TheAD9551acceptsoneortworeferenceinputsignalstosynthe-sizeoneortwooutputsignals.TheAD9551usesafractional-NPLLthatpreciselytranslatesthereferencefrequencytothedesiredoutputfrequency.Theinputreceiversandoutputdriversprovidebothsingle-endedanddifferentialoperation.
Referenceconditioningandswitchovercircuitryinternallysynchronizesthetworeferencessothatifonereferencefails,thereisvirtuallynophaseperturbationattheoutput.
TheAD9551usesanexternalcrystalandaninternalDCXOtoprovideforholdoveroperation.Ifbothreferencesfail,thedevicemaintainsasteadyoutputsignal.
TheAD9551providespin-selectable,presetdividervaluesforstandard(andFECadjusted)networkfrequencies.Thepin-selectablefrequenciesincludeanycombinationof15possibleinputfrequenciesand16possibleoutputfrequencies.ASPIinterfaceprovidesfurtherflexibilitybymakingitpossibletoprogramalmostanyrationalinput/outputfrequencyratio.
TheAD9551isaclockgeneratorthatemploysfractional-N-basedphase-lockedloops(PLL)usingsigma-delta(Σ-Δ)modulators(SDMs).Thefractionalfrequencysynthesiscapabilityenablesthedevicetomeetthefrequencyandfeaturerequirementsformultiserviceswitchapplications.TheAD9551preciselygeneratesawiderangeofstandardfrequencieswhenusinganyoneofthosesamestandardfrequenciesasatimingbase(reference).Theprimarychallengeofthisfunctionistheprecisegenerationofthedesiredoutputfrequencybecauseevenaslightoutputfrequencyerrorcancauseproblemsfordownstreamclockingcircuitsintheformofbitorcycleslips.Therequirementforexactfrequencytranslationinsuchapplicationsnecessitatestheuseofafrac-tional-N-basedPLLarchitecturewithvariablemodulus.