Tektronix 494AP service manual

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Digital Control Section

Operational modes and internal functions of the spectrumanalyzerare selectedand controlleddirectly from the front panel. The modesanOtunctionsthat are selectedare processedand activatedby the instrument mastermicrocomputerwhichtarksand iisten" to ail cir_ cuits over the instrumentbus. ttre progiammablever_ sion can also be remotelycontrolledfrim an external controilerthroughthe IEEE_4gg(GplB)connector. This connectorinterfacesto the instrumentmicrocomputer throughthe GptB.

Front panel control and sel€ctordata is processed by a front panel CpU that interrac"" *itt the master microcomputerover the instrumentbus. The master microcomputerreceivesand sendsall of its information over the instrumentbus to th€ intemal circuits. The programmable

version communicates

with other instru- m€ntsthroughthe GplB connector.The programmable controllanguagecorresponds

to tront_panelcontrofs.

Theory of Operation- 4g4A/4g4ApServlce,Vol. .l

Power Supply Section

The power supply s€ction provides regulateddc power and forced air cootingfor all circuitswithinthe instrument.The switchingsupptyis capabteof provid- ing regulatedvoltagesover a wide range of input line frequenciesand voltages.The coolingsystemconsists of an intake on the bottom of the cise, air passages withinthe instrument,a fan, and a rear panelexhaust. Air is routedto all sEctionsof the instrumentin propor- tion to th€ heat generatedby circuitswithinthoie sec- tions. Internaltemperaturevariationsare minimizedto providereliableoperation.

Other Sections

Interconnectionsbetween assemblies are made througha common Motherboard. Most circuit board assembliesplug into the top side of the Motherboard. Assemblieson the RF deck are connectedto the bot- tom side of the Motherboardthroughcablesand con_ nectors.

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