Theoryof Operation- 494A/494ApServlce,Vol | Tektronix 494AP specification

Theoryof Operation- 494A/494ApServlce,Vol. 1

+

EPnoti

+-

--7E@

+--+ +---

SYSTET fiA'U

I

I

I

?

DTA PFESENT DT REOUEST ilA GRANT

feoc

SWPCATE TMER sNO oPrg sRo SERRgO

PWRFAIL

lililililll

NEAR PANEL

GPa SIVITCH

(P VERStOfl ONLYI

-__ ,-7

Llllllull,l

ITISTRUTE'{T oATA 8US

INSTRUTIEilT

AOOfi€SS aus

TIMEN SRO

02CLocK

I/O SPACE MEMORY MAP

Figure7-31. UOaddr€ssspace.

Image 282

Tektronix 494AP service manual Theoryof Operation- 494A/494ApServlce,Vol, UOaddr€ssspace

Contents

Tektronix Page O o o o o o o o a o O o o o o o o o o Releaseof Copyright O a a o o o O oO o o o o O a o o A a a a a Warranty O o o o oO o A a a a A o a o o a o o 494A/494APServiceVot.1 O oA o A o o o o o o o O a o o o a oA o o 494A/494APservice Manuatis divided into two volumes O aO o o o o o o CONTENTSGonr O o o O a o o o O a o o o O o a o o o O o tA a o o o a o o O o o o o o O o a o A o o o o o A o PowersupplyO o o o a o RAM VOLUME2 494A/94APServiceVot.1 494A/494APServlceVol O o o oO t Preselector P3035on theVideoProcessorboard Rejection A o O o O o o494A/494APServtceVot.1 494A/494APServlceVot Simplifiedschematicof harmonicmixerBasicblockdiagramof a -rNsynthesizer Simplifieddigital-to-analog Converter O a o O o oO o a o Options 22 Waveguide 944/494AP Service VotSpEctFtcATloNs MtxERSCHARACTERTSTTCS O a o Do Not ServiceAloneO o o o This Manual O a o o o A o o o o Sedon 1 4g4l494Ap Servlce,Vol O a O o O o o o SELValue. WM 490E 490W Mlxer 18to 26.5to 33 to 490U 40 toWM 49OV 10to 170GHz O o A o Electricalcharacteristics O o o o O o a o o o o A o o o a Freouencyrelatedcharacteristics +tAF. x BEF + 20 + 4NHz Resolution FrequencySpan/Div OverallRange MinimumSpan/Div Characteristic Performance UirementNormal Accuracyand Ldentical Resolution KHz 300 Hz 30 Hz 3Hz Typically 30 psldivision of pulse tude Freouency Relatedcharacteristics E o oFreq O o a e O a o s33-s0GHz Accuracy Steps 10 dB/divLogMode DisplayDynamicRange Accuracy Accuracy Normal or Delta ModeReferenceLevel SEt With Data ENTRYpush Buttons RF Attenuatol Range Accuracy Dcto 1 GHz DecadeTransitions Delta a Mode provides differentialPerformancq Requirement 100 DBc C o A07-100Amplitude Inputsignalcharacteristics Externalmixer O a o e O o o a 1stLO OUTPUTpower Speclfication- 494Al4g4ApService,VolJ104 ACCESSORy Pin4 Pin5 UalrbratorcAL ouT Outputsignalcharacteristics Teristics O a o o Line CE03-15 kHzto 50 MHzpowerleads Speclfication- 494A1494APService,Vot.1Temperature Operatingand Humidity CS02-50 KHz to 400 MHz power F u l l l i m i t Leads O o o o o o o 15.05 28.85inches232x 392x 732.gmmSrDEvtEw O c o A a o o 494A1494ApService,Vot O a o o o o o a o o A o a o o oUnpackingand INITIALINSPECT6N Installation- 494A/494ApServlce,Vol ConnectingpowerPowerSourceand power Requirements O o o A o o o o o o o O o o o o o o O o o o o a o O o a o o o o o o O o a o o o O o o o o o o o O c o O a o o Secton 4g4Al494ApServiceVol O o o o o o o o o o o o o o o o o o o O o o o o o o o o o o o o o o o o O o o o o oEouipmentreouired LodB/soolttffi LrlrGtr O o o o o o o o o o o o o o o o o o o oAccuracy*0.2s dB io Eouipment Reouired Preliminarypreparation MAX 10DB/ 2ODB PerformanceCheck procedure 4g4Ll4g4ApServiceVol O o o o o o o o o o o o Performancecheckprocedure O o a o o a o o o o o o o o o o o o o o o o o o o o aTrME/DrV SPAN/DIV O o o o a o o a o a o o oTIME/DIVAUTO TME/DV Auto Vertcaldtsplay A a o o o o o o o a o o o o o a o o o o o O a o o o o oA a o o o Center Freouencyaccuracycheckpoints Autoresoln on O o a o a A a o o o o a o o o O o a o o o o o o a oCheckCenterFrequencyStabilitv Verticaldtsplay O o a o o o o o o a o o o o o a o o ResolutionbandwidthautoO a o a o a o a a TME/DVAUTO MarkerOutput SPAN/DrVTlmeMarkGenerator 50 Hz 20 ms Autoresoln Reflevel O o a a o o o a o a o a a o o a o a o o o oO a o a o 10dB/Dtv A a A o o o a o oA o a o o o a TimeAccuracy C.**qyegp WithinS%oftherateserecitJi O a o O o a o A o CheckResolutionBandwidth and shape FactorO a a o a o a Minnose Qlglptv to 50Hz, andVERlCnl.drdpr-ry SPAN/DV O o a o a A o o o o o o o o o o O o o o oA o o MIN RFATTENdB 0 GHz Nro sMAooaetenLowLosscoAxcABLEwrrx sul courEEroid 7.4 GHz 9,4 c O o a o o A o O o a a aA o o o o o a A o a a a o a o o a O o o o Narrow Videofilter PEAK/AVERAGE Part O o a o o o A oTmeidivauto Narrowvideofilter TME/DV Auto PEAK/AVERAGE PerformanceCheckprocedure- 4g4Al4g4ApServlceVol.1 O o a Testsettings O a aTrME/DtV Narrowvtdeofilter VerticalDisplay 21BIDIV TIME/DIV Auto 15.RF attenuato. test equipment setup for 50-60 dB step O o O o a Display A a a o a oCentebfreouency Widevideofilter O a a o o o o a o A o o o o o A o aDB/Dtv 12s O o aO a a 103 Trr PerformanceCheck procedure- 4g4A/4g4ApServiceVol Resolutionbandwidth 100 kHzFreorange Autoresoln O aCheckLO Emission -70 dBmor tess Otsplay PerformanceCheckprocedure- 4g4Al4g4ApServiceVol.1 18.Test equipmentsetuplor checklngharmonicdistortion @o@o@g@@ EXT Bandwtdth 1 MHz PerformanceCheckprocedure- 4g4A/4g4ApServiceVol. lResolutton Verticaldisplay LIN Triggering INT TIME/OIV Vertcal Dtsplay 2 dB/DtVTME/DV EXT I3fil l t r l RrttlOptioninstruments CheckHORIZOUTPUTSignatLevel Spanidiv DtsplayMAX Hold Verttcal Tmeidiv Auto PEAK/AVERAGE Comb Generator Mooule TR5O2 Q15oit rilineUll OutputLevel -dBm Var dB Dot Intensity Off A o o o c Gpibverificationprogram Return 5OOOREM49XPGPIBVERIFICATION Program O o o o o o o a oReturn 5O9OINPUTT$ O a o a o r 9040PRTNT10OMSGEOS 8070INPUT8100PRINT 1OO1OPRINTSELECTEDDEVICE-CLEAR...SDC A o o 16050PRTNTTALK Jtestng only 16060INPUT O c16000REM Talkonlymode 16010PAGE Onlymode Fail JJLrsrEN!TALKTLF OCrnoiness ANDRENREMorEJGPIBvERlFlcArtoH Cotapr-erEo Message Generator O o o o o o o O o o o o o o o19300cosuB19390 19530REMFAILUREDECISION Handler 19540PRINT!SELECT a UDK Semiconductor Many Componentsused StancdischargeCan Damage Thisinstrument Preparation Adluctment Procedure- 4g4A/4g4ApServtceVotA a o 7lu Selectorswttch Adjustment Procedure- 494A/494ApServiceVol O o o o o o o o o o o o o o o o o o o Rrqm O o o o o o o o o o o o o o o o o o o o o o o o oO o o o o o o o o o o o o o o TIME/DIV MNL AdJustmantProcedure- 494A/4g4ApServiceVot. .l TIME/DIVAUTO Triggeringfreerun Adiustment Procedure- 4g4Al4g4Ap ServiceVol Test equlpment setup for adiusting the D€flection Amplifier Adfustment Procedure- 4g4Ll4g4ApServtceVot O o o o o o o o o o o o o o o oTriggering EXT O o o o o o o o o o o o o o o o o o o o o o o o o o AdJustmentProcedure- 494A1494ApService VotO o o o o o o o o o o o o OuFur GarnR1033 nrors Adjustnent Procedure- 4g4A/4g4ApServtceVot O a o O o o o o o o o oO o o o o o o o o o o o o o o o o o o o o o RPlo3s-.8 Adjust Log Amptitier=?l R1012 R102S,R1030 AdrustmentProcedure 4g4Ll4g4ApServtce Vot @o.@@GPt O o a o o o o o o o a o o o a o Adjustment Procedure- 494l4g4Ap ServiceVot A o o o a o o a o o o o o o o o o o o oFilrcsotocr A a o A o O o o AdJustmentProcedure- 4g4l4g4Ap Service VotO o o o O a o o o LNorE-l O o a o o a o o a a o o o o o o o o o o o o o o O o o o o o o o a Tion Bandwidth SETTTNGS2Rcsct Tionbandwtdth O o a o a o a $o o3 $oO o o o a a O o o o o a o o o a o a o o o O o o o O o o o o Ai kHrOotHi MIN RF Atten O a a a a o o o o a o o o o o o o o o o oO a o a o a o o o Triggering Auto Adjustment procedure 4g4Al4g4ApseMce vot O a o o O o o a o OTi#i O o aFrequencyrange R1OS4 Frequency AdrustmentProcedure- 4g4A/494ApRplo?2/h To 3.0-7.1 GHz, set AdfustrnentProcedure- 4g4A/4g4ApServtceVol Ot o A o o o AdJustnentProcedure- 494A/4g4ApServtceVot A o o o o o a Snrnasrzen @ Lt. a o o a o O a o@@r Rpzoro O o o o o A o o o o AdjustOption0Z VR BandLevetino Optioninstruments Only AdrustmentProcedure- 4g4A/4g4ApServlceVot,I O a o a o o o o oRFouT OUr Introduction Sectfon6 494A/494ApServicc Vot.1Relativesusceptibilityto Staticdischargedamage Maintenance- 4g4Al4g4ApServlce Vot TroubleshootingAids Maintenance- 494A1494ApServiceVol367-028s,00 Page Diagnostics Troubleshooting Using Theerrormessagedisplay TUNINGFAILURE- 2NDLO Powersupplyoutof Regulation Malntenance- 494A/494ApServlceVol TUNINGFAILURE- 1STLOO a o a o TPl034 GroundRefer€nce 1STLO Setnngaccuracy DesiredMixerfreo Auxiliary Synthesizer O o o o o o O o o o o o a 2NDLO Controldiagnosttca Offset SET.540.672KHZ Ting Accu Racy Enabledpresshelpto Disable PHASELOCKFAILURE- 1STLOLockdisabled Press Help Enable Pressshifttoexit Strobefreq O o a o e Shift Pulsestretcher O o a o a TracemodesMalntenance- 4S4Al4g4ApServtceVol Corrective Maintenance Maintenance- 494Al4g4fupService Vof 1R1020 A46A1Rl015 446A1Rl010Selectedcomponents Mafntenance- 494l4g4Ap ServtceVot.1 O o o o o a o o o o o o A aSERvIcING TooLs FoRBoARDTffi3.i*,o= MouNTEDcoMPoNENTs HorAlr HepairTerminal Maintenance- 494A/494ApServtceVot.1 Diode Color Code O o c a o oO o o O a a O o E a o o o c o o A o o a o a o o Replacingassembliesand Subassemblies O o O o o o O o c o o o o a A30A57-GPIB Interface Maintenance- 494Al4g4ApServiceVot.1 Sr3- ltILlrxER O o o o o o o o o o o o o o o a o o o o o Page Main PowerSupplyModule Removal EAUloNl @o @@tr\ UpJ Maintenanceadjustments O o a o o o o o o o o o o O o o o o o o o o o o o o o o o Malntenance- 494A/494ApServiceVol EouipmentrequiredforreturnlossadjustmentO o o o o o o o o o o o o O a o o A o o o o o o o o a o O o o o o o a o o o Mafntenance- 494A1494ApServtceVot.1 O o a a o o o o O o o o o o o o o o o o a o o O o o o o o o o o o o 829lr/lHzCoaxial Band-passFitterAdjustmentO a a o a o o a a a o a o a o o o O o o o o o o o Maintenance- 494l4g4Ap ServlceVot Malntenance- 494A/4g4ApServiceVol O o o o a o oO a o o O o o o o a o o o o o o o o o o O o o o a o o 17.Corect re3ponsetor g29 MHr tlrlrd and fourth resonators O o o o o o o o o o o O a o oO o o o o o o o a o o Calibrated100 MHz.with +20 kHz accu O o a o o A o o O o oLr.fry- gllpin . DoNorposinouexy op Maintenance- 494A/4g4ApServtce Vot Ffiffi Maintenance- 4g4Al494ApService Vot.1 O a o a o a o o A a o a o o o Mafnfenance- 4g4Ll494ApServtceVot.1 O a a o o a o o A a o a o o o o o Maintenanco- 4g4A/4g4Ap ServlceVol R404OFine TuneRanee O a a o o o a o a o o O a o a A o o FREOSPAN/DV MAX Start Spur Amplttude O o o a a o oMalntenance- 494A1494ApServtceVol Capableof measuring0 Ogmit 2 t\,tt-tz O o o o a a o FREQSPAN/DV MAXFREQSPAN/DIV MAX Autoresoln on Minrf Atten RangeFREQSPAN/DIV MAX Autoresoln Reflevel Timeidv HhFr $ f Malntenance- 494A/494ApServlce VolF t Ll,l Malntenance- 494A/4g4ApServiceVotA o O o o o a o o a a o Jdl tt A a Maintenance- 494A/4g4ApServtceVot.1Settings O o o o a O a C o o o O o o o o O o o o o a oAddressBusTest Maintenance- 494A1494ApServiceVot..t 0xxx U302sSwtTcH O o o o o a o o Troubleshootingon InstrumentbusMaintenance- 494A/494ApServtceVol Instrument Bus Test Yr iffiI ADDRinstrumentbusaddresscommand Responseto ADDRquery DATAinstrumentbus data command DATAinstrumentbus dataquery CombinedADDRcommandand DATAcommandMalntenance- 494Alttg4ApService Vol InstrumentBusRegisters DB7-DBO X x x 0 0 X x x 0 1 X x x 1 0Instrumentbusregisters DB3-DBO O a o A e DBO DB5-DBO ExtmixerRfinput O o o a a DB2DB7-O DB3-DBO DB6 DB2 DBO VcoLSB DB2-DBO Front-PanelRegisters Maintenance 4g4ful4g4ApService Vol TapedatatransferprogramOFF A o o o A o o o a o o O o o aE o Finished O t a O o o o a o O o o O o o o a O o t O o o a a o O o a a o o o a o a o o o o a o o O o o o O o o a o o o o a o o o o o o o o A o o Secton 494A1494Ap Service,Vot Pr signals. Blockdiagrams ShowingmoreRollow O o o o O a o o a o o o o o o o o o a o o O o o o o o Page RF Input TheRFINPUT50 connectoracceptstheinputsig RF InterfaceCircuits Diagram28Lst ConverterDiagram12 O o o o o o o o o o O o o o o o o o o o o o o O o o o o a o o o o o o a o Andpassesdtttreiueniy oroPage Tabte7-1 O o o o o o O o o o o o oTheoryof Operafon 4g4A/494ApServlce,Vot 2NDCONVERTERIF Selection O o o a o o o o o o o o o o o 2472MHz2ND CONVERTERDiagram12O o o O o o a o o o o o o o o CAvrrYPosr+l+ Precision Theory of Operaton 4g4A/4g4ApService, Vol. .lOu-Too---l ExternalCabtes 2182MHz PHASELOCKED2nd LO Diagrams13 O o o o O o a o o o o O a o O a o o O a o O o O o o o O a o a o o o o o o o o a o o a o o Vco MHz 2nd Converter Diagrams15 O a O o a o A o a o O o a a o o oO o o o o o o a o o o a o o o Otr SwitchandamplifierA o a o o o o o o 829MHz2nd Conv O o a a a o o o O o o a Page O o a a o o a o o o O o a a o o o o o 110MHztF AMpLtFtERDiagram17 Theory of Operation 4g4Al4g4ApService,VolA o o a A o o o O o o o O a o 110MHzFTLTERS Diagram173rd CONVERTERDiagram17 A o o o o o o REFERENCELOCKDiagramS0 O o o O o a o O a o O o A o o o o o o a VariabfeResolution Diagrams19, 19,104Id,and Bandwidthselection A o o o o o O o o A a o o o o O o O o o BandLevetingCircuitDiagram20 GalnRequired A o o O o LOc AMp and Detector Diagram22 Fn9 input signat.at a nominal14 dBm, is applied 0316 O a o o a316 316 684 O o o A a o o Theory of Operation- 4gA/494Ap Servtce,Vol VIDEOAMpLtFtERDiagram23 FunctionaldescrtptionTheoryof Operation- 4g4ful4g4ApServtce,Vot Theory of Operaton 494l4g4Ap Servlce,Vol 12. Selecdon ol display positlon on lhe log scale VtDEo PROCESSORDAGRAM24 Theory of Operaton 4g4Al4g4ApService,Vol Ffi rffi O o Filtercomponentcombinations C3026C2016 DB2 DB3 DlclTAL STORAGEDiagrams25 Sytemis desisncto opn IT,r-.,.P.,9? O o LI l Theory of Operadon- 4g4N4g4Ap Servtce,Vot.lPage O o O a o o O o o o o O o o o o o a o o o Theoryof Operafon 494A/4g4ApServlce,Vol O o e O o a a A o a a o o DEFLECTIONAMpLtFtERSDiagram27 500 digitalstoragesweep positions. The resultingClisplayappearsas a bright dot sweepingacross O a o o O o o Axisand RF Interface DTAGRAM28 O o O o o a o o A o O o o o o o o o O a a Enables CurrentdriversTransferswitchdriver MHztF high Stateor F829 MHzlF towstate O o A o O o o o O o a o HTGH,VOLTAGESUPPLYDAGRAM29 Lf BLANKMSBof the characterdatais not set,the O o o o o O o o o o o oCRT READOUTDAGRAM30 GEN RUNNTNGflip-flopunassertsR/o oFF throughoR Theory of Operatlon 4g4A/4g4Ap Service, Vol Zo-cou I lF-€ Theory of Operation- 4g4Ll4g4ApService,Vot. lO o o o o o o o o o o o o o o o o o o o o o o o o o o o OFFTME u1055-d X o o o o o X x x x x xX x o o o o o X . o o o o o O o o o o o o o o o o o o o o o o o o o o o o o o o o o o Theory of Operation- 4g4l4g4Ap Servlce,Vol Oe o \i o Address of Ascii code Skip bit Blank character 58O O o o a o o o o a O o o o o o o o O o a Theory of Opera$on- 494l4g4Ap Servtce,Vol SWEEPDiagram31 Theory of Operaton 494A/494ApService,VolA o o o a o o o o o O a o o o o o a o o A o o O a O a o o o o o o o o 20ssldiv 100D4 D3 D2 D1 do Triggerselectionmodes Theory of Operation- 494A/494ApService,Vol Tabte7-12O a o o a o o o a o o o o o TriggerMode Freerun Internal Ext€rnal Line TableZ-13 A o a o o o o O a a o a o A o a o A a o o o o a o o o o o O o o o o O a A o o o o o o SPANATTENUATORDiagram32 A o A o o A o o o o a o o Theory ol Operaton 4g4A/494ApService,Vot 20kCalibrationcontrolselectioncodes Xl K3065 x0.1 K3075 x0.01 K4072 1st LO DRTVERDiagram33O o o o a a a o o A o o a a a X0.01 O o a o o o o o O a a o a O o o o o o o o o o a DB4DB7 Preselectordriver Diagram34 A oO o a a o o o A o U5031OUTPUTLINES O o a o a o a oO o a o o o o o Span Volts Span Volts Theory of Opera0on- 4g4\l4g4Ap Servlce,Vot TabteT-18Preselector Freouencybands Centerfreouencycontrol DIAGRAM35 O o A a a a o O o oA o a o o o o O o a O o o Figure Z-2S. DAC Varlance graph O o O a o o A o RuooeourTheory of Operaton 494l4g4Ap Servlce,Vot O o o a o ADDRESS70 Formats O o a o o o a o C o a o O o o o o o o o Theory of Operation- 4g4ful4g4ApService,Vot O o o O a aFunctionaldescription HARMONICMIXERDiagram36 AUXILIARYSYNTHESZERDiagram37 O a a A o oTheory of OperaUon- 494Al4g4ApServtce,Vot.1 Theoryof Operation- 4g4A/494ApService,Vol O o o o o O a o o o o o a A oPage COUNTERBOARDDiagram38 Phaselocksynthesizer Diagrams39 O e o o o o o o o o o o o O o o aO o o o o e U2025OUTPUTLINES O e o o a o o o o o o o o o Page DIGITALCONTROLDiagram9 Theory of Operation- 494l4g4Ap Service,Vot Theory of Operation- 4g4Ll4g4ApServlce,Vol 30. System memory map Theoryof Operation- 494A/494ApServlce,Vol 31. UOaddr€ssspace Theory of Operation- 4g4hl4g4ApService,Vol O o c oPage A a O o O o o o o o o a o PollbitsBit A o A a o O o a o o o o o A a O o o o o Ff nsrrrcrr qrs crccrLunr O a o O a o o o o o o A a O a o o O a o O o o o O o o a o A o o O o o o o o o o a o o o o o o o o O o o o o o o o o a o o 100 101 CoL 102 103 Main power Supplyfurnishes all the regulatedO o o o a o O o o o o o o Theory of Operation- 4g4A/494ApServlce,Vol O a o o o o o o o a o o o o o O o o o A o o o o oTheory of Operaton 494A/494ApServtce,Vot 105 Theory of Operation- 494l4g4Ap Servlce,VotOenanon*l Theory of Operation 4g4Ll4g4ApService, Vot 106 +5 V Voltage ReferenceSupply O o o o o O a a o a o o a a a o o o a o Determiningcomponents 107Page Opl ONs RcV/134Swtss O a o o o o o o o o o O o o O o o o CalibratorOutput cALOUT A o o o o o o o O a o o a o o o o o o o a o o o o o o o O a o O o o o o o o o o o o oFreouency Symmetry O o o o a o o o A o A aGoesanoffi OoultnGffi A o o a o o A o o o o a a o o o a o o o o GeneraltermsVtdeoimarker Frequencyterms Gfoesary- 494l4g4Ap Service,VolO o o o o o Amplitudeterms A o a o o Digitalstorageterms O o a o o o O a a o o a O o o o o Revisioninformation Manual Part No G556O-00 Flrst prtnilng JAN1987 Revleed O o a a o a o o o o o o a o o SENSmVTY O o o o a o12s Instrument O o O o a o o A o a o Efr/sN O o o A o a o o O o o o o o o O o O o o o Product see llst Dale2-22-88 Chg. Ref M63911 DeflectionAmplifiertestpointsandadjustments Page5 ol Productsee llst O a o o o o o o o o Performance Checkprocedure O o o o o O o aInstrument Manual A o o o O o O a a Instbument Partno ADD O o o o o O o o o a Perfrmanceheck pRocEDURE change stepr r as foltows Product494N4g4Ap ServiceVolDate 5/u9o O a o o o Date 6/1190 ServiceVot.1O a o o o o Product494N494Ap Vehticaldisplay RHILO0 Peaiaverage +i\ + \ MHz 10dB 100kHz Resolution Verncal Freouency Bandwidth Display SpaivdivMHz DB Bandwidth DB Bandwidth Limits 60dB Measured Minnoise Verticaldisplay ResolutionbandwdithParti Narrow Videofilter TIME/DIV Auto Peaivaverage A a a Peavaverage PerformanceVerification SpecificationTercfgnix CHANcE INFoRMAnoN Check1 dB Compressionpoint SpectrumAnalyzerUnder Test FREQUENCYSPAN/DIV 1 MHz RESOLUTIONBANDWTDTH1 MHzProduct SeeLlst Date11 Ref. C1-991 A o a o o O c