Application Note: AN2093 · ground circuit that is shared by all of the components ... Application Note AN2093 Page 2 Revision P01 ... 3.1 OCXO Theory of operation

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Application Note AN2093Page

1Revision P02Date 18 April 2018

OCXO Layout Guidelines

Application Note AN2093Page 1 Revision P01Date 10 May 2013

1.1 Introduction

Thetechniquesincludedinthisapplicationnotewillhelptoensuresuccessfulprintedcircuitboardlayoutusinganoven-controlledcrystaloscillator(OCXO).Problemswithlayoutcanresultinnoisyanddistortedfrequencytransmissions,error-pronedigitalcommunications,latch-upproblems,significantlyreducedfrequencystability,thermalinstabilitywithintheOCXO,andotherundesirablesystembehavior.

1.2 This document includes the following • Powerandgroundcircuitdesigntips. • Theoryofoperation/Characterization • Techniquestoachieveoptimalthermalconditionsusingmultilayerboards • Adesignchecklist

1.3 About this document

ThemethodspresentedinthisapplicationnoteshouldbetakenassuggestionswhichprovideagoodstartingpointinthedesignandlayoutofaPCB.Itshouldbenotedthatonedesignruledoesnotnecessarilyfitalldesigns.ItishighlyrecommendedthatprototypePCBsbemanufacturedtotestdesigns.

ForfurtherinformationpleasecontactConnor-WinfieldEngineeringDepartment.

Application Note: AN2093

Section 1: About this document.

Specifications subject to change without notice. All dimensions in inches. © Copyright 2012 The Connor-Winfield Corporation



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Application Note: AN2093 Page 2 of 20 Rev: P02 Date: 04/18/18© Copyright 2018 The Connor-Winfield Corp. All Rights Reserved Specifications subject to change without notice

All Rights Reserved. See website for latest revision. Not intended for life support applications.



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Section 2: Power & Ground Considerations.

2.1 Power Supply and Grounds. Allsystemdesignshaveapowersupplyandgroundcircuitthatissharedbyallofthecomponentsontheboard.Theoperationofonecomponentcanaffecttheoperationofothercomponentsthatsharethesamepowersupplyandgroundcircuit.

2.2 Power Supply. Thegoalofasystem’spowersupplyistomaintainastablevoltagewithinaspecifiedrangewhilesupplyingsufficientcurrent.Whileanidealpowersupplywouldmaintainthesamevoltageforanypossiblecurrentdraw,realworldsystemsexhibitthefollowingbehaviors:

• Achangeincurrentanditsassociated noisecausedbyonedeviceaffectsother devicesattachedtothesamepowersupply net.• Achangeincurrentdrawaffectsthevoltage ofthepowernet.

2.3 Typical Power Supply System. Atypicalpowersupplycircuitconsistsofthefollowing:

• Voltageregulatorsthatmaintainvoltage stabilitywithinarequiredrangewhile supplyingsufficientcurrenttoallcomponents served.• Bulk,decouplingandbypasscapacitors.• Powerandsupplycircuitrunnersorpower supplyplanesforpowerdistributionto components.• Localdecouplingandbypasscapacitorsat eachsupplysensitivecomponent.

2.4 Power Supply Management. Impropervoltageregulationcanresultininstabilityofmanysystemcomponentsorcompletesystemfailure. Periodsofinsufficientpowerareoftenreferredtoas“Brown-Outs”,wherepowersupplyvoltagedropstoaninsufficientlevel,or“Black-Outs”wherepowersupplyvoltagestotallydisappearforaperiodoftime.

FortheOCXOtopowerupandconfigureproperlyoninitialpower-up,Vccmustexceedthemaximumpower-onreset(VPOR)voltageinorderforthedevicetoproceedwithconfigurationandinitialization.TheVCCvoltageisinternallymonitoredonpower-uptoproperlytriggerthedeviceconfigurationcircuitry. Onsubsequentbrown-outconditionswherethedeviceisnotpower-cycled(i.e.,thesupplyvoltagesarenottakedownto0v),theVccvoltagemustbetakendownbelowtheminimumVPORvoltageinordertoclearoutthedeviceconfigurationcontent.Subsequently,thevoltagesmustexceedtheVPORvoltageonceagaininorderforthedevicetobeprogrammedwiththenewconfiguration. Abrown-outconditionisdefinedbytheVccraildroppingbelowitsrespectivedataretentionvoltagedefinedbytheVRAMinthedatasheet. PleaserefertotheOCXOdatasheetfortheminimumandmaximumVPORvoltagesandDataRetentionVoltages. Preventsystemmalfunctionduringperiodsofinsufficientpowersupplyvoltagebyusingexternallowervoltagedetectorlogicandsupplymanagementcircuitry.

2.5 POR/BOR Operation. Thepower-onresetoccurswhenthedeviceisstartedfromaVsslevel.Thebrown-outconditionoccurswhenapreviouslypowereddevicedropsbelowaspecifiedrange. ThedevicesRAMretentionvoltage(VRAM)islowerthantheVPOR/VBORvoltagetrippoint.WhenVPRO/VBOR<Vcc<2.7V,theelectricalperformanceoftheOCXOwillNOTmeetthedatasheetspecifications.2.6 Power-on Reset. Whenthedevicepowersup,thedeviceVccwillcrosstheVPOR/VBORvoltage.OncetheVccvoltagecrossestheVPOR/VBORvoltagethefollowingwilloccur: • Volatileregistersareloadedwith valuesformthecorresponding non-volatileregisters. • TheTCONFregisterwillloadthefactory programming. • ThedeviceiscapableofDigital/Analog operation.

Application Note AN2093Page 2Revision P01Date 10 May 2013






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2.7 Brown-Out Reset. Whenthedevicepowersdown,thedeviceVccwillcrosstheVPOR/VBORvoltage.OncetheVccvoltagedecreasesbelowtheVPOR/VBORvoltagethefollowingoccurs: • Factoryserialprogramminginterfaceis disabled. • Non-Volatileregisterarenolonger programmable.

IftheVCCvoltagedecreasesbelowtheVRAMvoltagethefollowingoccurs: • Volatileregistersmaybecomecorrupted. • TCONFregistermaybecomecorrupted. • OCXOovencoremaylosethermal equilibrium. • OCXOfrequencystabilitymaynotmeetthe datasheetspecifications. AsthevoltagerecoversabovetheVPOR/VBORvoltageseesection“Power-onReset”Onsubsequentbrown-outconditionswherethedeviceisnotpower-cycledproperly(i.e.,thesupplyvoltagesarenottakendownto0V),thepowersupplyvoltagemustbedroppedbelow1.6voltsinordertoclearouttheEEPROMdeviceconfigurationcontent.

2 8 Voltage Regulators. Avoltageregulatortakesaninputvoltagefromanexternalsourceandstepsitdowntoasuitablevoltagelevelthatcanpowercomponentsonthecircuitboard.Twocommontypesofvoltageregulatorsaredc-dcconvertersandlow-dropoutregulators(LDO).Whendecidingonavoltageregulator,alwaysreviewtheregulatordatasheetstomatchcomponentspecificationswithsystemrequirements. AsdigitallogicgatesofICsswitchfromonestatetoanother,theIC’scurrentdrawfluctuatesatafrequencydeterminedbythelogicstatetransitionrateor“rise-time”.Thesecurrentoscillationscausethepowersupplyvoltagetofluctuateasasmallvoltagedevelopsacrossthenetduetoitsintrinsicimpedance.Thecircuit’simpedancecanbeloweredbycarefullyselectingacapacitorthatprovidesalow-impedancepathtogroundforhighfrequencies.Asacapacitorchargesordischargescurrentflowsthroughitwhichitselfisrestrictedbytheinternalresistanceofthecapacitor.ThisinternalresistanceisknownasCapacitiveReactanceandisgiventhesymbolXCinohms.Unlikeresistancewhichhasafixedvalue,ie100Ω,1kΩ,etc.CapacitiveReactancevarieswithfrequencysoanyvariationinfrequencywillhaveaneffectonthecapacitor.Theloopfromthevoltagesupplypintodecouplingcapacitortogroundshouldbekeptassmallaspossiblebyplacingthecapacitornearthepowersupplypinandgroundpinofthedevice.


Section 2: Power & Ground Considerations continued






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2.9 Low-Dropout Regulators. Low-DropoutRegulators(LDOs)arelessefficientthandc-dcconverters,buttheyalsointroducesignificantlylessnoiseintothepowercircuits.

2.10 Power Supply Bulk Decoupling and Bypassing. Noisecanbeintroducedintothepowercircuitfromthevoltageregulator,fromICsconnectedtothenet,andfromelectromagneticnoisethatcouplesintothepowersupplytraceandplanes.Powersupply“bulk”decouplingcapacitorshelptominimizetheeffectsofnoiseandprovideotherbenefitstothecircuitaswell.Largevaluebulkcapacitorsimproveperformanceduringlowfrequencyfluctuationsinsupplycurrentdrawbyprovidingatemporarysourceofcharge.Manyvoltageregulatorsmaintaintheirvoltagebyusinganegativefeedbacklooptopologythatcanbecomeunstableatcertainfrequencies.Acapacitorplacedattheregulator’soutputcanpreventthevoltagesupplyfrombecomingunstable.Checktheregulator’smanufacturerdatasheetforrecommendedcapacitorspecifications.Bulkdecouplingcapacitorsshouldbeplacedascloseaspossibletotheoutputpinofthevoltageregulator.

2.11 Ground Circuits. Thegroundcircuitcanintroducenoisetoanembeddedsystemandaffectcomponents.Anidealgroundcircuitis“equipotential”,meaningthatthevoltageofthecircuitdoesnotchangeregardlessofthecurrent.Real-worldgroundcircuitshaveacharacteristicimpedanceandexperiencechangesinvoltagewithchangesincurrent.CarefulPCBdesigncanminimizethisnon-idealbehaviortocreateagroundcircuitthatprovidesalowimpedancereturnpathforcurrent.

2.12 Designing with a Ground Plane. WhilesomesystemsconnectcomponentstoagroundcircuitthroughwiresortracesmostdesignsuseagroundplaneinwhichthePCB’scomponentsconnecttheirgroundpinstoacommonconductiveplane.Designingwithagroundplaneishighlyrecommendedfortworeasons: • Thereturncurrentnoiseofonedevicehas lesseffectonothercomponentswhensharing groundinaparallelconfiguration. • Shortconnectionstogroundminimizecurrent returnpathinductance,whichcaninduce largevoltageswingsinground.

2.13 Ground Plane Fill. Agroundplaneshouldcoverasmuchoftheboardaspossible,eveninspacesbetweendevicesandtraces.

“Islands”ofcopperformedbetweentracesordevicesshouldalwaysbeconnectedtogroundandshouldneverbeleftfloating.Spreadingthegroundplaneacrosstheboardalsoaidsinnoisedissipationandshieldstraces.

Caution!CaremustbetakentothermallyisolatetheOCXOfromtheunderlyingpowerandgroundplanestoensuretheOCXOcanreachthermalequilibrium.Separatingtheanalogcurrentreturnpathfromthenoisierdigitalcurrentreturnpathcanimproveanalogmeasurements.Groundisolationcanalsoimproveperformanceinboardsconnectedtoindustrialornoisysystems.Separategroundplanesshouldbeconnectedinonlyonelocation,usuallynearthepowersupply.

“Caution!CaremustbetakentothermallyisolatetheOCXOfromtheunderlyingpowerandgroundplanestoensuretheOCXOcanreachthermalequilibrium.”

Section 2: Power & Ground Considerations continued





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3.1 OCXO Theory of operation. TheOCXOenclosesacrystalinatemperature-controlledchambercalledanoven.Frequencystabilityisachievedbymaintainingtighttemperaturecontrolofthecrystalwithintheoscillatoroven.CaremustbetakennotwickthermalenergyawayfromtheOCXOpreventingthecorefromreachingthermalequilibrium. WhenanOCXOisturnedon,itgoesthrougha‘‘warm-up’’periodwhilethetemperatureofthecrystalintheinternalovenstabilizesatatemperaturesignificantlywarmerthanambient. During“warm-up”,theperformanceoftheoscillatorwillnotmeetthespecifiedfrequencystabilityuntilnormaloperatingtemperatureisreached.Aftertheovenhas“bridged”thetemperaturewithintheovenremainsconstantasambientvaries. Theovencontrolleroperatessuchthatiftheinternaltemperatureoftheovendecreasesduetoanambienttemperaturedrop,theovencontrollerwillprovidemorepowertocompensateforthermallosses.Similarly,anincreaseinambienttemperaturecausesareductioninappliedpowerintotheovenandthecompensationtemperaturedecreases. Inmostmoderndesignsadditionalheatisconsideredaproblemandwillcausemostdevicestodegradeinperformanceoreventuallyfail.TheheatgeneratedwithintheOCXOovencoreisusedtomaintainaconstanttemperatureatthecrystal.

“During“warm-up”,theperformanceoftheoscillatorisdegradeduntilnormaloperatingtemperatureisreached.”

Section 3: Theory of Operation / Characterization.

Figure: 5-01

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Typical Thermal Performance Characteristics.Test Conditions: OCXO mounted in socket, VCC=3.3V, Temperature = @25°C unless otherwise noted.

The following graphs represent the typical characteristics of a DOC052F-010.0MHz OCXO. Consult with Connor-Winfield Engineering Department for characterization data on any of our existing models.

Time (s)

Vol

tage

(V

)C

urre

nt (

A)

Figure: 6-01 Figure: 6-02

Figure: 6-03

Section 3: Theory of Operation / Characterization continued







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Typical Thermal Performance Characteristics.

Figure: 7-01 Typical Warm-up Time (seconds) Figure: 7-02 Retrace after power cycle.

Figure: 7-03 Typical Warm-up Time (seconds) Figure: 7-04 Typical Warm-up Retrace Curves








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Figure: 8-01 Typical Warm-up Retrace Curves Figure: 8-02 Typical Oscillator Output Start Time

Figure: 8-03 Frequency Error vs Supply Voltage drift ( nom, +5,-5,nom)

Figure: 8-04 Typical Frequency Stability vs Temperature








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Typical Thermal Performance Characteristics.Test Conditions: OCXO mounted on 0.062” 6 layer board, VCC=3.3V, Temperature = @25°C unless otherwise noted.


Time (s)

Vol

tage

(V

)C

urre

nt (

A)


Figure: 9-03








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Figure: 10-01 Typical Warm-up Time (seconds) Figure: 10-02 Retrace after power cycle.

Figure: 10-03 Typical Warm-up Time (seconds) Figure: 10-04 Typical Warm-up Retrace Curves








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Figure: 11-01 Typical Warm-up Retrace Curves Figure: 11-02 Typical Oscillator Output Start Time

Figure: 11-03 Frequency Error vs Supply Voltage drift ( nom, +5,-5,nom)

Figure: 11-04 Typical Frequency Stability vs Temperature








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Typical Electrical CharacteristicsTest Conditions: OCXO mounted on 0.062” 6 layer board, VCC=3.3V, Temperature = @25°C unless otherwise noted.


Figure: 12-01 Typical Oscillator Output Start Time Figure: 12-02 Control Voltage Tuning Linearity

Figure: 12-03 Typical Phase Noise Plot Figure: 12-04 Available Tuning Slopes








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Typical Electrical Characteristics



Fre

que

ncy

Err

or

(pp

b)




See FIGURE 13-01.





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4.1 Layout Considerations ToachievethestatedfrequencystabilityspecificationstheOCXOmustbeabletoreachandsustainthermalequilibriumoverALLoperatingconditions.Observationoftheambientoperatingtemperaturerange,controlledairflow,thermalrampratesandminimizingthermalenergygains/lossesarecriticalforasuccessfullayout. “Anyattempttocool,ordispersethisheatwillcauseovencoretemperaturetodriftandOCXOwillnotmeetthespecifiedfrequencystability.Improperboardlayoutcouldalsoallowheattransferfromnearbycomponentstooverheattheovencoreresultinginlossofspecifiedfrequencystability.” Severalthermaldesignparametersmustbecarefullyconsidered. • BoardLayoutConsiderations • ControllingThermalTransfer/Transients • ControllingAirFlow

4.2 Board Layout Considerations Carefulselectionoflayerstack-ups,groundfills,andtraceroutingishighlyrecommendedforasuccessfullayout. “ToachievethestatedfrequencystabilitytheOCXOmustbeabletoreachandsustainthermalequilibriumoverALLoperatingconditions” Considerthefollowinglayoutconcerns. 1. AlwaysplacetheOCXOnearthetimingcircuitry, andkeepallPower/GroundandRFtracesas smallaspossible. 2. Alwaysadheretostatedloadingspecifications} forOCXORFoutput.OCXOsareloadsensitive andrequireanequivalentloadcapacitorwitha flatcapacitancevs.temperaturecurveto achievestatedfrequencystabilityspecifications. ConsiderNPO/COGcapacitorswhenpractical foroptimaltemperaturecharacteristics.See FIGURE13-01FrequencyResponsetovarious equivalentcapacitiveloads. 3. UsesecondarybufferstofantheOCXORF signaltomultipleinputsortimingcircuits.Avoid designsthatwould“switchin”additional capacitiveloads.Avoiddesignsthatwould “switchbetween”multiplecapacitiveloads. OCXOsareloadsensitivedevices. SeeFIGUREXXX

Section 4: Multi-Layer Board Design / Thermal Considerations

4. Alwaysusespeedratedleveltranslatorsor buffersinapplicationsrequiringcommunication betweendigitaldevicesoperatingfrommultiple supplyvoltages.Neveruseresistordivider networksonRFsignals. 5. Itisrecommendedtoplacea10uFto47uFbulk capacitorasclosetoaspossibletotheVCCpin ofthedevice. 6. Placeasmallceramicdecouplingcapacitor typicallyNPO/COG/X7Rwitha2to3ohm reactanceattheoutputfrequencyofthe OCXOtoshuntanynoiseonthesupplyrailto ground.Additionaldecouplingcapacitors canbeusedtofilteroutotherunwantedsupply noisegeneratedfromotherdevices. 7. Avoidusingseriescurrentsenseresistorsin OCXOmonitoringapplications.Improper selectionofresistorvaluescouldcreate significantvoltagedropswhencombinedwith thethermalcoefficientsofthesupply,andlarge currentdrawfromtheOCXO. 8. Carefulevaluationofviasizeshouldbe consideredwithallhighpowerdevicesto guaranteesufficientcurrent.Maximum currentthroughaviacalculationsshouldbe madeusinghighestexpectedboardtemperature insteadofmaximumambienttemperatureas heatgeneratingcomponentswilltypicallyheat theboardwellbeyondtheambienttemperature rangeoftheOCXO.Improperviasizeselection couldcausecurrentstarvationissueswhich wouldresultincurrentoscillationsastheoven coreisstarvedofpowerandunabletoreach thermalequilibrium.Viatopadstringers orrunnersshouldalsobeevaluatedforsufficient currentcarryingcapacityatmaximumexpected boardtemperatures.Viainpadworkbest,but addsadditionalboardcost.







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Multi-Layer Board Design / Thermal Considerations

9. Lowimpedancepowerandgroundplanes shouldbeusedinsteadofstringertracesorstar basedpowerdistributionmethodstoreduce inducedvoltagescausedbyhighcurrent devicessuchasOCXOs. 10. Forcontrolvoltageequippedmodelsitis importanttoconnectthegroundofthecontrol voltagesourceascloseaspossibletotheOCXO groundtominimizetraceimpedanceand thereforeminimizeanygeneratedvoltages whichcouldcauselargefrequencyerrors.Itis highlyrecommendedtousethesupplied OCXOVREFsourcetoderivethecontrolvoltage whenequipped.Formodelsthatdonotoffer theOCXOVREFsourceitisalsorecommended thatthecontrolvoltagesourcerunoffthesame railastheOCXOtoeliminatethesupplyvoltage thermaldrifterrorsthatcouldexistbetween multiplerails.Thiswillpreventlargeunwanted frequencyshifterrorsastherelativevoltagedrift betweenthesupplieswillbeeliminated. Scenario:Assumeacontrolvoltageequipped OCXOwithatuningsensitivityof7ppm/voltis poweredfroma3.3Vsupply(3V3_OCXOSupply) withatemperaturecoefficientof2.6mV/°C.

Freeviasizecalculationstoolsareavailableonline.SaturnPCBDesignInc.offersthisfreetoolavailableatthefollowingURL.

Figure: 15-01

TheDACwhichdrivestheOCXOCVpinis poweredfromasecondary3.3Vrail(3v3_ DacSupply)withathermalcoefficientof 0.2mV/°C(SeeFigure15-02Example schematicFigure15-03ThermalCoefficientvs. Temperature)InthisexampletheOCXOwas initiallytunedto0ppmat0°Cwithacalibration errorof±0.025ppm.Astheambient temperatureinthesystemchangesto70°Cover thenexthour,thevoltageerrorbetweenthetwo railswouldhavedriftedfrom0voltsbetween thetworails,to0.1834voltsresultingina 1.2838ppmfrequencyerror.

DNC

DNC

Figure: 15-03 Thermal Drift between Supply Rails

Figure: 15-02 Example Circuit

http://saturnpcb.com/pcb_toolkit.htm

Section 4: Multi-Layer Board Design / Thermal Considerations continued







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Figure: 16-01AsmallairgapshouldexistbetweentheOCXOandallthe

wallsofthecover.

Figure: 16-02OCXOPadLayoutw/TopLevelCopperpour.

11. Toreduceunwantedthermalgains/losses, openawindowinanytoplevelcopperpours underthedevice.Excessivethermallosses mayresultinhigherthannormalcurrent consumptionorcompletedevicefailure.See Figure16-03.Excessivethermalgainswill overheattheovencorecausingfrequency instability.FIGURE16-02illustratesahowto properlyfloodatoplayerwhilecreatinga “window”undertheOCXO.

Figure: 16-03

12. Optionallyathermal“moat”canbecreatedto preventthermalenergytransfersbetweenthe OCXOandboardasseeninFigure17-01.A thermalmoatisaroutedareaaroundtheOCXO andisveryeffectivemeanstopreventlosses fromtheoscillatorwhilestillallowingatoplayer pourontheboardtoheatsinkcoolother electricalcomponents. 13. Plasticandmetalcoverscanbeusedtofurther reducesmalltemperaturefluctuationsinthe systembyreducingvariableairflowacrossthe device.Aneffectivecoverwillshouldstillallow asmallairgapbetweentheOCXOandanywall ofthecover.SEEFIGURE16-01 14. DONOTCONNECTANYELECTRICALSIGNALS TOPINSMARKEDDNC.Thesepinsare reservedforfactoryuseonly,andconnectingor monitoringthesesignalscouldpermanently damagethedevice.








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Figure: 17-01

Optionallyathermal“moat”canbecreatedtopreventthermalenergytransfersbetweentheOCXOandthesurroundingheatgeneratingcomponents.








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4.3 Controlling Thermal Transfer / Transients ControllingthermaltransientsiscriticalforproperoperationofallOCXOs.Anysuddenchangesinair-flow,ortemperaturewilldrasticallyeffectshorttermfrequencystability.Ambientsystem,cabinet,orenclosuretemperaturemustalwaystransitionunderthemaximumstatedrateidentifiedontheproductdatasheet.Typical∆tºC/t(minutes)ratesforOCXOrangefrom0.5to1ºC/minute.Anyadditionalheatgeneratedbynearbyintegratedcircuits,ormechanicalpartscouldcausetheinternaloventemperaturetofalloutofequilibriumandnolongermaintainfrequencystability.TheOCXOcontrollercanonlyself-regulateovencoretemperatureifthermalenergyconductedthroughtheprintedcircuitboardiskepttoaminimum.

Avoidthefollowing,astheywillcausetheovencoretooverheatordropoutofthermalregulation.

• PlacingtheOCXOnearheatgenerating components(electricalormechanicalthatcould bleedthermalenergyintotheOCXOcausing theovencoretooverheat,evenwhenthe ambientairiswithintheoperatinglimits)

• PlacingtheOCXOintointermittentairflowpaths (ieswitchedorvariablespeedfans)asthiswill causetheovencoretofalloutofthermal regulation

• Onmulti-layerboardsavoidtoplevelcopper floods,poursandfillsundertheOCXOland patternthatwouldcontributetoexcessiveheat gains/lossesthroughtheprintedcircuitboard resultinginaninabilityfortheovencoreto maintainthermalequilibrium.

AvoidlayoutsasseeninFIGURE18-02

4.4 Controlling AirflowShieldingtheOCXOfromintermittentorvariablespeedairflowpathswillminimizesmalltemperaturefluctuationsandsubstantiallyimproveshort/mediumtermstability.ThiscanbestbeaccomplishedbyshieldingtheOCXObehindtallernon-heatgeneratingcomponentsormechanicalpartstocreateaphysicalbarrier,orwiththeuseofametalorplasticcover.

Seefigures18-01and18-02

Figure: 18-01GoodChoiceforOCXOplacement.OCXOisoutsideofairtunnel,notnearanyheatgeneratingcomponents

Figure: 18-02PoorlocationforOCXO.Variablespeedorswitchedfanscreatelargetemperaturefluctuations








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Section 5: Design Checklist

5.1 Power Supply Checklist • SelectaLOWnoisepowercircuit/source • Addalargebulkcapacitoratthevoltageregulator’soutputthatcanprovidecurrentforlocalcapacitorsand ensureregulatorstability. • Placebulkcapacitorsasclosetothevoltageregulatoroutputaspossible. • Thelargebulkcapacitor’scapacitanceshouldbe10to100timesaslargeaslocalICdecouplingcapacitors. • Addasecondcapacitoranorderofmagnitudeortwosmallerincapacitancerelativetothelargebulk capacitortohelpfilterhigh-frequencynoise. • PlacealocalcapacitanceascloseaspossibletothepowersupplypinofeachIC. • ThesideofthelocalcapacitorthatconnectstogroundshouldbeplacedasclosetotheIC’sgroundpinas possibleinordertominimizetheloopareabetweenthecapandthepowerandgroundpins. • Addafilter,suchasanL-CfilteroranR-Cfilter,tothepowersupplycircuit.

5.2 Ground

• Designusingagroundplaneinsteadoftraceswhenconnectingcomponentstoground. • Ifatoplevelcopperpourisused,itshouldcoverasmuchoftheboardaspossible,includingthespaces betweendevicesandtraces.EXCLUDETHEAREAUNDERTHEOCXO. • Separatingtheanaloggroundplanefromthedigitalgroundplaneimprovesanalogperformance. • Separategroundplanesshouldbeconnectedinonlyonelocation,usuallyclosetothepowersupply.

5.3 General • Keepanaloganddigitalsignalsasfarapartfromeachotheraspossible. • Avoidroutinganaloganddigitaltracesperpendiculartoeachother. • Avoidroutinganalogordigitalsignalsunderoscillators. • Tracewidthshouldremainconstantthroughoutthelengthofthetrace. • Turnsintracesshouldberoutedusingtwo45degreeturnsinsteadofone90degreeturn. • Tracelengthshouldalwaysbeminimized. • Useviasonlywhenabsolutelynecessary. • Avoidtheuseofviaswhenroutinghigh-frequencysignals. • Keeptracesassmallaspossible. • PlacetheOCXOascloseaspossibletothetimingcircuitry. • Neverleavethecontrolvoltagesignalfloating. • Designusingapowerplaneinsteadoftracesroutedfromthepowersupply.







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Design Checklist

5.4 OCXO Layout Considerations • AlwaysplaceOCXOasclosetotimingcircuitryaspossible.. • AlwaysadheretostatedloadingspecificationsforRFoutputofOCXOandconsiderusingNPO/COG capacitorstocreateequivalentloadingcapacitanceforoptimalcapacitancevs.temperaturecharacteristics. • Itisrecommendedtoplacea10uFto47uFbulkcapacitorasclosetoaspossibletotheVCCpinoftheOCXO. • PlaceasmallceramicdecouplingcapacitortypicallyX7Rwitha2to3ohmreactanceattheoutputfrequency oftheOCXOtoshuntanynoiseonthesupplyrailtoground. • CalculateviasizetoensuresufficientcurrentisavailabletotheOCXOattheanticipatedmaximumboard temperaturewhichmaybeSIGNIFICANTLYhigherthanambientairtemperature. • ConnectthegroundofthecontrolvoltagesourceascloseaspossibletotheOCXOgroundtominimizetrace impedance. • Openawindowinallgroundandpowerplanesunderthedevice.Excessivethermallossesmayresultin higherthannormalcurrentconsumptionorcompletedevicefailure. • DonotplaceOCXOinpathorturbulentairflowwhichcouldcausequicklychangingtemperaturefluctuations thatcouldcompromisetheovencoretemperature. • [Optional]Routeathermal“moat”topreventthermalenergytransfersbetweentheOCXOandboard. • [Optional]Useplasticandmetalcoverscanbeusedtofurtherreducesmalltemperaturefluctuationsinthe systembyreducingvariableairflowacrossthedevice. • DONOTconnectanysignalsto“DoNotConnect”(D.N.C.)pins.TheseareFACTORYuseonly.DNCpads maybesoldereddowntoelectricallyisolatedpadsforstructuralsupportonly.

Section 6: References

ChesterSimpson"LinearandSwitchingVoltageRegulatorFundamentals"NationalSemiconductorhttp://www.national.com/assets/en/appnotes/f4.pdf

HabeebUrRahmanMohammed,Ph.D"SupplyNoiseEffectonOscillatorPhaseNoise,"TexasInstruments,ApplicationReportSLWA066–November2011http://www.ti.com/lit/an/slwa066/slwa066.pdf

Section 5: Design Checklist continued

Documents

Application Note: AN2093 · ground circuit that is shared by all of the components ... Application Note AN2093 Page 2 Revision P01 ... 3.1 OCXO Theory of operation