Tektronix 494AP service manual O o

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Video Leveling

. -A^mjnor slopein frequencyresponse,causedby the 1.86GHz low-passfilter in the front end, is corr€cted with band I SlopeadjustmentR1012. Wirenoperatng in band 1, contacts6 and 7 of U3025are closed;therJ forq a portionof the PRESELECTORDRTVEsignatis applied to the VIDEO I output signal, providiig the ofiset necessaryto correct slope difference.

Video Leveler Circuits

. Mdeo levelingcomp€nsatesfor analyzerfront-end microwave circuit characteristicsthat cause unflat response.ln band 4 (5.4GHz to 1g GHz). Sinceband4 is a multipliedband, any unflatnessis ac"entuatecl. Levelingis accomplishedthroughprogrammablepertur- bationof the disptaybasetine[trai is opposatein direc_ tion to the flatnesserror. As the signaipower output decreases,the baselinerises an equil amountto com_ pensate,and as power output increases,the baseline falls an equalamount..Theperturbationis producedby

a normalizerintegrated that produces19 evenly

spaced valu€s of -circuit

the input voltage, with each valui

correctedto compensatefor unflatness.

The PRESELECTORDRTVEsignatfrom the tst LO drivercircuitsis appliedto a translationcircuitthat con- sists of two currentdrivers(U3O45Aand hatfof e303g, plus U30458 and the other half of e303g). The PRESELECTORDRIVE signat is direcfly r€tated in amplitudeto displayedanalyzerfrequency.The nominal +10V to -10V gxcursion voltage versus frequency

in maximumspan, relatesto the full bandwidth. 9urve1

This 20 V maximurnexcursionis scaled to a precise current(from1 mA at +10 v to o currentat _10 v) that is appliedto the normalizerlC to generatethe baieline perturbation.Actual signal scalingis done by current driver U3045A/Agmg. The outpui signatis apptiedto the normalizerSWp lN input, pin 5 of U20g9. The second current driver, Ug04SB/Og03g,generates a 2 mA referencecurrent for the normalizer]Horizontal Freq adiustmentRl069, in the inputtranslationcircuits, shifts the 19 evenlyspaced points up or down in fre- quencyto compensatetor unflatness.

NormalizerlC U2039operatesas a shaperand con- tains 19 transistorsthat turn on and off in sequenceas the currentinput to pin 5 decreasesfrom 1 rnA to 0. Each collector is connectedto a potentiometerthat allows outputtrimming. potentiometer

R1061is active with no current,and R1013is acfiveat I mA. The trim_ mingoperationis describedlater.

Theory of Operaton - 4g4AJ4g4ApService,Vol. 1

Fromthe normalizer,the outputis appliedthrougha jumper switch to bufferamptifierU2OS5B,which his a gain of five, then to offs€t amplifier U2OSsA. This amplifierhas a gainof two, but its primarypurposeis to offset the 0 to +5 V (normal),0 to -5V-(invert),bufferoutputto the levels requiredby th€ Log Amplifiercir- cuits. The range requiredby the Log Amplifieris O to

. J-0V. The output voltageis a seriss of linear interpo_ lations of th€ voltag€ betwe€n adjacenttrimmingreiis_ tors at the outprrts of the normalizer. Compensation adjustmentR1065setscorect interpolation.

Jumper ptug p2060 setectsthe input side of buffer amplifi€rU20558 and properofiset vottagefor U2055A. This providesthe meansto invert the buffer output dur_ ing the instrumentadjustmentprocedure.The adjust- ment procedure is describedin that section of this manual.

As previouslynoted,onlyband4 requiressignificant

compensaflonSelection. of band4 is indicatedby data bit 0 switchingto a 1 (seethe Lev€lingtabteat tire top

right of Diagram24). When DBOis a 1, pins b _corner

and 2 ot switch U2015 are connected,and the output from offset amplifier U205SAis supplied out as the VIDEOI signat.

Minor compensationis required for Band 1, to correcta minor slope causedby the 1.8 GHz low-pass filter and 2 GHz limiter. When pins 6 and 7 of switch U3025are connected,the PRESELECTORDRTVEsignal is ofrset by R4023and R4011and Band Stopeadjust- ment R1012 to provide an attenuated negative-going

ramp to the VIDEOI outputline. SwitchUg02Sis con- trolled by inverter 04025. O402Sis activatedby data bit 6 going low. As shownin the Video Blankingtable on the schematicdiagram,DB6 is 1 exceptwhenBand 1 is selected.

Video Filter Gircuits

Video filtering provides selection of onE of six bandwidths,under microcomputercontrol.As shownin the Vid€o Filtertableon Diagram24, dataits 1 through

4selectany of six bandwidths:30 kHz, 3 kHz,300 Hz,

30Hz, 3 Hz, and 0.3H2. Either wide or nanow-band filtering is selected at the front panel (Wide band is definedas 1/30th of the selectedresolutionbandwidth and narrow is defined as 1/30fth of the resolution bandwidth). The microcomputermakes the selection, basedon such factorsas sweeprate and total disper- sion. With no video filtering(all data bits are 0), the videosystembandwidthis 500kHz.

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