Tektronix 494AP service manual Functionaldescrtption, VIDEOAMpLtFtERDiagram23

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Theoryof Operation- 4g4ful4g4ApServtce,Vot. I

DISPLAYSECTTON(Diagram6)

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FUNCTIONALDESCRTPTION

The displaysectionconsistsof the followingmajor blocks:

oVideoAmplifier o MdeoProcessor

. DigitalStorage

o DeflectionAmplifiers

. Z-Axis

r CRTReadout

. The Video Amplifierprocessesthe detectedlF sig_ nal through appropriateamplifiersfor log and tineir displays, and provides pulse stretching for narrow pulsedsignals.

The Video processor provides band leveling to corr€ct front-endunflatnessthrough the bands,video filtering for noise averaging,out_;f_bandblankingto clamp the displayto the basetinewhen the sweep is outside the range of the selected band, and video markercapabilityfor use with a TV sidebandadapter.

. Tl" DigitatStorage digitizesth€ video and sweep signalsand stores the data in memory. Storeddata is thsn converted to analog signals for ttre Deflection Amplifierand Z-Axiscircuits.

The DeflectionAmplifierprovidesthe drivevoltages for the cRT. This includes vertical and horizontal deflectionsignals as well as readout charactersfrom the CRTReadoutboard.

The Z-Axis circuits receiveand decode data from the microcomputer;accept control levels ,rom the front-panelbeam controlsand generateunblankingsig_ nals to controlthe displayappearance,brightnesslaid focus;detect power failure;monitorthe inJtrumentvol_ tagesupplies;and recordthe elapsedoperatingtime.

The CRT Readout circuits generate the alphanumericcharactersfor the display.

VIDEOAMpLtFtER(Diagram23)

Video signals, from the log amplifier and ctetectorin the lF section, are received by the Video Amplifier. In the logarithmic mode, the signals are amplified linearly and appliedto the Video processor. In the linearmode, exponential amplification converts the logarithmic aain

characteristicto lin€arfunction. In eithermode, base_ line compensationfrom the Vid€o processoris applied to the videosignalto compensatefor any unflatnessin the front-endresponse.The pulsestretcircircuitat the output of the Video Amplifieratters narrow pulsesso data can be acquired and disptayedby the Digital Storagelogic. Signalamplitudeoffset ciicuits provide displayoffsetfor the "ldentify,,modeoperation.

Log Mode Circuits

The log mode circuits process the VIDEO signal from the Log Amplifier,andthey add ofrsetfor selecting that segrnent of the log amplifier gain curve to be displayed. The circuitsalso selectscreendisplaygain stepsfrom 1 dB/divto 15 dB/div.

The detectedVIDEOsignat,the VTDEOI correction signal, an offset signal, and reference current are summedat th€ inputto operationalamptifierU4050. A lowpass filter removes l0 MHz from the detected VIDEO signal.The VTDEOI signatis adjustedon the Video Processor board (A40) as compensationfor front-endunflatness.The signalis equaland opposite in amplitudeto the unflatness. Th€ lnput Ref Level adjustment,Rl 012, sets the referencelevel. Signal 9{"9t is suppliedby digitaf-to-anatogconverter(DAC) us160.

The DACconvertsthe microcomputercommandsto an offset signal that selects the location on the log amplifiercurve tor the disptay(see Figure 7-12). In dB/div or log displaymodes,a changein th6 Vertical POSITIONcontrol produces an effect after the log amplifierthat is the sam€ as a signal level or gain change before the log amplifier. Instead of using a large amountof lineargain beforethe log amplifier,the output of the DAC effectivetyofisets ths disptay up or down along the log curve. This offs€t producesthe sameeffectas varyingthe pOSITION

controlexceptthe displaypositiondoes not change,only the signallevel requiredto reachthe referencelevelchanges.

This processallowsthe lineargain to changewhile the top of the screenis kept constant,and it must also allow any 16 dB segment(in th€ 2 dB/div mode)to be displayed.Nominally,the log amplifieroperates with +6 dBm equivalentreferencelevel at the top of the screen.

The outputol U4050is equivatentto 20 mV/dB. Futl screenis 2.2V, as set by lnput ReferenceLevelpoten- tiometerR1012. FromU4050the signalfeedsthe log mode gain circuil andthe lin€armodegain circuit. The digitalcontrolcircuitselectsbetweenth€ log and linear modecircuits.

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