Theory of Operaton - 4g4Al4g4ApService,Vol. I

 

 

 

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Figure7-13. Functonal dlagramrhowing the SpectrumAnafyzcrand 1405TV SldcbandAd.picr System.

The product is converted twic€ more, amplifi€d, liltered,log amplified,and detected. This detectdOsig- nal is appliedeither directlyto the video amplifiersof the crt or to digitalstorage.

The SpectrumAnatyzerlst LO signatis appliedto the RF mixer of the sideband adapter. The 2.26525GHz signal from the tunableLO is subtract€d from the 2.26725GHz signal trom the Spectrum AnalyzerLO, yieldinga ZMHz product. This videofre- quencysignalis conditionedwith syncand blankingsig_ nals and applied to the video input of the T\/ transmitt€r.

Whenthe SpectrumAnalyzeris sweeping,the video signal starts at 3 MHz, lalls to 0 Hz, and rises up to 7 MHz. During this intervat,the analyzerdisplaysthe lower sidebandas it movestoward the carrier,diiplays the carrier,and then displaysthe uppersidebandmov_ ing awayfrom the carrier. Sincethe SpectrumAnatyzer and 1405 TV SidebandAdapter systemis similario a trackinggeneratorsystem,it rejectsnolseand uncorre- lated signal. This allows normal in-serviceuse of the transmitterby addinga tow tevel(1 to 3 tRE units)cw signalto the video or by using fuil levetswith a ilTS inserter.

The sidebandadapter can lnsert frequencymarkers at preselecteddeviations from the carrier frequency. Six selectablocrystal oscillators have their outputs mixedwith videosignaland appliedto a Z-Axiscircuit. This circuitproducestwo n€gativepulsesas the video signal sweepsthroughtho crystaloscillatorfrequency. These pulses are applied to the spectrum analyzer mark€r input, whers they appear on the crt as two notch€s on either side of th€ marker frequency. The sidEbandadapter allows the width and depth of th€ notchesto be adjustedwith the width and Intensitycon- trols.

Video Marker

The Z-Axis signalfrom the 1405SidebandAdapter connects to the MARKER input on the spectrum analyz€rrear panel. This negative-goingsignal flows through the Accessoriesand Mother boards to the Video Processorboard. Here, the signal drives the ernitterof Q4060and turnsthe transistoron, pullingthe VIDEOOUT line down. This produc€sa notch in the video signalof the displayto signifythe locationof the markeron the display.

7-36

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Tektronix 494AP service manual Ffi rffi, Theory of Operaton 4g4Al4g4ApService,Vol