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

Theory of Operation- 4g4[l494fupServlce,Vol. 1

drifts negative,the other half of UlO.l5conducts,caus_ ing reverseactionto occur.

- Normally,the inputsignalto the windowcomparator is attenuat€dby R2040, which reduces the voltage applied to U101Sto 0.9% of the output from U204-g.This allowsU2048to drift up and downwithoutimmedi- ately triggeringeither comparator. when R2043 is in the circuit,it is called ,'wide

window,'operationWhen.

phase lock is de-selected,the microcomputerselects narrow window (which bypasses R2049). The Center Frequency Controt circuit is then instiucted by the microcomputerto move the 1st LO frequencyuniit tne windowcomparatorindicatesthat the FM coil currentis near zero. This preventsthe lst LO frequencyfrom shiftingtoo far from the lock point when phaselock is canceled.

The F ERRORsignal is used by the Counterboard (A51) for diagnosticsso that the microcomputercan determinethe relationshipbetween.tst LO irequency and the strob€ line. The F ERRORsignalis generatei from thE outerloop ERRORsignalfrom the phaseGate Detector(A24). The circuit consistsof an active low- pass filter U2065and Schmitttrigger U1Og5.This cir- cuit ftlters and squares the incomingERRORsignal. The ERRORsignatis apptiedthrough CZO67to an nC 500kHz low-passfilterand amptifierU2O6SAfter. fitter- ing, the signal is applied through Enor Count Break_ pointadjustmentRl061 to the inputof u1035,a schmitt trigger circuit. The squared output signal is then appliedto circuitson the Counterboard.

The STROBEENABLE signat enabtesthe strobe generator in the Strobe Driver circuit (ASOA2).Shift register U2025reads the instrumentbus latch on the counter board (A51) to determinethe status of the STROBEENABLEsignat. O20gOinvertsthe signatand convertsit to TTL levelto drivethe strobegenerator.

controltedoscillator (vcol. The controiledoscilla- tor (VCO) is a voltage-controlledcrystal oscillator whosefrequencyis controlledby the outputof the Error Ampllfier. The oscillatorgeneratesa referencesignal that asusedto stabilizethe lst LO frequency.Refel to the block diagramadjacentto Diagram40 for a func_ tionaldescriptionof this part.

The controlvoltagefrom the Error Amplifier,which is a functionof th€ difierencebetweenthe microcom- puter controlled +N signal and the Offset Mixer differencefrequency,is appliedto the VCOon the Con- trolled Oscillator board to regulate its frequency of operation. The circuit has two outputs:the first, which is part of the innerloop of the phaselockcircuits,is fed to the Offset Mixer, where it is used to derive the differencefrequencythat is comparedagainstthe +N signal. The secondoutput, which is part of the outer loop, is led to the Strobe Driver circuits.where it is

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divided down to becomEth€ STROBEsignal that is comparedagainstthe l st LO signalin the phaseGate.

The VCO consists of five major circuits, four of whichare connectedin a positivefeedbackloopto sus- tain oscillation.Thesecircuitsar€ the resonatorstage, the differentialamplifier,the bandpasslilter, the isola- tion amplifier, and the output amplifier. The resonator stage operates at a frequency ot 2S.OS2MHZto 25.094MHz. Th€ output signal from the resonatoris applied to the input of a differentialamplifierwhich drivesthe outputamplifierand the bandpassfilter. The output from the output amplifieris fed to the Offset Mixer and the Strobe Driver. The bandpassfilter strips the signalof any spurii €ither side of csnter frequency and feeds th€ signal to the isolation amplifier. This stagefurnishesthe positivefeedbackdrive to the reso- nator stage and isolatesthe bandpassfilter from the resonatorstage.

The resonator stage consists of crystal y1012,

varactordiodesCR1011anclCRl012,and r€latedcom- ponents. The stage operates within the frequency rangeof 25.032MHz to 25.094MHz,which is controil€d by the voltageappliedto varactordiodes CR1011and cRl0l2. Feedbackenergy for sustainingoscillations comesfromthe isolationamplifierby way of coil Ll025.

The resonator output signal is applied to a differentialamplifierQ2033and Q2041. The Q2033side drivesthe outputamplifierand servesto isolatethe out- put load from the feedbackloop. Gainfrom this sideis less than one. The signal is fed from the collectorof Q2041,followingamplification,to the band-passfilter.

The band-pass filter consists of passive com- ponents,and is usedto stripthe signalof anyfrequency componentsmore than about 40 kHz away from the center operating frequency, which is approximately 25.06MHz. CapacitorsC1041and C1042ar€ adjusted at th€ factoryto set the bandwidthand cent€r the fre- quencyof the filter.

The isolationamptifier,01028, is a common-base configuration,in order to match the impedanceof the filter to the resonator. Output current from the stage furnishespositivefeedbackfor the resonator.

The outputamplifier.consistingof transistorsQ2025 and Q2026,is connectedas a differentialamplifierwith 02026 drivingone side of the OffsetMixer and Q2025 drivingthe inputof the StrobeDrivercircuit,for eventual applicationto the PhaseGatecircuits.

StrobeDrlverClrcult. The StrobeDrivercircuitcon- sists of counterU1022,bandpassfilter FL2064,sourc€ follower02091, andANDgatesU1091AandU10918.

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