Szoncsó, F

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Assessment of EMC Parameters of LHC Front End Electronicsby Fritz Szoncsó CERN TIS1) IntroductionEMC parameters are an issue for LHC detectorelectronics because package density and manmade noise interfere strongly with thesusceptibility that is a by-product of thedynamic range required by many detectorsystems.The parameters include relevant noise emissionsby switch mode power supplies, noise transferby linear power supplies, noise behaviour andemission by the electronics itself. It will beshown that radiation of noise is generally smalland, in case this turns out to be a problem, mayeasily be screened. However, conducted noisemay affect electronics via all connections andstray capacitances. Conducted noise handlingwill depend on system grounding, powering,cabling, connector quality. Screening effortsmay interfere with conducted noise whenscreens become major stray paths.Knowledge about system immunity and thelevels of conducted noise to beexpected, all over the entire frequency range ofthe electronics, will enable designers to reactearly in the system design stage.EMC parameters allow for sound systemchoices whilst avoiding over engineering ofpower systems. Common mode current loadalso determines the layout of the groundingscheme. The ground configuration may then bechosen such that inevitable common modecurrents may enter and leave the system wherethey do no harm.2) Measurement of common mode noiseof switch mode power suppliesSwitch mode power supplies (SMPS) generatedifferential mode noise that in general is calledripple. Ripple may easily be filtered to reachany value desired. However, SMPS alsogenerate common mode noise. Pollution of theground system becomes inevitable if this part ofthe noise is allowed to stray. Figure 1 showshow the SMPS couples to the mains and to theoutput lines.fig. 1 Interference paths of a switch mode power supplyLarge voltage swings occur in the primarywinding of the power transformer T5.Connections to the switching transistors sharepart of the problem. Power supply designerswill try to have capacitive currents circulate asmuch as possible inside the power supply. It is,however, impossible to decouple the powerstage entirely from the case. Output rectificationadds on to the problem because of diode noiseand imperfect screening of the cabling and thetransformer. Thus, in addition to the ripple thatremains after the smoothing filter L1/C106, plusC110 and C109, the radio frequency potential ofthe entire output section varies somewhat withrespect to ground. Due to the inherentasymmetry of the output section the radiofrequency potential will not be the same whencomparing positive and negative line. If oneoutput line is grounded the resulting groundcurrent will not be equal for positive andnegative polarity grounding.Standardised EMC measurements are doneusing 150  terminations (valid for thestandardised conducted noise spectrum) whichis supposed to simulate the mains impedance.The result is not particularly relevant forphysics detectors. Firstly, mains impedances atCERN are one to two orders of magnitudesmaller because of the abundant presence ofother power circuits and the close vicinity oflarge power transformers. Secondly, there is awidely spread tendency to ground the connectedloads, i.e. the detector electronics, at thedetector side. Both boundary conditions requireentirely different impedances for measuring theactual common mode current.Measurements in this paper were done using theconfiguration drawn up in fig. 2.fig. 2 Measurement configuration for SMPS noise assessmentThe mains impedance is simply neglectedwhich reflects the conditions in the caverns to alarge extent. One output polarity is grounded atthe supply output, be it directly or via a resistor.This is, in terms of noise current, a worst caseassumption. Low impedance grounding of theinput circuitry will largely limit or even makeimpossible the development of any sizeablecommon mode voltage between mains input andground. This effect "pushes" the common modetowards the load. For the common mode noisemeasurements a wide range current clamp isused to have access to the spectrum of theground current under various conditions.Measurements shown in fig. 3 show a slightcommon mode dependence on the polarity andsome dependence on the load. The aim of themeasurement is to determine whether or not aparticular SMPS will be compatible with theimmunity level of the electronics or systemsupplied. Measurements 4 and 5 (see fig. 3)detect the common mode noise when the SMPSis idling. The output asymmetry is visible butnot dramatic. Bad quality SMPS would showmuch larger differences. Under load(measurement 8 and 9 in fig. 3) there is abroadening of the SMPS power pulses, aresulting different switching waveform and amore saturated operation of the magneticelements. As a consequence the lower part ofthe spectrum, say, between 10 kHz and a fewMHz, shows much higher noise generation.Whilst remaining within standards dictated byEuropean Community decrees common modenoise now increases considerably. Commonmode noise is conducted the better the lower thefrequency becomes. It is this part of thespectrum that needs to be taken care of whentreating the common mode problem. Inparticular, matching between the immunityspectrum (see section 4) and common modegeneration would be advisable.fig. 3  Conducted common mode noise of 5V 100 A SMPS grounded via a short lead.horizontal 5 MHz/div, vertical 10 dB/div a top line with reference level of  –58 dBm (calibration: –60 dBm correspond to 0.45 mA)The higher elements of the common mode noisespectrum do not show apparent differencesbetween idling and loaded operation of theSMPS. A small difference for positive andnegative grounding remains, however, visibleeven under load. This is the proof that, whenthis SMPS was designed, the output asymmetryhas been thought of thoroughly.SMPS of different manufacturers will have verydistinct patterns of common mode generation. Asecond series of measurements, not shown indetail in this paper, indicated a strong polaritydependence of the noise spectrum. This secondtype of SMPS was evidently designed fornegative grounding via short leads.3) Measurement of common mode ofswitch mode power supplies followedby linear regulatorsSMPS followed by linear regulators or longcables "see" a radio frequency impedance toground that may be as high as a few k. Theresulting common mode currents now dependmainly on the impedance to ground seen by thenon-regulated lead. This lead could have, for theradio frequency range, impedances in the k-range. If used in an intelligent way the linearregulator following an SMPS may well be usedfor reducing common mode currents as much atit may reduce differential mode. Small commonmode voltages may then develop withoutprovoking currents above the immunity level ofthe front end to be supplied.fig. 4  Conducted common mode noise of 5V 100 A SMPS grounded via 5 khorizontal 5 MHz/div, vertical 10 dB/div with a top line reference of–58 dBm (calibration: –60 dBm correspond to 0.45 mA)Again there are four measurements, 10 and 11(fig. 4) when idling, 12 and 13 (fig. 4) for twothirds of the nominal load. Grounding is doneby connecting a 5 k resistor between thenegative lead and ground (measurements 10 and12 in fig. 4) or between the positive lead andground (measurements 11 and 13 in fig. 4).Common mode generation is the same as inchapter 2 but no common mode currents areallowed to develop. Any load supplied by a longcable may easily be kept free of SMPS-generated common mode noise if theasymmetric impedance (impedance to ground)is kept high according to common moderequirements established by a system policy orby immunity measurements.It is stressed again that noise voltages have littleeffect provided they are not allowed to driveany noise currents over large loops. Noise andstray currents are easily accessible for precisemeasurement.4) Common mode immunitymeasurement of an electronics deviceFront end boards are part of a system and needto reply to many boundary conditions.Grounding is necessary for many reasons.Grounding serves at the same time for safetyequipotential, high level system reference,screen connection. It should, if ever possible,not be used for any power return.The radio frequency potential differencebetween metallic enclosures and the board willbe one of the ingredients for common mode.However, the largest “user” of the commonmode budget, i.e. the ground connection(s), isthe power supply system because of its inherentgalvanic connection throughout the caverns andcounting rooms. Some common mode on powersupply lines inevitably strays everywhere,including on to the screens of signal inputcables. Board analogue inputs and thresholdlines are the most sensitive points.Fig. 5  shows the sensitivity of a board's testpulse input to common mode noise. It does notmatter where this common mode comes from.Measurements 1, 2, 3, 4  give the noisesuppression capability of the board. The board'sanalogue output is connected directly to aspectrum analyser. Common mode is introducedon the test pulse line with a level of 0.1 V,0.4 V, 2.5 V and 5 V respectively, over thefrequency range of 0 - 100 MHz, on the currentprobe now used as a broad band signaltransformer. A distinct pattern of noisesensitivity in the range of a few dozens ofmegahertz is clearly visible.fig. 5    Common mode immunity of front end test pulse inputfrequency 0 - 100 MHz, 20 dB/unit, top line reference level –88dBmUsing the same method the noise sensitivity ofthe board was tested on all other connectionsleading to or from the board, such as analoguepower return, digital supply lines, thresholdinput line. All other lines were found to havehigher immunity to common mode. Theweakest point remains at the board input wherecommon mode must be kept below a level thatmay be derived from the dynamic range and themeasured board immunity. The level ofcommon mode noise present after finalinstallation inside a detector depends on theground impedances present at the connectionsof the board. Ground impedances are often to beconsidered like parallel impedances, hence thelarge confusion once the grounding systemneeds to be modified in order to achieve betternoise performance. Stray ground currentsoriginating outside the system under test usuallyare quickly found.A radio frequency radiation test showed verylittle board radiation of max. 78 µA/m in thelower MHz-range. As a consequence the boardwould be immune up to even higher field levelsdue to reciprocity. Radio frequency irradiationover large frequency ranges may only be testedin screened rooms. This test was not performed.5) ConclusionThe paper intends to establish a directrelationship between the noise generation bypower supplies and the noise immunity of frontend systems.Both noise generation and immunity parametersmay easily be determined using slightlymodified EMC-procedures that reflect thesituation inside the physics experiments.EMC parameters allow designers to use costeffective SMPS even in delicate environments.Matching of SMPS and delicate load must bedone over the entire frequency range that fallsinto either bandwidth. In case common modenoise is allowed to drive currents some powerline radiation will occur.Linear power supplies also generate noise. Thecommon mode part is generally small becauselinear power supplies actively generate verylittle common mode noise. However, quite closecoupling to the mains is established by thecapacitance of the windings of the mainstransformer ( 2 nF). Electrostatic screensreduce this coupling by a few orders ofmagnitude. Electrostatic screens therefore aremandatory for common mode and mains noisedecoupling. It may also be shown that linearpower supplies with long distance remotesensing show systematic limitations on theachievable noise performance.

Assessment of EMC parameters of LHC front end electronics

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