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COUPL1A.docE.M.C. COUPLINGS MAR 2004COMMON IMPEDANCE COUPLINGHot linesCold line (0V)Basic amplification lineEach amplifier stageamplifies the voltage differencebetween a hot line and the cold lineknown as reference or groundIf a high current or fast signal circuit shares this ground it may develop in it some parasitic voltage which adds to the useful one , and spoils the complete amplification line .Parasitic voltage is added to the useful oneThe reference is no more cold or quietUseful voltageload The ground lines must be as low impedance as possible , care must be taken to separate noisy circuits from sensitive circuits at gound level .Good ground planes are necessary .countergroundclocksupplyCHASSIS TO BOARD CAPACITIVE COUPLINGCapacitive coupling: Two neibouring conductors build a capacity (electricity reservoir)sourcesourceSubmitted to a direct voltage sourcethe capacitor charges with a transient current which automatically stops when the capacitor voltage is equal to thesource voltageSubmitted to a variable or alternatingvoltage source the capacitor charges and discharges at the rythm of the source , there is a continuously varying current in the circuit .The higher the frequency the higher the current in a capacitor How to make one of the capacitior plate inactiveSimply by inserting an intermediate plate (screen) connected to the plate which becomes inactive (the upper one here), this creates a secondary capacitor through which the current flowsOne can understand that any track on a printed circuit may act as a capacitor with the chassisEspecially when considering high frequency disturbances .Sensitive track0vChassis or surrounding groundsParasitic sourceTo avoid this sensitive track to chassis capacitor a screen connected to the 0v must be installed . (see micro computeurs)Sensitive track0vParasitic sourcescreenChassis or surrounding groundsDisturbance generating deviceINDUCTIVE CROSS TALK COUPLINGA variable current flowing in a loop (nearly always the case) generates a magnetic field , when this field circulates through another open loop , it creates a voltage at the terminals .voltageThis is the transformer principleThis kind of schematic may happen unintentionally on boards and also between cables . useflat independant loops(with reduced surface)One return for each loopBetter situation if conductors are againstthe conductive structure(with reduced surface)The conductive structure may surround the conductorsWorst case:Large loop circuits having acommon return wireAn additional short circuit (closed loop ,connection at both ends) will cancell the magnetic field , no induction will take place in the victim loopUse ground plane and supply plane between uncompatible signalsand paths at right angle crossingsCAPACITIVE CROSS TALK COUPLINGThis crosstalk may appear between two independant circuits .It may appear between two circuitshaving a common groundWorst case:Close circuits having aremote common return wireScreening the parasitic capacitorwill avoid picking up by victimcircuit . The screen needs only oneend connection to the groundUse ground plane and supply plane between uncompatible signalsand paths at right angle crossingsELECTRIC FIELD TO WIRE COUPLINGIEThis effect is used in radio antennas which are opened wires nevertheless a current is induced in this conductor by an inpinging variable electric field.EEComplete shielding or immediate ground proximity reduces this kind of couplingGenerally boards do not collect electric fields very well and usual metal casings and racks are sufficient shieldings .Only cables must be treated properly .ferritefilterscopeDisturbance generating devicescopeShield boxCoax cableDisturbance generating deviceMAGNETIC FIELD TO LOOP COUPLINGVHVHThis effect is used in radio ferrite antenna or frame antennas , in which a variable magneticfield induces a voltage To avoid this coupling the receiving surfaces must be as small as possible (flat)It means that every conductor must be rout