PicoQuant MultiHarp 150 - Page 33

PicoQuantGmbH MultiHarpSoftwareV.3.1.0.0
tectormaybeatriskatsuchhighcountrates.Youcanthenalsotryoutresponsivitytoilluminationchanges.For
SPADdetectorsthattypicallydeliverpre–shapedpulsesofconstantamplitudethesettingofthetriggerlevelis
verysimple.Justsetthetriggerleveltoapproximatelyhalfofthepulseamplitude.
Toactuallycollecthistograms,selectameasurementrangelargeenough(determinedbythechosenresolution)
and  a  display  range  to  cover  your  sync  interval  (i.e.  1/f
sync
)  if  possible.  Set  Offset=0  and  StopAt=
4,294,967,295.Startameasurementinoscilloscopemodewithe.g.,1secondacquisitiontime(seethenext
sectionforinstructions).Oncecountsarecomingin,youcantrytosetanupperlimitforthecountaxissothat
thehistogramisscaledforbestviewing.Startwithlogarithmicdisplaysothatweaksignalscanbeseen.
Ifyouuseduncorrelatedlightsofar,youmaynowwanttomoveontotime–correlatedmeasurements.Witha
detectorsignalthatise.g.,inducedbyalaserora‘fake’electricallyderivedfromthelasersync,youcantryto
obtainahistogramthatshouldbeanarrowpeak,asopposedtotheflatdistributionofuncorrelatedlight.This
requiresanexperimentalsetupthatdeliversthesignalsatthetwoinputchannelswithamoreorlessconstant
relativedelay.InhistogrammingmodethisdelaymustbesuchthattheChannel1..N(detector)signalcomes
laterthanthesyncsignal(forwardstart–stopmode).Inaddition,thedelaymustbechosensothatitfitsinthe
measurementrangeofthehistogrammer.Toobtainsuchatimingitmaybenecessarytoadjusttheinputoffsets
ortherelativecablelengths,whilealsoconsideringanyopticaldelays.
Iftheroughsetupiscompleteandatime–correlatedsignalpeakispresentinthehistogramyoumaywantto
switchtoahigherresolution.Foroptimizationpurposesyoucanthentrytoslightlyvarythetriggerlevels(in-
cludingsync)forbesttimingresponse.TheFullWidthatHalfMaximum(FWHM)ofthesignalpeakisdisplayed
on-lineasafigureofmeritforinstrumentresponse.Ifthereisnoclearoptimumforanylevel,returntothecen-
terofitsstableworkingrange.Youcanthenalsoadjusttheinputoffsetstoplacethesignalproperlywithinthe
timeaxisboundariesofthesyncframe.Youcanalsoadjustyouraxislimitstooptimizethedisplay.Notethat
youcanusethemousewheelforquickchangesofanyspincontrol.Notealsothatyoucandouble–clickallthe
ratemeterstoenlargethemasseparatewindows.ThisisusefulforopticalalignmentworkifthePCscreenis
somedistanceawayfromyouropticaltable.
Input Troubleshooting
Wheneverthereisaproblem,firstcheckyourcabling,detectorpowersupply,andsyncsource.Tobeabsolutely
sure,checkthesignalswithafastoscilloscope(50Ωinput!).Never use TTL signals directly.Nevertrytode-
liverthesignalstomultiple50Ωloadsinparallel,usingsimpleT–pads,useproperpowersplittersinstead.If
youhavemultipledetectors,trytotestyoursetupwithonedetectorfirsttokeepthingssimple.Neverforgetto
letdetectorsusinghighvoltagedischargebeforeconnecting/disconnecting.
Ifyoucannotgetastablesyncratereadingwithexpectedvalues,theremaybeseveralreasons:
● thereisnopropersyncsignal(voltage,polarity,pulsewidth,frequency)
● thesynctriggerlevelsettingisinappropriate
● thesyncdividersettingisinappropriate
● Notethatatsmallratesthemeterdisplaymayfluctuatebetweensomediscretevalues.
Reasonsforzeroinputchannelcountsare:
● wronginputselectionfortheratemeters
● noorinappropriatesignal(voltage,polarity,pulsewidth,frequency)
● inappropriatetriggerlevels
● preamplifierordetectorfailure
Notethatallmetersareupdatedatthedisplayrefreshrateyouhaveselectedinthegeneralsettingsdialog.In
oscilloscopemodetheupdatefrequencyisequaltotheacquisitiontime.Ifnothingseemstohappenatall,you
maybeinoscilloscopemodewithaverylongacquisitiontimeset.
Oncecountrates>0arebeingdisplayedyoushouldthenalsoseecountsappearinginthehistogrammingrate
meter,ifyourmeasurementrangeislargeenough.MakesuretheoffsetvalueintheAcquisitiontabissetto0.If
therearehistogrammingcountsbutnohistogramisbuildinguponthescreen,checkthetimeandcountaxis
bounds.Itisbesttostartwithawidedisplayrangeandthennarrowitdown.Similarly,alogarithmiccountaxis
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