Tektronix 494AP service manual 2NDLO Controldiagnosttca, Offset SET.540.672KHZ Ting Accu Racy

(b) lf the -e1o-r-9cnr:with frequencyspans greater than 5 MHz/div,ttre t si t-O is iroO- abtydefective.(E)

(4) lf the measuredand calculatedcurrentsare not equal,th€ problemis likelyin the finalstage of the LO Driver.(E)

4. Measurethe lst LO tuningvoltageat edgecon- nector 47, of the CenterFrequeniyConirot boarO. Ttris voltageshoutdbe within 200 mV 6t tne tisteoDAC Set value.

?. lffl".vottage is within this timit, faitureof the l st LO Driverboard is indicated.(E)

b. tf th€ vottageis not withinthe limit,failureof the CenterFrequencyControtboardis inOicateO.

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TUNINGFATLURE- zNDLO

The 2nd LO is set by a combination hardware/software loop. There are two distinct hardwareblocksin the loop; the blockwhich measurEs the..oscillatorfrequencyanOtne Uoct wfrictrsets the oscillatorto frequency.The micropro""".o, closesthe roop by d€termininghow far the oscillator must be movedto bring it to the desiredfrequency. setting is an iterativeprocesswhereinthe microprocesso,counts the oscillator frequency, moves it as needed, and counts again. The error messagels displayedif the 2nd LO is not set to the Oesiid trequlncy after a numberof iterations,dependingon instrumentlettings.

The 2nd LO Controt DiagnosticAid displaysdata which can be used to deternrinewhich part ot t'tretoop has.fait€d._11typicat disptayis snown below. press <Blue-SHIFT>PULSE STRETCHERand setect #3 from the menufor the DIAGNOSTICAIDS, then select #2tor the 2nd LO Control.

2NDLO CONTROLDIAGNOSTTCA'-

TUNE VOLTS O.O1V 0 . 19V

2NDLO FREQ 2.182OOOGHZ 2.182140c4z OFFSETFREQ 18.OOOOOOMHZ 17.860MHZ

OFFSET SET.540.672KHZ TING ACCU.

RACY

PRESS "SHIFT' TO EXIT

Malntenance_ 4g4A/494ApServlceVol. 1

The Tune Volts is the voltage that would be expected at the output of the 2nd Lo section of the Cent€r FrequencyControl. The Nominalvoltageis the valueneededfor the Desiredfrequencyof the 6scillator

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in a perfecflycatibrat€dsystem. beC Set vonale should be producedby the presentseuingof the 2;d LO tuning DACs. The DAC Set vottage may ditrertrom the Nominal value because the system may not fully calibratedand the DACswitf be movedto tryto set th; oscillator.

The Desired2nd LO Freqis the frequencyto which the microcomputeris trying to move the osciitator. The Counted 2nd LO Freq is that frequencythe microcom_ puter has calculatedfrom the CountedOfset freq.

The Offset Freq is the frequency of the tow- ofrset VGOin the 2nd LO Asiembly. ln the

lr9^e-u9ncy

2182 MHz LO, this frequ€ncyis the differencebetween

22OAMHz and the LO frequency. 719 MHz LO is -aniFtre

derived from the Z1g2MHZ LO, the frequencyreta- tionshipsare more complex.) Again,the DesiredFreq is--thefrequ€ncythe microcomputlr is trying to set the offset, and the CountedFreq is the value iead by the internalcounter.

.If-j,Offset€etting Accuracyis the maximumpermat_ ted differencebetweenthe actual and desiredotriet fre- quencies. The setting process ends when the differencebeeomesless than or equal to this value. The tolerancedependson frequencyspanand band.

The followingprocedureshould localizethe failure to the 2nd LO assembly,the hardwaresettingblock,or the hardwarecountingblock.

Troubleshooting Procedure

1.Displaythe diagnosticinformationfor the 2ndLO control loop as outlinedabove.

2.lf the CountedOffsetFreqand the DesiredOffset Freq are withinthe OffsetSettingAccuracy,the 2nd LO probablyhas failed.

3.lf the CountedOffsetFreq is within 100kHz of the DesiredOffsetFreq,make sure that p1049is prop_ erly seatedon J1048. lf the finetunegroundleadis not making .good contact, the tuning voltage can shift sufflcientlyto causesettingfailures.(E)

4.lf the CountedOffset Freq readout is within 100 kHz of the DesiredOffsetFreq, the oscillatormay be out of calibration. Return to normal operation by pressing <Blue-SHIFT>. Try to calibratethe 2nd LO by pressing <Blue-SHtFT> PULSE STRETCHERand selecting #1 from the menu for the FREQUENCY

LOOPSGAL, then setecting#4 {2nd LO). Now,foilow the instructionsof the displayedmessages.lf you are able to completethe calibrationroutine,checkto see if the error condition sflltexists. lf the error is stiilthere or you whereunableto completethe calibrationroutine. proceedto the nextstep.(E)

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