Tektronix 494AP service manual Desired, Mixerfreo, 1STLO Setnngaccuracy, Auxiliary Synthesizer

		Mafntenance-	4g4ful4g4Ap
			ServtceVot. 1
1STLO CONTROLDIAGNOSTIC
	AID
TUNEVOLTS	NOMINAL		DAC
	-6.79 V
			-6.80 V
SENSEVOLTS
	3.43V		3.43V
lST LO FREQ	DESIRED
	2.720504c-1.z		2.719735cHZ
MIXERFREO
	45.896MHZ
			46.665MHZ
1STLO SETNNGACCURACY			4.981MHZ
AUXILIARY
SYNTHESIZER			212.800MHZ
'SHFT"
PRESS	TO EXIT

The Desiredl st LO Freq is th€ frequ€ncyto which llre qroge.ssor,is trying to move the oscillator. The uounted l st Lo Freq is the frequencythe microcom- puter has calculatedfrom, the internaltycountedhar- monicmixeroutputfrequency,the Auxiliiry Synthesizer frequ€ncy, and the assumed harmonicnumOerof the AuxiliarySynthesizer.BEcauseof this lasi assumption, if the lst LO is not near the DesiredFrequency,the counted Frequencywiil not be the actuarosciilatorfre- quency,eventhoughthe counteris functioning.

The DesiredMixer Freq is the difierencebetween the the Desired1st LO Fr€q and the nearestharmonic of the Auxiti.arySynthesizer.(fhe AuxitiarySynthesizer laryolig wiil atways be higherin frequencythan rhe desiredlst LO frequency.)

The 1st LO SettingAccuracyis the maximumper_ mitted difference.between

the actual and desiredLO frequencies. The setting process will end when the differencebecomesless than, or equal to, this value. Thetolerancedependson frequencyspanandband.

.. -T" Auxiliary Synthe.sizerFreq is the frequency that is programrnedintothe +N synthesizer.

This troubleshootingprocedureshould localize a problemto the oscillator,the oscillatorsettingblock,or the.oscillatorcountingblock. lf the failur€ is not in the oscillator, it is further localized within one of the hardwareblocks.

Troubleshooting procedure

1.Press <Btue-SHtFT>PULSESTRETCHERand#3 to displaythe DiagnosticAids menu,thensefect#1 to display the 1st LO Control DiagnosticAid informa- tion.

2.lf the counted lst Lo Freq is withinthe 1st Lo

Setting-Acgyracyof ths Desired Freq readout, press <Blue-SHIFT>to retum to normal operation. Now detennine if the error occurs, for the same center fre- quency,at frequencyspans/divisionaboveS MHz only, or at spans less than 5 MHz/div. (Frequencyrange mustbe 0 - 1.8GHzor i.fl(em 5.5GHz.)

a.lf the frequencycontrol error occurs only at fre_ quencyspans of 5 MHz/div or more, the capacitor switchingrelay,on the 1st LO assembly,is probably shorted.(E)

b.lf the error occurswith a frequencyspan/divof 5 MHzor less,the 1st LO is probablydefective(E).

3, Measurethe voltage across th€ sense resistor (R1040)on the lst LO Driver board. tf this vottageis within 50 mV of the DAC Set value,measurethe fre- quencyon the l ST LO Output connector. This meas- ured frequencyshould be within S0 MHz of the fre- quencycalculatedby multiptying800 MHz/Vby the vot-

tage that was measured across the sense resistor R1040.

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