Tektronix 494AP service manual O o o o O a o o a o o o o o o o o o a o o O o o o o o

Theoryof Operation- 494A/494ApService,Vol. 1

mainlF sectionfor processingand detection.

lF Section

This section processes the signal for frequency resolution. Several functions are performed here: bandwidth filtering, amplitude calibration and loga- rithmicconversion,andsignaldetection.

The 10 MHz lF signal is processedthroughone of sev€ralband-passfiltersselectedby the RESoLUTIoN BANDWIDTHcontrol. In the auto modethe microcom_ puterwill sel€ctthe best combinationof bandwidthand sw€eptime for the selectedspan, unlessoverriddenby the operator.

Weak signalscan be amplifiedby a set of switch- able amplifiersso the dynamicdisplay range (v€rtical window) ls shifted up or down. The REFERENCE LEVELcontrolselectsth€ gain and inputRF attenuation to framethis windowbetweenthe referencelev€lat th€ top of the displayscreenand the bottomof the display.

Alevelingcircuit helpsprovideflat frequencyresponse across the range. The signal is amplifiedby a loga- rithmicamplifierto producethe verticalsignalcalibrat€d in dB/div.

The detector produces a voltage that corresponds to th€ input signal strengthin decibels. The detector outputis then sent to the verticalchannelof the display sectionto drive the verticalaxis of the crt and display the signal.

Display Section

The display section draws the signal on the crt screen. Vertical deflectionof the beam (y axis) is increased as th€ output of the amplitude detector increases.The horizontalposition(X axis)of a signalis controlled by th€ frequency controt section and correspondsto the frequencyof the detectedsignal. The Z axis, or brightness,is controlledby th€ INTEN_ SITYcontroland the Z axis blankingcircuits.(However, markerbrightnessis actuallycontrolledby stoppingthe digital storagesweep for a period of time to brighten the spot.)

As the spectrumanalyzerspans from low to high frequenciesthe beamsweepsfrom left to right. When the spectrumanalyzertunesthrougha signalfrequency,

averticaldeflectionshows the strengthof the signal. The result is a signal displayedat a positionon the span that correspondsto its frequency,or in other words,the displayshowsamplitudeas a functionof fre- guency.

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The video amplifierscales the detectoroutput for vertical deflectionin dB/div or performs a log/linear conversion,dependingon the vertical display mode. The video processor provides additional bandwidth filteringit eitherthe wide or narrowfilteris selected.

The display section also providescrt readout to show control settings and measurementdata. This readoutis basedon data from the microcomputerwhich is readingthe settings of the front panel controlsor dataon the instrum€ntand GPIBbuses.

Digital storagecircuits providetwo functions;they provide a flicker-freedisplay at slow sweep rates, and they store the display for later viewing. Up to nine different displays with their readouts can be stored in the battery-poweredmemory. The stored display data can then be transmittedthrough the IEEE-488port to a plotter,or for programmableinstrumentsto GPIBcom- patiblecontrollersor instruments.

Frequency Control Section

The spectrumanalyzersweepsthrougha frequency rangethat is set by the frequencycontrolsection. The CENTER/MARKERFREQUENCYcontrol sets the fre- quencythe l st and 2nd localoscillators.

The output of a sweep generator is scaled by a span attenuatorto sw€ep a rangeor span of frequen- cies. The output of the span attenuatordrivesthe lst LO for wide spans and the 2nd LO for narrow spans. The output sweepalso deflectsthe crt beamacrossthe horizontalaxis as the local oscillatorsar€ swept so the displayis a spectrumof powerversusfrequency.

The frequency control section also tunes the pr€selector so it tracks the signal frequencybeing detectedoverthe 1.7to 21 GHz range.

Counter and Phase Lock Section

The Counter,HarmonicMixer, and AuxiliarySyn- thesizer form the nucleus of the frequencycontrol hardware.Both the l st LO and 2nd LO frequ€nciesare controlledvia the firmware-basedcontrol loop. Oata from the Counteris used as feedbackto control the oscillatorfrequency. Accuratesignalfrequencymeas- urementis possibleby countingthe frequencyof the 3rd lF.

The PhaseLock system stabilizesthe 1st Lo fre- quency. This minimizes display jitter and increases resolution.

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