Tektronix 494AP O o o o o o o o o o, RF InterfaceCircuits Diagram28, Lst ConverterDiagram12

Theory of Operation- 4g4Ll4g4ApService,Vot. 1

DETAILEDDESCRIPTION

The followingdescriptionis arrangedby sectionsor systems;such as lst Converter,2nd Converter,etc., followedby circuitanalysisof the circuiiswithinthat section. Eachsystemor sectionis introducedwith a descriptionof the systemusing the sectionblock diagramfound in ihe oiagrams sectionof the Service Manual,Volume2. Thisis followedby a descriptionof Lach cireuitboardor miajorcircuitwithinthe system. The appropriateblock or schematicdiagramnumberis includedin the text h6adingsfor each sectionor part.

1STCONVERTERSECTTON(Diagram2)

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The 1st Converter consists of the 0-60dB step Atenuator, PreseleCtor,lst Mixer, .lSt LO, power Divider,Transfer Switch, 2.OT2GHz DirectionalFilter. Diplexer,and two 4.5 GHz Low-pass Filters. Externai circuitsthat control or drive the assemblieswithinthe l st Conv€rter are: the preselector Driver, 1st LO Driver, Counter and phase Loek system, and the RF Interfaceboard.

The 1st Converterconvertsthe incomingRF signals to the lst lF. lncomingsignalsare appliedthrougha calibrated0-60dB decadeattenuator(AT10)to a fitter select switch (S12). Signats in band t (t Ok-tz to 1.8GHz) route througha Limiter(A10) and 1.gGHz Low-PassFilter (FLl0) to the .tst Mixer (A12). Signats in bands 2 through S (1.7 to 21 GHz) route througha tunable Preselector (FL12) and a g dB Attenuator (4T11)to the mixer.

The RF signalsmix with the output from a tunable localoscillator(A16)to generateproductsat oneof two intermediatefrequencies,dependingon the bandin use. The 1st Mixer output goes to DirectionalFilter FL16 through Transfer Switch S1B. The transfer switch allows input from the EXTERNALMTXERinput,except in Option 07 and 08 instruments.In Option0Z anO0g instruments, the EXTERNAL MTXER capability is deleted.

The EXTERNALMIXERinput permitsan externallF source (externalmixer)to be connectedto the instru_ ment. The lF signalsfrom externalmixers are routed through the Transfer switch to the DirectionalFilter. This feature allows much higher input frequenciesby usingwaveguidemixers.

The direetionalfilter separatesthe 2OTZMHzand 829MHz intermediatefrequenciesfor the 2072MHz 2nd Converter(A18)or 829 MHz 2nd Gonverter(A23). The 2072MHz lF is apptiedthrough a 4.5 GHz Low_ Pass Filter(FLl1) to the 2072MHz 2nd Converter.The 829MHz lF is fed througha Diptexer(A14)and another 4.5GHz Low-PassFitter(FL1S)beforeit is apptiedto the 829MHz lF stages.

The spectrumanalyzer uses two intermediatefre- guencies(2472MHz and 829 MHz) to preventbasetine rise caused by local oscillatorf€edthroughand cross- over of intermodulationproducts. The 2072MHz lF is selectedfor band I and for bands 5 and above. The 829MHz lF is select€dfor bands2-4.

RF InterfaceCircuits (Diagram28)

The RF interfacecircuitsreceiveinstructionfrom the microcomputerand produce control signalsfor the RF Attenuator,the Transfer Switch, and the lF Select. TheseRF controlcircuitsare located on the Z-Axis/RF Int€rfaceboard (A70)and their operationis described underthe z-Axis boardpart of th€ Displaysection.

lst Converter(Diagram12)

RF Input

TheRFINPUT50() connectoracceptstheinputsig-

nals in bands 1 through5. Higherfrequenciesrequire external waveguide mixers that use the EXTERNAL MIXERinput,alongwith the LO outputs.

Option07 instrumentshave a75A input in placeof the EXTERNALMIXERconnector. TransferSwitchS13 selectsbetweenthe 50O and75O inputs.

The RF input signal goes through a 0-60dB Step Attenuator(AT10)consistingof relay-controlled10d8, 20 dB, and 30 dB sections. The relaysare actuatedby controlsignalsfrom the RF Interfacecircuit.

PreselectorCircuits

Coaxialswitches311 and 512 are relaysthat select eitherthe low-passfilter (FLl0) and Limiter(A10)or the Preselectorand 3 dB attenuator(AT11)for the RF sig- nal path. The relay coils are drivenby circuitryon the PreselectorDriver board. The low-pass filter path is used for band 1, and the Preselectorpath is used for bands2 through5.

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