ConductedA very frequent cause of conducted interference coupling is the existence of a commonimpedance path between the interfering and victim circuits. This path is usually thoughnot invariably in the ground return. This topic has been covered extensively in Chapter1 and you are referred back to there for a full discussion. A typical case of commonimpedancecoupling might involve impulsive interference from a motor or switchingcircuit being fed into the 0V rail of a microprocessor circuit because they share the sameearth return. Because of resonances, the impedance of earthing and bonding conductorsis high at frequencies for which their length is an odd multiple of quarter-wavelengths.Another coupling route is through the equipment power supply to or from themains. The power supply forms the interface between the mains and the internaloperation of the equipment, and as we have seen many emissions standards regulate theamount of interference that can be fed onto the mains via the power input leads. Powersupply design was considered in more detail in Chapter 7. A great deal of work has beendone to characterise the impedance of the mains supply, and perhaps surprisinglymeasurements in quite different environments show close agreement. This has allowedthe development of the CISPR 16 artificial mains network (LISN) referred to in the lastsection. The impedance can be simulated by 50Ω in parallel with 50µH with respect toearth (Figure 8.7).Power cables tend to act as low loss transmission lines up to 10MHz so thatinterference can propagate quite readily around the power distribution network, mainlyattenuated by the random connection of other loads rather than by the cable itself.
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The Circuit Designer’s Companion
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NewnesAn imprint of ElsevierLinacre
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vi Contents2.2 Design rules 452.2.1
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viii ContentsChapter 5Analogue inte
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x Contents7.5.3 Secondary cells 256
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xii Contents
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2 The Circuit Designer’s Companio
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10 The Circuit Designer’s Compani
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Grounding and wiring 25Cable type R
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Grounding and wiring 27Shielding an
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Grounding and wiring 29successive h
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Grounding and wiring 31(a) Two audi
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Grounding and wiring 331. Side-by-s
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Grounding and wiring 35ABV pZ out =
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Grounding and wiring 37times will d
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Grounding and wiring 39of Z o . Thu
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Printed circuits 41material. The co
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Printed circuits 43board cost you s
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Printed circuits 45Board ABoard ABo
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Printed circuits 47which they can w
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Table 3.3 Survey of capacitor types
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334 Indexde-rating 310dielectric ab
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336 IndexFlash ADC 209Foldback curr
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338 IndexMultilayer PCB constructio
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340 Indexlimiting element voltage 7
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