Bán, J

Chase, B E

Citterio, M

Lanni, F

Makowiecki, D S

Parsons, J

Radeka, V

Rescia, S

Sippach, W

Takai, H

CERN Document Server

Characterization of the Coherent Noise, ElectromagneticCompatibility and Electromagnetic Interference of theATLAS EM Calorimeter Front End Board*B. Chase, M. Citterio, F. Lanni, D. Makowiecki, V. Radeka, S. Rescia, and H. Takai,Brookhaven National Laboratory, Upton, NY, USAJ. Ban†, J. Parsons and W. Sippach, Nevis Laboratory, Columbia University, Irvington, NY, USAAbstractThe ATLAS Electromagnetic (EM) calorimeter(EMCAL) Front End Board (FEB) will be located incustom-designed enclosures solidly connected to thefeedtroughs. It is a complex mixed signal board whichincludes the preamplifier, shaper, switched capacitor arrayanalog memory unit (SCA), analog to digital conversion,serialization of the data and related control logic. It will bedescribed in detail elsewhere in these proceedings. Theelectromagnetic interference (either pick-up from the onboard digital activity, from power supply ripple or fromexternal sources) which affects coherently large groups ofchannels (coherent noise) is of particular concern incalorimetry and it has been studied in detail.1. COHERENT NOISE IN CALORIMETRYPick-up due to external causes can impose a seriouslimitation to the resolution attainable in calorimeters atcertain levels of shower energy even when the level ofpick-up in individual channels is small compared with therandom noise. This is due to the fact that signals from anumber of channels must be added to determine totalshower energy. Pick-up signals tends to be correlated overa large number of nearby channels and, therefore, addlinearly when sums are formed, whereas the random noise,being uncorrelated from channel to channel, addsquadratically. Thus, a pick-up signal at the level of 10% ofthe random noise becomes of equal importance to thenoise when signals from 100 channels are added.The coherent noise can come from a number ofsources:1. Noise on power supply lines, in particular thatassociated with switching power supplies.2. Digital noise associated with the operation of logiccircuits and analog-to-digital converters on the sameboard or nearby boards.3. External electromagnetic interference which entersthe enclosure through apertures in the shielding orwhich is conducted on power supply and controllines which penetrate the shielding.1.1. Power Supply NoiseIn the case of the Atlas EMCAL FEB enclosures,power supply noise has been reduced to an acceptablelevel by using resonant charging type switching powersupplies in which the switches open only at zero currentlevel. No increase in coherent noise has been measuredwith respect to linear power supplies. A separate paper inthese proceedings will describe the power supplies.By injecting radio frequency (RF) directly on thepower supply lines by means of a loop antenna, it has beenfound that RF can couple through the power supply cables.Shielding of the cables is being considered and itseffectiveness will be studied.1.2. Digital NoiseIt is the noise being generated by:• Control signals carrying commands into the FEB (andspurious signals from the digital activity of the controlcards).• Digital activity on the FEB itself (40MHz clock,digitizer clock, serializer clock, SCA control lines.Digital noise can be controlled by careful shielding ofthe input amplifiers by means of a local Faraday cageenclosing the preamplifiers and by shielding of the inputconnectors.Good grounding, including ground continuity of theconnector shield to the baseplane and additional groundcontacts from the FEB to the baseplane further reduce thecoupling of on-board digital noise to the input of thepreamplifiers.Use of fiber optic transmission of the control signalsboth eliminates the coupling of unwanted signal from thecontrol cards to the FEB and breaks ground loops createdby the connection of the control electronics to the FEB.1.3. External EMIThe effect of external electromagnetic interferencedepends on the enclosure design, the intensity and natureof the electromagnetic field and its frequency spectrum. Ingeneral, in the closed environment of a colliding beamdetector, we will be concerned only with so-called nearfield radiation coming from signal and command cables* This research was supported by the U. S. Department of Energy: Contract No. DE-AC02-76CH10886.† On leave from the Institute of Experimental Physics, SAS Kosice, Slovakia.associated with other detector systems that pass close tothe FEB enclosures.2. SHIELDING PROPERTIES OF AMETAL ENCLOSURE [1,2]The enclosure housing the FEBs is designed to shieldthe FEBs from external radiated EM fields.Its shielding properties are defined by the “shieldingeffectiveness”, defined as the attenuation ratio (in dB) ofthe field inside the enclosure with respect of the fieldoutside as depicted in Fig1:Several effects play a role, and they will be discussedin the subsequent paragraphs.2.1. Absorption lossIt is proportional to the ratio of the thickness of theenclosure to the penetration depth of the EM wave:2.2. Reflection LossThe reflection loss depends on the mismatch betweenthe wave impedance Zw = E/H and the shield impedance2.3. Total Shielding EffectivenessIt is the sum of the absorption and reflection loss andquantifies the quality of an enclosure. It is plotted in Fig. 2.For near field magnetic fields, the most likely scenarioin the densely packed environment of the calorimeter itincreases with frequency. The rejection of electric fields isabout 60dB higher than the one for magnetic fields at10MHz due to the mismatch between the high waveimpedance and the low shield impedance of the metallicbody of the enclosure.2.4. Effects of Apertures in an EnclosureThe shielding effectiveness of a closed box is reducedby the apertures necessary to feed control lines, powersupplies etc.The shielding (in)efficiency of n apertures is:The shielding effectiveness is also reduced by theseams where the front panels of the FEBs butt wheninserted in the enclosure. It is improved by the presence ofthe cooling plates, which provide additional shielding(they can be modelled as a transmission guide belowcutoff, limiting penetration of an EM wave underneath).In conclusion, the metallic enclosure body presents avery effective reflecting mismatch to the high impedancenear field environment so that electric fields are stronglyattenuated before entering the enclosure.The most troublesome potential source of interference,is likely to be near field RF magnetic fields produced bycurrents in nearby cables.3. MEASUREMENT OF COHERENTNOISEThree techniques were used in an effort to quantify theeffects of external RF magnetic fields. In all cases, amagnetic field was generated by a small loop (about 15cmin diameter) driven from a 50W broadband powerFig. 1: The shielding effectiveness of a closed box isdefined as the attenuation ratio of the field inside theenclosure (Ft) with respect to the external field (Fe).tBInternallyreflectedFe External Incident waveFr Reflected waveFt Wave TransmittedInside the EnclosureSE db( ) 20Log10FtFe----  =AdB 131.4 t µ f⋅ ⋅( )0.5⋅= t in mm( )A Absorption loss=t thickness=µ relative permeability=σ relative conductivity (with respect to copper)=f frequency in MHz=Zshield ω µ⋅( ) σ⁄=RdB 20 Log10Zw4 Zshield⋅--------------------------  ⋅= in dB( )Fig. 2: Total shielding effectiveness of a closed box. It isdefined as the sum of the absorption and reflection losses.1 10 100 1000 10000f[MHz]6080100120140160180Shielding Effectiveness [dB]Near Field Total Shielding Effectiveness A+R vs. frequencyAluminum t=5mm d=20cmElectric Fields (high impedance)Plane Wave (377Ω)Magnetic Fields (low impedance)SEdB 20 Log10λ2 L⋅----------  ⋅ 20 Log10 n λ 2⁄( )⋅–=L maximum linear dimension (e.g. diagonal)=λ wavelength=n number of slots within λ\2=amplifier excited by a variable frequency source andmounted about 25cm from the power bus side of theenclosure. The driving coaxial cable was passed through anumber of ferrite toroids, acting as baluns to reduce theradiated electric field strength. A calibrated loop antenna(COM-Power AL130) was placed 25cm in front of thetransmitting loop. The signals from the 128 channels oneach FEB are summed in four groups of 32 channels fortrigger purposes. In one of the techniques, these sums wereused, rather than the individual channel signals, to increasethe level of coherent noise relative to the random noise.3.1. Measurement of Coherent noise with aNetwork AnalyzerThe first technique makes use of a network analyzer(HP4395). The RF output of the spectrum analyzer isconnected to the power amplifier (see Fig. 4) as the sourceof electromagnetic interference. The calibrated antennaoutput is connected to the “A” input of the analyzer andthe amplified sum signal is connected to the R (reference)input. The network analyzer is swept over the range of1 to 40MHz, using a narrow filter bandwidth (100Hz).The narrow bandwidth suppresses all frequencies exceptthe exciting frequency, and the instruments displays a plotof the RF level measured on the sum signal, normalized tothe excitation field as measured by the receivingantenna.The results, normalized to the coherent noise perchannel assuming an equal contribution of each channel,are shown in Fig. 5a as a solid line for the “slice” of theboard (i.e. the sum top and bottom of one quarter of theFEB) nearest to the power supply bus (worst case). Fig. 5bplots the coherent noise of each slice: the noise decreasesas the distance from the transmitting antenna and the3.2. Measurement of Coherent Noise with theDAQ and Non-coherent ClockThe second technique makes use of the complete FEBdigitizing and data transfer capability of the FEB dataacquisition system (DAQ). The 40MHz digitizing clock isasynchronous with respect to the electromagneticinterference frequency. The RMS fluctuation of thepedestal values for all 128 channels over a large number ofsoftware generated triggers (10,000), each 32 samplesdeep, taken by the switched capacitor array is measured.Channels more sensitive to pick-up than the average (due,for example to less effective grounding or shielding) showhigher levels of total noise. Fig. 6 shows the plot of therms noise versus channel number for a frequency of28.5MHz, at which the average coherent noise ismaximum. It can be seen that the first half of the FEB(ch.1-64) is barely affected. The worst coherent noise isfor the channels nearest to the transmitting antenna andalso nearest to the slots for the power supply, the timing,trigger and control (TTC) cable and the SPAC cable.This coherent noise is due to EMI coupling into theinput of the preamplifiers: it is greatly reduced when thecapacitors connected to the input of the FEB on thepedestal are removed.The plot of Fig. 6 shows also an odd- even effect,related to the length of the pins of the right angleconnector at the input of the FEB. This technique is usefulFig. 3: Magnetic Field H measured by the monitoringantenna 25cm in front of the transmitting loop.6.577.588.599.51010.51111.50 5 10 15 20 25 30 35 40 45Frequency [MHz]H [mA/m]Fig. 4: Block diagram of the experimental setup tomeasure the coherent noise due to an externalelectromagnetic field by means of a network analyzer. TheTektronix AWG2020 is used to provide an external timebase phase locked to the 40MHz clock to the HP4395 tophase lock the RF interference signal when necessary.HP 4395ASlice #1Slice #2Slice #3Sum SignalCOM-Power AL130Receiving LoopTransmitter Loop10MHz Time Base25cm25cm50dB PowerAmplifier28dB40MHz CLKto DAQTriggerPower SupplyBus BarTektronix AWG2020Slice #0for finding defects in the board design and did, in fact,show that additional spring contacts between the two FEBexternal ground planes and the ground planes of theback-plane are very helpful, but it cannot detect a coherentnoise less than about 10% per channel of the randomnoise. To increase the sensitivity, digital sums over largegroups of channels are formed. The rms noise in the digitalsum of the data from n channels is compared with thesquare root of n times the average rms noise per channel. Ifthese are equal, there is no coherent noise. The quadraticdifference is a measure of the coherent noise, but it givesno indication of the source of this noise nor of itsfrequency spectrum.The coherent noise of the digitallygenerated sum of the “slice” nearest to the power supplybus are shown as solid squares in Fig. 5a. Table 1 showsthe coherent noise of various subsets of the FEB for thenoise sources intrinsic to the FEB itself and for an externalFig. 5: Coherent noise vs. frequency of the external RFinterference normalized with respect of the measured Hfielda: Noise of “slice 3” (i.e. sum of 1/4 of the FEB channels,top and bottom nearest to the power supply) measuredwith various techniques. Solid line: Network analyzerSquares: DAQ with non-coherent clock and trigger.Circles: DAQ and coherent clock and trigger.b: Comparison of the coherent noise summed by “slice”:the noise decreases with distance from the FEB sidenearest to the transmitting antenna0246810120 5 10 15 20 25 30 35 40Frequency [MHz][(%)/ch]/ [mA/m]Non-coherent clockCoherent clock and triggerNetwork AnalyzerNormalized Coherent Noise per Channela0246810120 5 10 15 20 25 30 35 40Quarter Slice 0Quarter Slice 1Quarter Slice 2Quarter Slice 3Frequency [MHz][(%)/ch]/ [mA/m]Normalized Coherent Noise per ChannelbTable 1: FEB Coherent NoiseSum typeIntrinsicFEB noise[%/ch]External RFf=28.5 MHzH=7.2 mA/m[%/ch]Top     3.76 mA/mBottom     4.03 22.40Left Half     3.69 11.21Right Half     4.50 48.87Top Left Quarter  4.04 10.61Bottom Left Quarter  3.80 12.25Top Right Quarter  5.13 34.95Bottom Right Quarter  4.86 63.02Slice 0, Quarter     4.54  9.06Slice 1, Quarter     4.00 14.07Slice 2, Quarter     4.39 24.38Slice 3, Quarter     5.64  75.01Total (128 ch)     3.62 29.78Fig. 6: rms noise vs. channel number measured for anexternal RF at f=28.5MHz inducing a field H=28.5MHz.05101520250 20 40 60 80 100 120Channel No.ADC Counts [RMS]RF at a frequency of 28.5 MHz and a field intensity asmeasured at the monitoring antenna of H=7.2 mA/m. BothTable 1 and Fig. 6 show that the pick-up decreasesapproximately exponentially with distance from the sideof the enclosure, the 1/e distance being about 8 channels.This shows that the assumption made, in the other twomeasurement modes, that the pick-up is uniform over 32channels is not really valid.3.3. Measurement of Coherent Noise with theDAQ and Coherent Clock.The technique described in the previous paragraph issensitive to coherent noise down to the level of 1 - 2 percent of the random noise.To increase the sensitivity, the RF interference sourceis phase-locked to the 40MHz sampling clock. Also thetrigger is synchronized to the RF frequency. The 32successive samples saved in the SCA plot out thewaveform of the interference signal. Averaging over manyevents (10,000) suppresses the random noise and leavesonly the coherent noise. Coherent noise measured with thismethod is shown as circles in Fig. 5.4. AN EXAMPLEThe measurement of the effect of an externalinterference on the FEB allows an estimate of themaximum currents allowable in nearby cables.For example assuming a sensitivity of10%/(mA/m)/ch, the maximum field allowable to induce a10% coherent noise per channel would be 1 mA/m. Themaximum magnetic field created by two parallel wiresnormal to the plane of the wires is [1,3]:where I is the current in each wire (in oppositedirections, of course), d is the distance between wires and ris the distance from the wire pair. As an example, thedifferential current flowing in parallel wires 1mm apartnecessary to create a magnetic field of 1 mA/m at adistance of 10cm is ~30 mA. However in a practicalsystem the wires are twisted and the field will be muchlower, and it will be lower still if the wire pair is shielded.A system test has been performed to measure thepossible interferences between the LAr read-out and theTransition Radiation Detector (TRT) read-out.A board emulating the TRT driver and measuring thebit error rate at full speed was brought at the LAr test standat BNL. Four TRT cables, each with 20 twisted pairs werelooped on the side of the FEB enclosures, in the locationwhere the TRT cables will be in the final setup.The effect of the EMI emitted from this cable on theFEB was exceedingly small, with an increase of theaverage coherent noise of ~0.3%/ch at 20MHz, and only~0.08%/ch at 40MHz.Also the effect of the FEB and associated controls onthe TRT bit error rate was so small not to generate anyerror over a 60 hours run at 40 Mbit/s. This corresponds toa BER<10-14.5. CONCLUSIONSThe FEB enclosure provides an effective shieldingagainst EM fields: no effect is measured unless strong EMfields are generated with a power amplifier. The maximumeffect is of the order of 10%/(mA/m)/ch at a frequency of28.5MHz.Its shielding effectiveness is lower for magnetic fields:the most potentially troublesome source of EMI aretherefore near fields from currents in cables nearby theFEB enclosures. The openings on the enclosure and thepenetrations to carry external signals to the FEB are weakspots. Possible improvements are:• Use optoelectronics transmission whenever possible.• Extend the Faraday cage to include the power supplybus bars, cables and switching power supplies crate.To limit the coherent noise of the digital activity on theFEB itself careful shielding of the input connectors andground continuity from the FEB to the baseplane aremandatory.Low noise switching power supplies using a resonantcharging technique do not add to the coherent noise in ameasurable way.6. ACKNOWLEDGEMENTThe TRT test was made possible by the efforts ofM. Mandl and M. Newcomer, who were the principalcontributors throughout the test.7. REFERENCES[1]: H.W. Ott, “Noise Reduction Techniques in ElectronicSystems”, New York: Wiley Interscience, 1988.[2]: A good practical reference is the product catalog ofInstrument Specialties, Delaware Water Gap, PA[3]: S. A. Shelkunoff, “Electromagnetic Fields”, NewYork: Blaidsell, 1963.H 12pi----- I2dr2----- for d r«⋅⋅=

Characterization of the coherent noise, electromagnetic compatibility and electromagnetic interference of the ATLAS EM calorimeter Front End Board

Characterization of the coherent noise, electromagnetic compatibility and electromagnetic interference of the ATLAS EM calorimeter Front End Board

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