Tektronix 494AP service manual A o o o a o o o o o O a o o o o o a o o, SWEEPDiagram31

Theory of Operaton - 494A/494ApService,Vol. 1

FREQUENCYCONTROLSECTION(Diagram7)

o o o o a o

The FrequencyControlsectionperformsthe tuning and scan functionfor the preselector,l st LO, and 2nd LO. lt also provides the sweep voltage for the deflectionamplifiersin the Displaysection so the crt display is coincidentwith the frequencyscan and tun- ing. This sectioncontainsthe followingmajorcircuits.

Sweep

Circuits on the Sweepboard accept trigger inputs from line, internaland externalsources,and the normal free-runmode of operation.Theyalso receiveexternal horizontaland manualsweepinputs. The circuitspro- duce a PENLIFTsignalfor ehartrecorderapplications,

aSWEEP GATE signat for crt disptay btanking,a SWEEPsignalto drivethe crt beamacrossthe horizon-

tal axis and drive the horizontalportion of the digital storage circuit. plus a ramp (OSCSWEEP)that is fed throughthe Span Attenuatorto the preselectorDriver. the 1st LO Driver,andthe2ndLO.

Span Attenuator

This circuit attenuatesthe ramp signalas required, to swe€p the frequencyof the 'lst

and 2nd tocaloscilla- tors, and tune the Preselectorso it tracks the center frequency.

Center Frequency Control

The CenterFrequencyControlcircuitprovidesa tun- ing voltagefor the 1st and 2nd LocalOscillatorcircuits that resultsin a linearcenterfrequencychangeas the front panel FREOUENCYcontrol is changed. The cir- cuit is directly controlledby the microcomputer,so remote controlof the frequencyis possible,by way of the cPlB rear-panelconnector. The COARSETUNE VOLTSsignalfrom this circuitis appliedto the 1st LO Driver circuits for summingwith the SPAN signal to drive the lst LO. The FINE TUNE VOLTSsignatis appliedto the PreselectorDriverfor summingwith the lF Offsetvoltages,and to the 2182MHz phase Locked 2nd LO circuit for summingwith the 2nd LO SWEEP signal.

l st LO Driver

The 1st LO Driverperformsthe following:

oCombinesthe COARSETUNE VOLTS signatwith the sPAN signaland outputsa currentto drivethe 1st LO.

7-60

. Producesthe tuning and sweepingsignal for the PreselectorDrivercircuits.

I Producesthe mixer biasvoltages.

oProducesa reference voltage that is used in both the 1st LO Drivercircuitand the Pres€l€ctordriver.

o Producesa supplyvoltagefor the 1st LO.

Preselector Driver

The PreselectorDriver combinesthe FINE TUNE VOLTSsignal,from the CenterFrequencyControl board with the PRESELECTORDRIVEsignaland the SPAN VOLTSsignallrom the l st LO Driver. This combined signalis offset,to compensatelor the selectedl st lF, then shapedso the Preselectortrackswith the lst or 2nd Lo as it is tuned by the output current. The PreselectorDriver also drives the Filter Select switch that selects either the Preselectoror the Low-pass Filter,dependingon the frequencybandselected.

SWEEP(Diagram31)

The circuits on the Sweep board (A72) provide the ramp voltage that drives the horizontal deflection amplifier,the l st LO Driver,the PreselectorDriver,the 2nd LO, and a voltageusedto alignthe frequencycon- trol system with the digital storagemarker positions. The sweepboard also providessignalsfor the Z-Axis circuitry,an externalplotterpen,anddigilalstorage.

The majorcircuitson the Sweepboardare:

oSweepGenerator

. Trigg€rCircuits

. SweepControl o DigitalControl o MarkerDAC

The sweep generator generatesthe voltage ramp that drives the DeflectionAmplifiers,Digital Storage, Preselector,and the swept oscillators.

The trigger circuitsproc€ss and multiplexthe three triggersignals.

The sweep control circuit generatesthe SWEEP GATEand PENLIFT signalsand determin€sthe holdoff time tor the sweepgenerator.

O

o o a o o o a o o o o o

O

o t o a o o o o o a o o

O

o o o o o o o o o o o