PicoQuant MultiHarp 150 - Page 32

PicoQuantGmbH MultiHarpSoftwareV.3.1.0.0
Ifthesignalissatisfactory,connectthesourcetothesyncinputandstarttheMultiHarpsoftwareifitisnotyet
running.Adetectorsignal(CH1,CH2,...)isnotrequiredatthispointbutitdoesnoharmifitisalsoconnected.
OpentheMultiHarpcontrolpanel.Leftmost,thereisagroupofcontrolsforthesyncinput.Findtheeditboxand
spincontrolforthetriggerlevel.Thelevelshouldbesettoavaluearoundhalfoftheamplitudeofthesync
pulses.Next,settheappropriatesignaledge(typicallytheleadingedgeofyoursignal).Thecode0meansfall-
ing,1meansrising.Thenlookatthesyncdivider.Itmustbesetsothatthesyncratedividedbytheshowndi-
vidervalueremainsunder78MHz,e.g.,foralaserwith80MHzrepetitionrateitmustbesetto2.Foraslow
sourcesuchasaflashlampitmustbesettoNone.Tdeadisaprogrammabledeadtimeforsuppressionofaf-
terpulsingartefactsofsomedetectors.Onthesyncinputitisrarelyneeded.Leaveitat0fornow.
Ifthesyncsourceisactive,thesyncratewillnowbedisplayedatthebottomleftratemeterintheMultiHarp
mainwindow.Notethatthesyncratemeterinternallycorrectsforthechosendividersetting,sothatthemeter
alwaysshowstheundividedinputrate.Themeterdisplayshouldthereforematchtheratedeliveredfromthe
source.Notethatmeaningfuluseofthedividerrequiresperiodicsyncsignals.Theratemeterwillberefreshed
every0.1to1seconds,asdeterminedbythevalueintheGeneralSettingsDialog.Thesyncrateshouldbedis-
playedveryaccuratelyunlessitisverylow.Largefluctuationsoroccasionalzerosindicateanincorrectdiscrimi-
natorlevelsettingoranunstablesyncsignal.Tryvaryingthediscriminatorleveltoobtainastablesyncratedis-
play.Iftherateisstableandattheexpectedvalue,youcanproceedtosettinguptheotherinputs.Alastfine
tuningofthetriggerlevelcanbedonewhenthedetectorinputsareupandrunning.Notethatitmaybeimpossi-
bletogetastablesyncratereadingatverylowsyncrates.Thisisbecausetheratecounterusesaprescaler
andagatetimeof100msandifthesignalperiodistoolowthereadingswillfluctuate.Youwillthenhavetoset
thelevelbasedonwhatyouknowaboutthepulseamplitudeandverifyitinanactualmeasurement.
Setting up the Photon Detector Inputs (Channel 1…N)
Dependingonthemodel,yourMultiHarpmayhaveupto64inputchannels.Asnotedbefore,theMultiHarpin-
putchannelsareelectricallyidenticalbydesign.Theinputsignalsmustconsistofpulseswithsteepnessand
amplitude matching the specifications  and the trigger  edge shouldbe  set to  theleadingedge (rising=1,
falling=0).Forexample,aNIMtypesignalisappropriate.Thisisasteepnegativepulse(0.5to10nswide,active
edgefalling)oftypically−800mVinto50Ω.TheMultiHarpcanactuallyhandle±1.2Vbutlargeamplitudesmay
causeexcessinterferenceandcross-talk.betweentheinputs.Amplitudesaround100to200mV(onallinputs)
arebestintermsoftimingaccuracyandlowesthistogramripple.Itmaythereforebeadvantageoustoattenuate
NIMpulsesby10or15dB.Lowestcross-talk.istypicallyachievedbyusingsignalsofsimilaramplitudeonall
inputs.SMAin-lineattenuatorsofsuitablebandwidthcanbeusedtoadjustthis.NotethatpopularTTL–SPAD–
detectors(e.g.,Perkin–Elmer/ExcelitasSPCM–AQR)deliverpositivepulsesof~3Vandmustbeconnected
throughanattenuatororapulseinverterwithattenuation(PicoQuantSIA400).Connecting TTL signals di-
rectly will cause damage to the MultiHarp!
PMTsshouldbeconnectedthroughapreamplifier(10to20dB).MCP–PMTdetectorsshouldbeconnected
throughanamplifierwithslightlyhighergain.AllaccessoriesareavailablefromPicoQuant.Besuretoswitch
thehighvoltagesupplyofPMTsoffandallowtheirelectrodestodischargebeforeconnecting/disconnecting
them.Theirhighvoltagechargemaydamagethepreamplifier.Observetheallowedinputsignallevelsincluding
thoseofthepre–amplifier.Again,inanewexperimentalsetup,tobeabsolutelysure,pleasecheckyourdetector
pulsesaswellasthepreampoutputwithafastoscilloscope.Starttimingisontheleadingedge,soitshouldbe
steep.Ringingandovershootshouldbeassmallaspossible.Donotover–illuminatethedetectortoavoiddam-
agingit.
Ifthesignalsareappropriate,connectadetectoratchannel1.Startingwithonlyonedetectormakesthefirst
stepseasier.Preliminaryadjustmentscanactuallybedonewithuncorrelatedlight,e.g.daylight.Toprotectyour
detector,usestronglyattenuatedlight,evenwhenthedetectorisoff.StarttheMultiHarpsoftwareandopenthe
ControlPanel.LookattheControlPanelsectionforthechosenchannel.Initiallysetthetriggerleveltohalfthe
expectedpulseheightandsetthetriggeredge(rising=1,falling=0)tomatchtheleadingedgeofyourdetector
pulses.Tdeadisaprogrammabledeadtimeforsuppressionofafterpulsingartefactsofsomedetectors.Leaveit
at0fornow.
Usingtheinputselectoratthefarrightoftheratemeters,selectthechannelyouarecurrentlyusing.Therate
meternexttothesyncratemeterwillnowshowtheinputrateatthatchannel.Makesurethatthereisactuallya
signalcomingin(somestronglyattenuatedlightonthedetector)andtrytoadjustthetriggerlevel.Tomonitor
this,watchthecountratemeter.PrecisetuningofthelevelsettingsisonlycriticalforPMTsorHPDs.Forsuch
detectorsaninitialsettingofthetriggerlevelof-50mV(assuminganon-invertingamplifier,otherwise+50mV)
shouldbeapproximatelyrighttosuppresselectricalnoise.Movingthisleveltowardszeroshouldresultinanin-
creasedcountrate(probablymostlynoise).Ifyoumovethetriggerlevelfartherawayfromzero,youwillseea
reducedratethatwillincurlossesoftruecountsifyougotoofar.Ideallyyoushouldstayjustslightlyabovethe
noiselevel.Ifyoustillseemillionsofcountspersecondat-100mV,reducethedetectorillumination.Yourde-
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