Tektronix 494AP service manual O o O o o, A o o O o

Theory of Operation- 49{A/4g4Ap Service, Vol. I

The output of u3038Ais apptiedthroughedgecon- nectorpin BB to the gain controlplN diodein the Band Levelingcircuit. For example;when band 1 is selected (U3023pin 1 low), current through Band 1 Gain poten- tiometer,R2031,and the emitterof e3036 sets the vol- tagethroughR2033to the summinginputof operational amplifier U3038A. The increas€d output of U303gA increasesthe currentthroughband levElingplN diode CR2021and increasesthe gain of the stage according to the settingof Band 1 Gainpot€ntiometerR2031. In similar fashaon, the other potentiometers (R9034, R3030,R3019,3A22,R3024,R3026,R3032,R302g,and R3028)allow adjustmentof the currentfor eachof the otherbands.

An additionaldiode may be added to eachdecoder output,for bands4 through10, to transmitthe low. via edgeconnectorpin OD,to the gain controltransistor,in the Band Levelingcircuit, and increasethe gain more lor these bands. These diodes are CR3022,CR3O23, CR3024,CR3025,CR3031,CRg027, and CR3026.tf needed these diodes are installed during instrument calibration.

The +5 V regulatorcircuit,U9041,suppliesa noise- free +5 V source for the VR system. This ls required becauseof noisein the +5 V mainsupply.

2nd Filter Select Circuits (Diagram 2l)

Circuits on the 2nd Fitter Setect board (46gAg) operatein conjunctionwith the circuitson the lst Filter Select board (A68A4) to set the overall systern bandwidth.Banksol filtersare select€dunderthe mas- ter microcomput€rcontrol. Data bits 0, 1, and 2, lrom the data bus, are appliedto decimal decoder U9070 (whichoutputs a low on th€ appropriateoutput pin to enable th€ selectedfilter). Bandwidth selectionsare 10 Hz to 1 MHzin decadesteps,plus3 MHz.

Filter selection is accomplished as previously describedfor the 1st FilterSelectcircuit exceptfor the 3 MHz,1 MHz,and100Hzl10Hz selections.

The input signal,from the Band Levelingcircuitvia jumper EE, is routed throughthe selectedfilter to the Post VR Amplifiercircuit,via jumper JJ. Nominalloss throughthe filter circuit is approximately12dB, with internal adiustment compensation for variations between the filters. The output level is nominally -12dBm.

The filter for eachbandwidthrangeslrom no filterat all to a temperaturecompensatedcrystal filter. An important differencebetween the i st and 2nd filter selectcircuitsis the additionof a gain adjustmentin all exceptthe 100kHz circuit. This adjuststhe amountof att€nuationthrough the other filters and matchesthe outputlevelto that of the 100kHz filter. since the Band

7-28

Levelingcircuit furnishescompensationgain to obtain equal signallevelsfor all bands,this adjustmentcom- pensatesfor variationsbetweenthe filters.

The 3 MHz and 1 MHz bandwidthsignalsuse the same path through this board. No filter is required here, becauso of filtering in previous stages. When

€ither 3 MHz or 1 MHz is selected,the signal goes through a simple attenuator with a gain control for matchinglevelswith th€ othersections.Pins2 and 9 of U3070aretied togetherto selectthe 3 MHz/l MHz path for eitherbandwidth.

The 100kHz filter is a double-tunedLC circuit designedfor a good tima-domainresponseshape. The filter is tuned with compositevariablecapacitorscon- sisting of small air variablesparalleledwith switched fixed capacitors. A third variable capacitor may be adiustedlo establishthe desiredbandwidth.ForOption

07instruments,a similar 300 kHz filter replacesthe 100kHzfilter.

The 10 kHz filter uses a two-polemonolithiccrystal filter. The impedances at the input and output are matchedto 50 ohm by T5047and T7050. An attenuator that containsGain adjustmentR3039is includedat the filterinputfor filtervariationcompensation.

The 1 kHz filter is also a two-polemonolithiccrystal with impedance matching transformers T40/,4 and T:7043. A Gain adjustmentis also part of th€ attenua- tor.

The 100Hzll0 Hz filter is a temperature- comp€nsatedhigh-Q crystal filtsr. The actual filter bandwidthis about 200 Hz. This filter augmentsthe filter in the 1st Filter Select circuit and reducesnoise producedin the interveningstages. FreqAdjustR4025, in a voltagedividercircuit,sets the centerfrequencyof th€ crystalfilter.

The 100Hzl10Hz path is selectedby Q2020and 08035throughdiodescRl017 andcR8016on the input and outputrespectively.when 10 Hz is selected,pin 7 of U3070goeslow turningon Q2020and Q8035.When 100H2 is selected,pin 6 of U3070 fonarardbiases cR3068,thus enablingthe path.

Gain control R3015 adjusts the 100Hz level. The

10 Hz levelis set by a controlon the 10 dB GainSteps board(A68A5),as previouslydescribed.

Post VR Amplifier Circuit (Diagram 21)

The Post VR Amplifiercircuit providesthe final VFt systemgain to bringthe signalto the requir€d+6 dBm output level and providesthe final band-passfiltering. The circuitconsistsof two stag€s of gainfollowedby a filter.

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