Tektronix 494AP O o O o o a o o A o O o o o o o o o O a a, Enables Currentdrivers, R 1

o o o o

o

o o o o o a o o

o

o o a o

a

o o o o o o o o o o

O

o a

I

o a o

O

a o o a a

?

changein input power frequencyor power supplyvol- tage and notifiesthe microcomputer.' etapiid time

en

meter is also located on the board to give a indication of total instrumentoperatingtime.

RF lhterface Circuits

The RF interfaceincludesthe digitalcontrolcircuits that receive the addr€ss and instruition data from the microcomputerand decodeit to control the RF Attenua- tor, TransferSwitch,and lF selection. The powersup- plies. that are required to drive the att€nuator and switchesarealsoinclud€d.

Address decoder U2045 latches the data at the input of U3O46wheneverthe microcomputerselects address4F. Table 7-7 lists the purpose'ofeachdata line from the buffer.

Table 7-7

RF INTERFACELINES

		Purpose
o1	Enables10 dB attenuator
Q2	No connection
o3	Enables30 dB attenuator
Q4	Enables
o5	currentdrivers
	Enables	e2O2Sande302g
	transferswitchdriver
Q6	MHztF (high		state)or 2022
	9.:1.""F_829
	MHzlF (towstate)

o7 Enables 20 dB attenuator baseli

When Q4 of U3046 goes tow, e2025 and e302g conduct. This raises the Vcc of attenuatordrivers U3034,U3029,and U30Ogto +16V for approximatety 100 ms to energizethe attenuatorsolenoi'dsA. diode protects each attenuatordriver output line lrom the inductivevoltage surge that occurs when the solenoids changestat€.

The TransferSwitchoperationdependson the out_ put of O302S/O9024. The e5 ouiput of U3046 is applied to the input of operationalamptifierU4O2g, which drives differentiatamptifierO2025/O3O24When.

Q5 goes high, Q3025is biased on and the Transfer Switchselectsthe externalmixer. Whene5 goes low, Q3024is bias€don, and the internatmixeris selected. Diodes CR3018 and CR3017 protect the transistors from voltagespikesinducedwhen the TransferSwitch changesstate.

Z-Axis Circuits

The Z-Axis circuits providethe drive currentsand bias voltage to operatethe crt. They consist of the intensity control logic cireuits, which control the crt bearncurrentfor normalsignatdisplayoperations,and

Theory of Opera{on - 494Ll4g4ApServtce,Vot. 1

the unblankinggates, which furnish cuffent to the Z_ Axis drive amplifierto drive the crt control grid.

The Z-Axis Drive Amplifier is an operational amplifierthat consistsof transistorse3047, e4125, and 04065. and retated components. R1OS0is the input resistancefor the amplifier,and R2066is the feedback lej;is-tor. The output is ctampedby diodes CR3059and CR3066to protectthe amptifierfrom transientsurgesin case of crt arcing. The amplifier is driven Oy two sources,exclusiveof each other;U203gBpzA42-drives

llt_e^^Tplifi€r during readout disptay periods, and U2038AO2O44 drives_ the amptifier'during sweep displayperiods. U2039is an AND-NORgate that pro_ vides_th€logic to one input of NANDgaie U2O38Ato turn Q2044on or off. The R/O OFF line and the output of U2039 must both be high for U203gA to fumish current to Q2044.Table 7€ lists th€ conditionsunder whichU2039wiffoutputa highto U2oggA.

Table 7-8

U2039TRUTHTABLE

	Gonditlon
U3046output(lineQ8)	0 0 0 1 1 1 0 0 0
CLIP	0 0 0	0 0 0 1 1
Z-AxisBlank	1 1 1 1 1 1 1 1
StorageOfi	0 0 1	0 0	1 0 0
SWPGATE	1 0 r 1 0		1 1 0
	0 0 0	0

Only the combinationsshown in Table 7-g plus a highon the RIO OFFtinewiil gatea tow out of U203gA. Whenthe U2038Aoutput is low, emittercurrentis fur- nished to Q2044, which in turn furnishes current throughR2051(the input resistanceof the Z-Axis drive amplifier)to Q3047. U20g4Bis a single-shotmuttivibra- tor that produces a 3 micros pulse to blank the crt beam duringtrace return, betweenreadoutand signal display.

The othersourceof inputcurrentto the Z-Axisdrive amplifier is Q2042. This transistor is turned on by U20388when R/O UNBLANKis high and R/O OFF ia low.

Ql028 is the current source for divider Rl030iR1025 that establishesthe operatingpoint for 0.2042 and Q2044, which s€ts the intensity level. DiodesCR1045and CR1043, connectedfrom the base ot Q2042and Q2044 to the emitter ot eZ02Z, timit the dtsplayintensity.These diodespreventthe bases from goingmore positivethan approximatety0.6 v abovethe emitter voltage of Q2022. This circuit,which includes lnt Limit adjustmentRl027, sets the maximumcurrent for both Q2042and Q2055.

7-49

O