Tektronix 494AP service manual U2025OUTPUTLINES, Llne

U10508 witt
Amptifier,s ctock first, placing a high at the Error
	invertinginput. This iamps it,"					ort-
put low untit U1050A switches d				"*ptiR",
U20508resets both				"iiort	time tater.
		flip-flopsanOtne inner toop error
amprifierwiil stop rampin! untir the next correction
cycle. At the next col-1clion cycle, the error amplifier
will have reduced the VCO freqfuency,						reduc-
ing the mixer differencefrequen.y.				it
				"i"for
tinues			fnl.-proceEs con-
	untit the two signati appfilO to the
lh".:"/F.r."q.uencyDetector ar6 eOge				"6ilcioent,		mean-
ingthattheirfrequencies
		and ptras6,"i"t.

Error Ampilfier.The.Eror Amptifierboarct(A50A4) provides the inner 3F oglgl robp error amptifiErs, enablesthe Strobe Driver (A5OA2),'andgeneratesthe UP/DOWNandF ERRORsignals.

, The inn-erloopamplifierintegratesthe error signals from the OffsetMixer and produJesa cJriection vottage to pullthe VCOto a frequencythat is synchronous

t!" -r-[rlsignat. Th€ Output f"rii", with (A5OA3)

phasefrequency dete-cJoroutput drives integrating diff€r.entiatamptifierU3075. As the signaisdrivingthe amplifiercontinuetowardone directionlthe outputcon_ tinues !o change the oscillator frequency in the appropriatedirection.ZenerdiodeVR2O6SandCRgO6g glamptl: jgel loon amptifieroutput io $,"t it stays abovg +5..V. This prevents tonrrard biasing th€ VCO varactordiodes.

. - _4. digital control circuits consist of shift register U2025and quad anatogswitch U2Og7.Data trom the microcomput.ris fed seriaily.via th€ counter boardcir- cuits,into.the.shiftregister,

thentransfer;; to the out- put finesby the CONTROLLATCHsignat. TabteT-21 liststhe purposeof the outputlines.

Tabte7-21

U2025OUTPUTLINES

Llne		Low
Q1	Windowdisabteda	Widewindowa
Q2	Lock	Unlock
Q3	Wideloop	Narrowloop
Q4	Strobeenabled	Strobedisabled
Q5	Narrowwindow	Widewindowb
rWith OSlow.
bwit ot	tow.

..-^fl" outerloop amplifiercircuitconsistsof amplifier U2048and surroundingcomponents.The ERROdsig_ nal from the phaseGateDetectorand ErrorAmplifieris appliedthroughLOOP GAIN adjustmentR0082to the invertinginputof U2O4g.The signal(ERROR)is a result

Theory of Operation- 4g4ful4g4ApService,Vol. 1

of the comparisonof th€ 1st LocalOscillatorfrequency and the nearestmuttipleof th€ STROBEsignattrdm tn6 Strobe Driver circuit. The ERRORsignat varies from zero to about S00kHz, and is up to 4 V peakto-peakinamplitud€.

When phase lock is not required,data into U2OZ' sets output e2 and e4 tow and e3 high. This op"ni the connectionbetweenpins 11 and ld of U20g7and the connection between pins 2 and 3. STROBE ENABLEtine to th€ Strob€ Drivergoes high and dis- ables the strobepulse.The FM coil of the oscillatoris openedby U2A37whichopensthe outertoop.

.. lo establishphase lock, th€ microproc€ssorsets the lst LO near the d€sired lock point and loads the proper N number into the synthesizer. The S MHz strobe is then turned on (e4 and e2 output ot U2025 set high)and the microprocessortunesthe 1st LO up or down 750 kHz eitherside of the desiredtockpointLt a 10 Hz rate. Whenthe oscillatorfrequencycrossesthe desiredlock point,the ERRORfrequency'isreducedto a dc voltagewhichresuttsin U204gpuilingthe 1st LO in th_edirectionrequiredto maintaina consiantfrequency. When the microprocessormeasuresthe lst LO fr€- quency and finds it hetd constant,at the desiredfre_ quencyrit then sets Q3 output ot u2025low to reduce the bandwidthof the phasetocktoop.

The UP and DOWNsignatsalertth€ microcomputer that the drive currentto th€ 1st Lo FM coil is reaining its limit in holdingthe 1st LO in phaselock The microl computerthenactsto bringthe 1st LO frequencywithin llt:-?pperrange. A window comparator,ionsiittng ot U1015 and thg associatedcomponents,sens€swhen U2048 has approachedits operatinglimits. Whenthe microcomputercausesthe e2 signaito closethe path from U2048 to the FM coil, U2O4gbeginsto fumish currentto the coil whichcausesthe 1st LO to trackthe stablestrobe signal. That is, eachtime the 1st Lo fre_ quency drifts, the ERRORsignal changesand U204g shiftsthe FM coil cunentto bringth€ lst LO backto its original frequency. At the same time, the microcom- puter causes lines el and e5 to be low, closingthe contactsthat connectthe output of u204gto the input of the window comparatorthrough a dividernetwork. Now, as the l st LO frequencydrifts,the loop amplifier will compensatefor the drift. lf the drift is excessive, however, V2048 will approach its limits and wiil be unableto furnishanymorecurrentto the FM coil.

_ Window comparatorU101S is a dual comparator that sensesa deviationof *15 mV. For example,if a frequencyshift forces U2049to move positiveenough

(approximately V), upper half of the comparator 3 the

conducts,and the Up line goes high. This triggersthe servicerequestcircuitson the Counterboard.whichin turn alertsthe microcomputerso it beginsadjustingthe TUNE voltagefrom the Center Frequencyiontroi cir- cuits to reduce U204Aoutput to zero. lf the output

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