Tektronix 494AP service manual O o o O o a o O a o, REFERENCELOCKDiagramS0

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Calibrator

u2022 and relatedcomponentsregulat€a 100 MHz signatto -20dBmfor the front-panetill bUf connec_ tor. VRI0S1 serves as an aecurate6.2 V reference, which is dividedto approximately

1.eV anOappliedto pin 6 of U2022.Theexact tevetii set by R1041the Cal Leveladjustmsnt.

The 100MHz signal_enters pin 1 and passes througha pin diode variableattenuator.The signal is th€n amptifiedand passedthrougha low-passfitterto removeany harmonics.The signalthen entersa peak detector-and comparatorwnere the peak amplitude of the 100MHz signalis comparedto the 1.2V reference on pin 6. An operationalamplifierthen adiusts the att€nuationlevelof the pin dlodeto maintaina constant signatlevel. Theoutqq of this operationaiampliRercan be measured on Tp301l. A small fortion of the 1^0-MHz0signal is attenuated tnrou'ghRzO.ll to -20dBm.R1021and R1022suppfyUia-s-current

to the p€ak detector circuits. fne vottale on pins 7 and g shouldtypicailybe +5 V.

c2023, C2011, and related componentsform a high-passfilterto allow harmonicsof iOOMn. to pass throughto the front panet. The nna resuttis a calibra- tor signal rich in harmonicswith an accurate100 MHz amplitude.

In Option0Z instrum€nts,the CAL OUT signalgoes thr:ugh a set of-relayswitches. tn 50O moOe,the out_ put goes straightto the CAL OUT connector. In the 750.mode, the outputis routedthrougha 50O_to-ZsOmatchingpad and the output is +ZOOg-mV.

REFERENCELOCK(DiagramS0)

The ReferenceLock module (A36) consists				of a
		'(A3erc),
".tlblg 10 MHz crystat osciilator			reference
detector, frequency	.synchronizer, phaie/frequency
detector,andtunewindow		detector. Eittrertne internal
10 MHz reference

or an external1,2,5,or 10 MHz refer- encefrequencyis routedthroughthe referencedetector

1o-^tlg-frequencysynchronizer.The local oscillator,s 100 MHz output is dividedby 100 and apptiedto one inp.ut a pha.se/frequency

-ofdbtectorwtrich'tomparesit with the i MHz reference frequency. Th€ resultant error signal is amplified by the tune amplifier and applied, as a correctivevoltage, to the voltage con_ trolled3rd LO.

Theory of Operation- 4g4Ll4g4ApService,Vot. 1

External Reference Detector

_ Buffer amplilier02014 convertsExtemalReference signals,within the rangeof -iSdBm to +15 dBm, into TTL compatiblelevel. Whenan ext€mal signal,within !!e tgvet rangE, is apptied, it triggers muttivibrator U20468.. The output of U2046BenaOlesext€rnatsignat control NAND gate U20g2D,and disablesthe inte-rnalsignalcontrolgate U2032A.lt alsodisablesthe internal 10 MHz referEnceosciltatorby turningel0g1 on, which biasesQ1033off, and r€movesthe +5 V" supplyfor the osciltator. The output of U20468,pin 9i is sent to the processor, on the EXT REF line, to indicate that an externalreferencefrequencyis in use. Duringa diag_ nostictest, the microprocessorcanalso pultthe INTEi_ NAL SHUT-DOWNline down to turn the lnternatRefer- ence Oscillatoroff and checkfor loop unlock. lJ2Og2B gateseitherthe 10 MHz from the internalgateu2o32A, or the external referencefrom ltZOgZD,to ihe frequency synchronizerU2046A.

Frequency Synchronizer

MultivibratorU2046A,synchronizesits 1 MHzoutput with any of the allowed input frsquenciesby edge_ triggeringthe time-outperiod. The I MHz ouiput fre- quencyis set by the timingcomponentsR2039,c2o3g, and adjustmentR2042.Witha 10 MHz signatappliedto U2046A,adjustrnentRZO4Zis set for a lrrs period,with 65 ns betweenthe fallingedgeat Tp2046and the next fallingedgeat Tpl044.

Phase/Frequency Detector

The 100 MHz from the 3rd Local Oscillator is clividedby 100 and convertedto a TTL level by pres- caler U2020. The I MHz from U2020,is fed to the clock input of D-typeflip-flopU1O44A.The 1 MHzfrom U2946A,is appliedto the clock inputof D-typeflip-flop U10448. The two ftip-flopsand NAND gaie UZOO2C, form the Phase/FrequencyDetector. R1Og4,Rl0gS, and C1037,along with it'scounterpart,on the outputof U1044A,form a low-passaveragingfilterfor the outputs of the flip-flops. When the two input frequenciesare equaland in phase,the compositeoutputof the averag- ing filter is +2.5 Vdc.

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