Tektronix 494AP service manual O a a A o o, AUXILIARYSYNTHESZERDiagram37

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Theory of OperaUon- 494Al4g4ApServtce,Vot.1

Output

Amplifier

LO AIIPLIFIER

4416-200

Flgute 7-27.Slmplified schematic of harmoolc mixer.

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Twoadditionaldirectionalcouplersareusedto cou- ?f..lh" 1-9c!. signatintothe mixercircuit.A power ll,ll"l,lll .12, Rsl sptitsthe sisnarinto two paths. FFcnstgnat(approximately_6 dBm each)is then cou_ pled through these coupleis to ttre mixer. Tfrettrroujtr portsare terminatedin 50 ohms. Thusthe 2-6GHziig-nal is couptedinto the mixer Oinerentiatty a power levelof about-16dBm.

The 200-220MHz referencesignalis also coupled differentiallyinto the mixer circuit, since the output ot ::11^.j:jlT, r1_is:pptied across*," uJ terminatins restsrorsR4 and R5. The level of this signalis higi enoughto drive the snap_offdiode into its-operationalregion. Harmonicsof this 2@-Z2OMnz silna mix with th.e. GHz signalto generatenumerouJlF products

?-6

:vhich.ar:detectedby diodesCR2 and CR3ano fed to rneoutputamplifier.

The outputamplifieris a two stagecommon-emittercascade amplifierwith dc couplinj between stages. The standingcurr€ntthroughttre sjconO stage (e4) is higher than in the first stlge (a3) to provide better power and intermodulationperformance.Th€ amplifier is designedfor a 10 to gOnlHz response. Signals above80Jr4Hzare rejectedby a tow-passfilter in the

AuxillarysynthesizerOutput. levelof signalsin the 10_ 99Yt-rltg.eis typicalty-20dBmrorinprisisnattev- els as described.

AUXILIARYSYNTHESZER(Diagram37)

The Auxiliary Synthesizeris part of the spectrum analyzeis Direct FrequencyReadout (DFR) systern. This,alongwith a harmonicmixer,countercircuiti, sup- portingfilters and amplifiers,and appropriatefirmware, make up the DFR. The DFR provides th€ ,neanstor measuringand determiningthe frequencyof all oscilta- tors and the centerof the lF, so the cenier screenfre- quency is always known. Since the lF signal can be counted,this allows direct frequencymeasurementof any signal applied to the input port of thE spectrum analyzer.

Afunctionalblockdiagramof a simpleor basicsyn- thesizeris shownin Figure7-2g. The VCOfrequencyis

dividedby "N" in a programmable down-counterwhich

outputsa-pulse every Nth input putse. This frequency along with a frequency reference is then fed- to i phase/frequency

detector. The differencebetweenthe two signalsis filteredand fed back as a controlvoltage to the VCOto phaselock the oscillatorto the r€ference. VCO frequency is related to the reference by, Fret- NFrer_As N is changed,the VCO trequencywill changeby Frerfor each step in N. This producesout_ puts separatedby Fr"r. To get closelyspacedchannels, in tuning the VCO, the referencefrequencymust be relativelylow.

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