Tektronix 494AP service manual O o o o a a a o o A o o a a a, 1st LO DRTVERDiagram33, X0.01

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The 2" sideof U4025is contro[edby databits,DBS and DB6,on the e6 and e7lines trom deotS. The,,2y, outputs of U4025are applied through buffers in u4042 to select the appropriateattenuationfactorfor the out- put sweep. Table7-1Slists the stat€s requiredto sner_ gize the attenuationrelays. A diode acrosseachrelay coil protectsthe ddung circuitfrom inductivefeedback transients.

Tabte 7-15

ATTENUATIONSELECTION CODES

xl (K3065) x0.1 (K3075) x0.01 (K4072)

x 1

100 K3065

x0.01

4 4 1 & r 2 3

Flgwe 7-2,4.Simplifi ed span decade attenuator.

Theory of Operation- 494Al4g4ApServtce,Vot. 1

1st LO DRTVER(Diagram33)

The 1st LO Driverperformsthe followingfunctions:

ocombines the spAN voLTs with the coARSE TUNE VOLTSand outputs the combinationto the PreselectorDriver (the combined signal is also appliedto the OscillatorDrivercircuits,whichdrive the 1st LocalOsciilatorcoit).

. Selectsand outputsthe appropriatebiasvoltageto the internalor external1st Mixer.

oOutputs a voltage to the preselectorDriver that peaksthe Preselector.

o Controlsthe oscillatorlilter switch.

oProducesa stabteand precise-10V referencefor both the 1st LO Driverand the preselectorDriver circuits.

The majorcircuitsandtheirfunctionare:

oThe digilal control circuits decodethe addresses, latch the incomingdata from th€ data bus, select the requiredmixer bias,connector disconnectthe TUNE VOLTS and SPAN VOLTS signatsto the summingamplifier,energizethe filter switchin the l st LO assembly,and controlthe driveandfiltering of the oscillatordriverstage.

oThe oscillator filter switch driver furnishes drive current to the capacitorswitchingrelay in the 1st LO assembly

oThe inputswitchingcircuitconnectsor disconnocts the SPANvoLTS and COARSETUNEVOLTSsig_ nalsto the inputof the summingamplifier.

. The summingamplifierfurnishesthe drive signalto the oscillatordriver. The summingamptifiersums the SPAN VOLTS ramp signal, from the Span Attenuator,with the coARSE TUNEvoltage,from the CenterFrequencyControlcircuit. In less than maximumspan,a sweepvoltageof *10V sweeps the oscillatorat a rate of 333 MHz/division.As the TUNEVOLTSsignalvariesfrom -10Vto +10V, the oscillator'scenterfrequencyis movedover its full range.

oThe oscillatordriver furnishesthe currentdrivefor the 1st LO coil.

oThe -10 V referencesupply producesa precise -10Vre[erencefor the 1st LO Driver and the PreselectorDriver.

7-67

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