Tektronix 494AP Rollow, Secton 494A1494Ap Service,Vot, Pr signals. Blockdiagrams Showingmore

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Secton 7 -

494A1494Ap

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Service,Vot.1

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THEORY

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OF OPERATION

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This sectiondescribesthe spectrurn

the maiorcircuitblocks'

analyzercircuitry.The sectionbeginswith a functionaldescription

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of

Thisis fotloweJ6y'ror"detaileddescriptionsof the circuitrywithin

eachblock.

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while readingthese descriptions,

serviceManual'

referto the corespondingblockor schematicdiagramin volume2 of the

The description'tittes

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useir,e oi"grar nam-esand numbers

for easyreference.

The FunctionalBlock diagram,located

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at the front of the Diagramssection

in Volume2. shows how the

majorsectionsin the instrumentrelateandihe.paths_of

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rollow

*o.1 r-"pr signals. Blockdiagrams

showingmore

ttreFunctiinat

L::*;ffi?#:llr:*tions

Brockoiagr;;. Gircuitschematicliasrams

ror6wthe

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Adjacentto eachschematic

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is a third levelof blockdiagram,a circuitboard parts

cross-reference

locationillustration,

and

look-uptables.The third levelblockdiairam'snows

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on the schematic'

the functionof the Lornpon"nt"

shown

The partslocationillustration

schematicor circuitboard.

ano tooi-uft"or". aid in findingcomponentson eitherthe

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FUNCTIONAL

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DESCRIPTION

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What lt Does

First, Second, and Third Converters

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The spectrumanalyzeracceptsan electricalsignal

Swept-frequency

as its input and displaysthe signal,sfrequency

anatysis is achievedby a triple'the-

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com_

conversionsuperheterodyne

nn:t:.9n

a crt. Signatscan beapptieddirecilyto the

technique. Each of

three frequencyconvertersconsists

of a rnixer,a local

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RF fNPUTor throughan externalmixer.

oscillator,and appropriatefilters. Onlyonefrequencyis

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The displayof the inputsignatappears

converted in each mixer to

pass through bind_piss

a graphwherethe horizontal

on the crt as

filtersto the detector. This frequencycan be changeO

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axis is'frequ€ncy

by tuning the local oscillatortrequencyin the first or

and the

v.ertjcalaxis is arnptitude.The disptaycan be ptotted,i,

second convErters.The third converteruses the fixed

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desired,by connectinga chart recorderthrougn rear_

100MHz calibratorsignal as a stable local oscillator.

panelconnectors.The displaycan also be transmitted

An externalsourcemay also be usedas a reference

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digitallyvia a EEE4gg Geneiarpurpose

InterfaceBus

for

(GPIB)to a GptB-compatibleptotter.

the 100MHz calibratorand third converter.

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The programmable

The first converter. usually referred to as the front

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end,.converts the input signal frequency

versioncan be operatedeither

T:H"lll.*jih

front-panetcontrots,or remotetyvia rhe

mediatefrequency(tF)of eitherg2g

to an inter-

MHz or ZO72MHz,

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\rFtEwtrnan easy_to_useprogramming

language.

dependingon which band is in use. The internalmixer

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convertssignals over the input range of 10kHz to

How lt Works

21 GHz. Externalmixers may be used for signalsinto

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the millimeterwavelengths.When the internalmixeris

The Spectrum Analyzer

used, a preselectoror low-passfilter is insertedin the

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signal path to reduceunwantedsignalsor imagesand

narrow-bandreceiver.The crt operates as a swept,

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as a range of frequencies

beammovesnorizontatty

spuriousresponses.

quencycomponent

is spanned. When a fre_

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of an input signatis detected,

One of

two second converters is automatically

the

beamis deflectedverticallyas a fuiction oi inprt power

selectedfor each band so the input fr6quencyrange

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at thatfrequency.

does not overlapthe first lF frequency. Eachsecond

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Frequency

converterhas its own local oscillator(LO),mixer,and

is measuredby counting

filters. Bothdown-convert

tor lrequencies

the localoscilla-

the signalto 110MHzwhich

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againsta reference.

Amplitude

is sentto the third converter.

ured by catibratingthe REFERENCE

is meas-

LEVEL and RF

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attenuator.A master microcomputerperformscontrol,

The

thirdconverteramptifies

storage,signal processing,and communications

the 110MHz lF signal

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tions.

func_

and converts it to the final intermediatefrequencyof

10 MHz. The third converterpassesthe signalto the

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7-1

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