Tektronix O a o o a o o o a o o o o o, Theory of Operation- 494A/494ApService,Vol Tabte7-12

Theory of Operation- 494A/494ApService,Vol. 1

Tabte7-12

TRIGGERSELECTIONMODES

TriggerMode

Freerun

Internal

Ext€rnal

Line

TableZ-13

SWEEPHOLDOFFSELECTION

Sweep Holdoff

Short

Medium

Long

Sweep Generator

The sweepgeneratoris an integratorcircuitconsist- ing of operationalamplifierU10S5with one fixed and two switchablecapacitorsin the feedbackcircuit. Fixed capacitorC1061 is used for the faster sweep rat€s. The other two capacitors,C1065or C1062,are added to change the time constant when either e2069 or Q2064 are switched on by comparatorsU1060Aor U2050A. These comparatorsare driven by register U2030, which interfacesto the instrumentbus. For manualsweep operation,02060 is turned on and the integratorbecomesan amplifier.

Multiplexer U3060 connects timing resistors betweena -12volt referenc€,out of UO050B,and the inputto integratorU2060. Data bits D2, Dg, and D4 of address0F.1 drivethe sElectinputs of the multiplexer. The voltage referenceof -10volts out of U4055 is boostedto -12voltsby U30S0B.A voltagedividersets the non-invertinginputof U1055to -gvolts. Therefore, thereis about4 voltsdifferenceacrossthe timingresis- tors. The timing current through the resistorsvaries overtwo decadessuchthat 1/l is proportionalto a 2-5- 10 sequence.

Switchingin feedbackcapacitorsC1065and C2060 each changesthe sweeprate by a factor of .l00 times. Sweep Accuracyadjustment,R1062,compensatesfor differencesin timing voltage or timing circuit values. The timing capacitorsar€ match€d so that one adjust- mentcompensatesfor smallvariationsin eachset.

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Trigger Circuits

The sweep circuit can be triggersd by an extemally applied signal,the int€rnalvideo filter slgnal,or from the powerline. Eachtriggersignalis convertedto TTL leveland thenappliedto triggermultiplexerU2026,part of the trigger control circuit. The trigg€r control circuit selects th€ desired triggering signal and triggering mode or rejects th€ trigger to let the sweep circuit free run, be manuallycontrolled,or let the externalsweep modebe used.

An externaltrigger signal applied to the external HORIZrrRIGconnectoris convertedto TTL level by 02030. CR2030limits any voltag€ surges that may be on the line.Line triggersignals,from the powersupply. are applied through comparator U3025Ato the multi- plexer. VideoFiltsrOut signalsfrom the VideoProces- sor board are bufferedby Q4037 and convertedto TTL levelby 03030. Boththe externalandvideotriggersig- nals are appliedto multiplexerU2026throughSchmitt triggerinvertersin U1015C.

Under control of the data (D2 anclD3) from register Ul035 (at extendedaddressOof address0F),the multi- plexersel€otsthe triggersignaland passesit to flip-flop Ul0168. After retraceand holdofftime,Ul0168 allows

atrigger to pass through V2026 and U2020Cto r€set the SweepState Controlflipflop U1025. When U1025 is resetthe integratorstartsa n€w sweep.

Sweep Output Circuits

The sweep ramp from the integrator is applied throughbufferamplifiers,U3045and U4050,and a bus on the Motherboardto the DeflectionAmplifiers,Span Attenuator,and DigitalStorageboard.The sweepout of U3045is an 11 volt peak-to-peakrampcenteredaround

0voft. The sweep out of u4050 is a 22 volt peak-to- peakrampfor the oscillators.

The sweep signal also drives pen-lift comparator U3010A and the end-of-sweepcomparator U30108. The thresholdfor the pen-liftcomparatoris +7.4 volts. The thresholdfor the end-of-sweepcomparatoris +8 volts. The sweep ramp, from the integrator,starts at -8volts and rises towards+8 volts. When the signal reaches +7.4 volts, th€ p€n lift comparatortoggles. This output is gated through u30158 and the pen lift signal goes hlgh. When the sweep ramp reaches+8 volt, the end-of-sweepcomparator,U30108, toggles. The resultantlow output is appliedthroughU1015Ato becomethe EOS(end-of-sweep)signal.

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