一个法国牛人的射频培训资料 Electromagcompat1

Electromagcompat1.docELECTROMAGNETIC COMPATIBILITY February 2002REMINDERSDIFFERENTIAL MODEDM signal transmission (to and from isolated wires)DM disturbance: if two wires are close to each otherbut far from a disturbance source ,DM disturbance may be neglected .DM measurementI1+I2COMMON MODECM signal transmission (the return signals flow on a common conductive path)CM disturbance: the disturbance is shared by every conductor in the same directionand the total return takes place in the groundThe ground is a dustbin for IcmEvery cable is pollutedCM measurementI1+I2一 THE SIX ELECTROMAGNETIC COUPLING PROCESSES1/ COMMON IMPEDANCE MODEInterfering voltage VeREMEDIES Z Ie ZExternal current Ie = IcmLow frequencies Z = R R = r l / s(Copper R = 17l / s mW m mm)(Coppert = 35m R = 0.5l / d mW)High frequencies Z = L 2 p f (Z 1 mH / m)UiIe = IcmVeZ = L and RUi + VeNote: Z = L 2 p f = L 2 p 3 108 / l p 1880 (p=l/l) Ue = Z IeSine Ue=LwIePulse Ue=LDIe/Dt80 W24 W8 W2.4 Wlength l l l l 1000 300 100 30 GROUNDING: wide conductor: good Short conductor: better Boards with ground planeCommon mode currents must be limited galvanic isolation for low frequencies Grounding to chassis for high frequencies2/ CAPACITIVE COUPLING BETWEEN BOARD AND CHASSISVe = VcmIe = Icm0v shakentracksChassis orenvironmentalgroundREMEDIES C DVe/Dt0v connected to chassisThe 0v is shaken with respect to the chassis , hence capacitive coupling through sensitive tracksIcm returns through C and other cables it should be by the groundIntrinsic capacity / environmental grounds Ci = 35 D (C pf , D diagonal in m)Plane capacity / close conductor Cp = 9 S/H (C pf , S surface m, H height m)Total capacity Ct = Ci + CpVictim track capacity Cv = n% CtSine Iv = Cv w UPulse Iv = Cv DU/DtIn case of high impedances U is high, 0v must be connected to ground or shielding .Electric shields protect sensitive circuits against these RF voltages .Hand effect: hand = ground.0vIcmVcmThe electric shield intercepts the board-ground capacitive currents every input must be as close as possible to each otherRF currents flow easily through galvanic isolation: transformers 500pf , relays 10 pf , opto couplers 1 pf .Limit quick varying voltages between electronic circuits and their environment3/ MAGNETIC CROSS-TALK COUPLINGHI H U MSine U = M w IPulse U = M DI/DtIUDifferential mode worse case conditions: common length lVv = n% Vg case of high impedance Iv = n% Ig case of low impedancen% = 80 % case of flat cables with only one zero volt returninterference duration 11ns/m = 2T but if tr 2T: reduction coef= 2T/trWith one zero volt return for each signal Dmax = 8%With twisted pairs Dmax = 2%A shield does not reduce low frequency magnetic cross-talks up to 1 MHz; low impedances give high currents then cross-talks at low frequencies .duIHh0Common moded h0 Dmax = (h0/d)interference duration 7ns/m = 2T but if tr 2T: reduction coef= 2T/trDcm DdmUse shielded cables . Move sensitive cables away from interfering cables . Use filters and ferrites (RF). Use symmetrical connections , galvanic isolations (low frequencies 100KHz). Right angles crossing . .4/ ELECTRIC CROSS-TALK COUPLINGIvUgU E I CSine I = C w UPulse I = C DU/DtDifferential modeSame as inductive cross-talk from 80% to 8%interference duration 11ns/m = 2T but if tr 2T: reduction coef= 2T/trAt low frequencies a shield is efficient againstelectric fields . High impedances give high voltages then cross-talks at low frequencies .dh0UgCommon modeSame rules as inductive cross-talksd h0 Dmax = (h0/d)interference duration 7ns/m = 2T but if tr 2T: reduction coef= 2T/trSeparate different types of conductors in different shieldings5/ ELECTRIC FIELD TO CONDUCTOR COUPLINGE IIELow frequencies: E is unstable (masks)High frequencies: reflections f(l) , attenuationsDifferential mode not measurableCommon mode Low frequencies case: l l/10 Imax = E l / 240E 1 V/m may cause troubles in analog systemsGood design will stand more than 10 V/mReducing effectsCable installed on a ground conductorGlobal shiedingInput protections Global shielding 360 filtering through shield ferrites on low Z6/ MAGNETIC FIELD TO LOOP COUPLINGHGround loopH U I Loop impedanceSine U = m0 H S wPulse U = m0 S DH/DtSmall loops case L l/4F = B S cosq B = m0 H REMEDIESSGround planesGrouped input connectionsHigh frequencies case distance between in-out conductors d l/4U = 150 H lL and d l/4 and d l/4 U = 0.4 E lL , d: loop length and width二 DISTURBANCE SOURCES1/ LOW FREQUENCY DISTURBANCES (F1s)Affect the supply , the signals by common mode cross-talk , especially low levelelectronics .Flickers (due to strong current sink)Main frequency variations (1% in Europe)Harmonics due to non linear loads (can trigger SCR , resonances may happen)Common mode switching residual (damped oscillations at a few MHz interfere by cross-talk and common impedance)TRANSIENT CONDUCTED LOW FREQUENCY DISTURBANCESAffect analog circuits, but digital circuits correct.Supply voltage fluctuations (+6% , -10%) a good electronic design stands +/- 15%Supply voltage dips and disconnections (T/2) (wind , storm )Slow overvoltages (DM) (network inductance / compens capacitors , fuses)Test IEC 1000.4.5Damped sine overvoltages (CM) (circuit breakers) low energyLightning strike (average 25 KA , low frequency) H loop UTransient currents (DM , CM) load switching , fluorescent lamp ballast (10Ap)CONTINUOUS RADIATED LOW FREQUENCY DISTURBANCESTransformer field leakage (100A/m) CRT picture damage , noise in cabling loops . ( HBF 1/d3 local source)Overhead lines radiations (trains , mains) ( HBF 1/d2 for a line)Ground electrical heating recently H0 S0Leakage current to earth (filters on digital systems)CTR deflection system (dH/dt)Induction ovens (100HzF10KHz , 10KWP1MW ) HBF 1/d3 localTRANSIENT RADIATED LOW FREQUENCY DISTURBANCESDisturbing if strongShort circuits (strong Isc give strong H) case of fuses Isc = 5 to 50InProblems on low level electronicsOverhead lines switching (overvoltage 2Un and oscillating current giving Hthen overvoltages about 1KV in neighbouring loops for several ms Electronic flashes (two meters away H 0.1A/m with f 100KHz)Lightnings (several 10KA , tr 1MHz)Each one of the six coupling processes is efficientGalvanic isolations are not efficient , even unfavourableGround strap lengths are criticalCables become efficient antennas , short cables may resonateCommon mode prevails . RF disturbances are delicate to measure; precision: 30%CONTINUOUS CONDUCTED HIGH FREQUENCY DISTURBANCES（EMC测试中的传导连续骚扰30MHz）Affect low level analog electronics , rectification .Electric engine commutator noise (10ns edges CM cross-talk)DC/DC converters CM currents (damped sine 100mA , 5MHz F 50MHz)TRANSIENT CONDUCTED HIGH FREQUENCY DISTURBANCESShort edges ( 30MHz .Coils switching (relays, contactors, engines, transformers) inductiveEnergy stored in coil, charges line capacitor, overvoltage spark etc.IEC 1000-4-4 testESD 20KV on 200pF some mJ 10Ac duration 200ns , rising edge 100A , tr 1KW , 1MHzF3GHz) must be shielded .Hertzian transmitters from mW to MW create noise in analog circuitsMicrowave weapons directional antennas and pulse modulation (1MJ)Create thermal effects Km away 3KV/m peakEquipment sensitivity depends on frequency, antenna position, modulation type .Reflections may double field amplitudes.TRANSIENT RADIATED HIGH FREQUENCY DISTURBANCESESD 8A/m 25cm awayHigh voltage switching 100KV pulses generate 50KV/m 1m away , tr 3000 Wm) since it is the electric potential difference which is dangerous , one must know many KV may appear between two distant earth connections . Only an equipotential site is safe , one must reduce the impedance between every point of a site . Electronics need equipotential references , a low earth resistance is useless for them .Earth connectionMetal pipemeshbeltGoose footEarth networkOnly one earth network is the law.EQUIPOTENTIAL REFERENCESThey are all the accessible conductive structures connected together , this contributes to good electronics operation . They are connected to earth for conventional safety reasons .Close to electronic circuitry its an RF reference and efficient shielding .Safe devices , different earthsDefective deviceSafe deviceSHOCK HAZARDSHOCK HAZARDEvery chassis must be connected to earth GY must never be cutReferences must be connected to earth in parallel not in serieGY diameter 35mmThese rules are compulsory but not sufficient for a good EMCREFERENCE NETWORKMechanical reference for protectionSignal reference = functional reference = 0VGROUND LOOPSSurface between an active conductorand the nearest reference conductorGround loopFor RF a ground loop is closed and may be resonating if not damped . CM currents in ground loopsare due to the first and the sixth coupling modes (bad equipotential state)Surfaces between reference conductors are called loops between grounds. This contributes to good equipotential state (Zcm reduced).Nevertheless a connection to earth is necessaryneutralLightningconductorONLY ONE EQUIPOTENTIAL REFERENCEStar connection to earth: max common Z , ground loopAcceptable for not interconnected equipmentsConnection to the closest GY: good immunity for CM LFBut not for RF currents YGShortest connection to the closest equipotential referencePersonnel safety small loop surfaces , very good referenceadditional equipotentials between interconnected equipments is good .Link the chassis directly by means of screws , remove the paint locally , use wide braids , reference grids , short link between grid and earth bar . Every metal structureshould be connected to earth . Reference meshes in three directions against lightnings magnetic fields .Transient plate couples with concrete bars by a few nF in insulating environment; S S equipmentIEC 1000-4-4 testEquipotential reference is the condition for fast digital electronic good working .SIGNAL REFERENCE OR ELECTRONIC REFERENCE OR 0VSuch a reference fears noise and dU/dt with regards to the environment .NOYES(the close mechanical reference )MAINS POWER SUPPLIESThis DM supply delivery must be a star connection type to avoid common impedance , while GY must be connected in CM to mechanical references .DC POWER SUPPLIESConnect shortly the DC power supplies 0V to the chassis .This will avoid board to chassis coupling , and it will allow residual RF currents to evacuate into mechanical reference. (These currents have come through the transformer capacity) .boardsupplyMother boardchassisDIGITAL BOARDS0V plane not slotted0V plane connected shortly to the chassis close to each connector0V mother board: not slotted , srews to chassisMany 0V pins between boards and mother boardDC supply should be delivered by means of planeDM filtering 100F/A and RF capacitorsNo IC farther than 5cm from decoupling capacitorsDirect connection between supply 0V pins and planes.ANALOG DIGITAL BOARDSsupplyDigitalor poweranalog0VClearly separate the two zonesAnalog supply via digital supplyDecoupling to 0V in common zone (ADC , DAC)Analog supplies decoupling near converters No analog 0V mesh with chassisANALOG BOARDS0V plane without slotssupply from the noisiest stagedaisy chain supply to the most sensitive stageDM decoupling on low level stages supplies: OP ampSymetrical power supplies avoid noise in 0VLarge dynamic signals should be carried on symetrical pairs , instrumentation amplifiers have large CMRR avoid CM LF problems .REDUCING EFFECTSEfficient and cheap , they limit the interferences collected by cables (CM coupling reduction: common Z , cross-talk , field/cable)a conductive gutter or any conductive structure close to cables from end to end offersa better reference meshing and a reducing effect by mutual inductance (efficiency 5)A cable is a wide band efficie