Tektronix 494AP service manual VtDEo PROCESSORDAGRAM24

backloopfor U5030is now open. DiodeCR5035turns on to preventU5090from slewingtoo far negative.

ldentily Circuit

This circuit provides a vertical offset on alternate lllcg: t9 helpidentifytruEand fatsesignats.Whenthe 'ldentify"

featureis in op€ration,it allolvsthe operator to distinguishbetweenresponsesthat result from sig- nals at.the desiredspectrumanalyzerinput trequenjy (true signals)and those that are producedby'othe? spuriousor harmonicconversions(falsesignats;.fatse signalsshift horizontallyon alternatetrac6s while true signalsremainin position.

The horizontaloffset is accomplishedelsewherein the instrumentby movingthe 1st and2nd LO frequency an equal and oppositeamount,relatedto the ist LO harmonicused, or by movingthe lst LO twice the lF divided by the harmonicnumber(N), on every other sweep. The result is that any conversionpioducts causinga false responsewill shift a significantamount horizontallyon the displaywhiletrue signalswill remain closeto eachother.

The identifyoffset circuit describedhere shifts the alternateor'ldentify"sweepvefiicallyas a furtheraid to identifyth€ true signals. The microiomputersets DB6 high during'ldentify"

sweepso the open collectorout- put of U4160A(pin1) goes from -15Vto open. This remov€sthe currentnormaltyflowingin R5154, R51SS, and R5158 from the summing node of U5030 and causesa shiftin the VIDEOil outputlev€t.

Digital Control Circuit

The digital control circuit providesthe control sig_ nals that select the variousVideo Amplifierfunctioni. Addresses78 and 79 are decodedby U6160and sent throughinverterU6170as clock or enablingsignalsfor gainlatchu5010and modetatchu4010.

Gain latch lc u5010 is an g-bitlatch that suppties commanddata to the 8-bit DAC, USl60, to offset the Log Amplifieroutputsignat.ModetatchU4O1Ois an 8_ bit latchthat suppliescommanddatathroughthe com- paratorsin U4020and U4160to selectthe resistorsin the dB/divswitchingcircuitand to sel€ctidentify,pulse stretch,andlog or linearmode.

Theory of OperaUon- 494A1494ApService,Vot. I

vtDEo PROCESSOR(DAGRAM24)

The Video Processor performs four functions.The ftrst is compensationfor flatness variationsin front-end response. The secondis vldeofiltering,whichprovides the selectionof six video bandwidths(90kHz, OkHz, 300 Hz, 30 Hz. 3 Hz, and 0.3 Hz) under controlof the instrumentmicrocomputer.The third functionis out-of- band blanking,which blanksth€ upp€r and lower ends of the local oscillator swept frequ€ncyrange to provide

aselectedwindowfor the display. This functionis also controlled by the microcomputer. The fourth is the capabilityto genEratea negative-goingditch markeron the video displayfor interfacingwith a 1405 TV Side- bandAdapter.

Interface with 1405 TV Sideband Adapter

The TEKTRONIX1405 W SidebandAdapter is a specializedtracking generatorthat is used with the SpectrumAnalyzerto analyzethe responseof a televi- sion transmission system. The Spectrum Analyzer monitorsthe RF outputof the transmitterwhilethe side- band adapterdrivesthe videoinputof the system. The

video inputmay be at the transmittersite,the headend of th€ studio-transrnitterlink, or the video switcherin the studio. ThE sidebandadaptermust be connect€dto the lst LO of the SpectrumAnalyzerby a shortlength of coaxialcable.

Th€ systemin Figure7-13 depictsa TV transmitter operating on Channel 10 with a video carrier at 193.25MHz. The sidebandadapteris tunedto Channet

10.The SpectrumAnalyzeris tuned to 195.25MHz with a span settingof 1 MHz/Div(for purposesof iilus- tration,the sweepis assumedto be haltedat the center frequencyof the analyzer).

The sidebandadapterappliesa2MHz signalto the AM modulatorof the videotransmitter. Th€ modulator producesa lower sidebandat 191.25 MHz, a carrierat 193.25MHz, and an upper sideband at 195.25MHz. This signalis amplified,filtered,and combinedwith the

FM aural signal. The compositesignalis sensedby a RF pickupand appliedto th€ RF Inputot the Spectrum Analyzer.

The 1st Converterappli€s the compositesignalto

'l mixer. The compositesignal is mixed with a the st

2.26725GHz signalfrom the 1st LO, formingthreepro- ducts. The subsequentstages of the analyzeraccept

onfythe 2.072GHzproductand rejectthe rest. Forfre- quenciesused in this example,the acceptedproductis

the differencebetweenthe 1st LO and the upperside- bandof the TV signal.

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