Tektronix 494AP service manual O o o a o, ADDRESS70 Formats

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Theoryof Operaton - 4g4ful4g4ApServlce,Vol. 1

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.Datafrom U4035,the databuffer,is clockedinto the registerseach time a difierenttune voltageis required. U3050feedsthe lowesteightbitsto thEliw-order DAC, u2055; U3070 teeds the higlgs1eight bits of the high- orderDAC,U2060. RegisteiU3060ieeOsthe remaining bits of both units.

Tabte7-19

ADDRESS70 FORMATS

DBO FineTunelow byteenable

DB1 FineTunemiddlebyteenable

DB2 FlneTunehighbyteenable

D83 FineTunehold

DB4 CoarseTunelow byts enabfe

DB5 CoarseTunemiddlebyteenable

DB6 GoarseTunehighbyteenable

DB7 CoarseTunehold

Dlgital-To-Analog Converters. Since both the coarse and fine tune circuits operate similarly, only the coarse tune section of the board will be discussed here. Figure 7-26 is a functional block diagram of the circuit.

Eachside of the converterhas two DACSsummed log€therto producean output of approximately*10 v. The DACsar€ programmablecurreni generatoisdriving the preamplifier-integrator

circuit. ttre higtr-orOerDAd provides 0 to 2 mA of current to the circuit via the buffer,whilethe low-orderDAC providesapproximately +2.5 mV at the invertinginput of the pr€amdifier.fn6 prearnplifierthen drives the integrator via the storage gate.

An isolated ground system for each half of the cir- cuit minimizessusceptibilityto noise and €xtraneous signals. This is becausethe convefiers providethe dc voltagesthat tunethe oscillators.

Track/Hold Amptifter

. The amplifierconsists of high_orderDAC U2O6O, low-orderDAC U20S5, buffer U2O5O, preamptifier Ul065, storagegate01065.and integratorJ21TA.

. -Thecircuit output is requiredto tung approximately *10 V for the fuil-scalerange of U2060. ialnenU2060 is off and the outputof u2050 is at 0 v, the +10 v out- put levelis set by 1 mA of curr€nt throughR1055,and the combinationot R1092,R1053,anctR1070. When u2060 is fuily on, and output of u2050 is at +10 v. the -10V output level is set by 2 mA of currentthrough R1052, less the I mA constanfly flowing in R10S5. Full-scalegain is adjustedby R1032. ResistorsRlOS2, R'053, and R1055 are matched for temperature coefficientto minimizeoutputvoltagedrift as a function of temperature.

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TabteZ-20

DACTUNINGCODES

Polnt	Results
Positivefull range
	Enablesall latches,track mode
Mid-range	Loadszerosintoall positionsof both DACs
	Enablesall latches,track mode
	Loads zeros into all positions of both DACs
	Enables high byte latch, track mode
Negativefull-range	Loads 80 into DACs. Midrange value
	Enables all latches, track mode
	Loads FF into all positions of both DACS

7-75