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

Theory of Operalion- 4g4A/4g4ApService,Vol. 1

Low-orderDAC U2055 tunes approximately +2.5mV at pin 1. and its gain is adjustedby R1O2S.

gain oj p-r_elmplifier

l!e^U1065is set at approximatety 10,000by Rl056 andthe 5O combinationof R2O59ana R2060. The combinationof CR1056,CRlO58,RlO54, and R1059 limits th€ gain of U1065when the output exceedsapproximately0.7 V in eitherdirection.

U1065is connectedto integratorU2OT0via storage gate Q1065,which is on in the track mode. Transistor 01065 is turned off any time a DAC is being tuned to allowthe DAc outputto settle beforetuningthe output of U2070. lt is also turned off duringthe intervalwhen

acarry from the low-order DAC to the high_orderDAC occurs. Transistor Ql065 is controlled by e1061. WhenQ1061is on, CR1064is reverse-biasedThe- vol- tage at the gate of e1065, which is developedby R1064,R1065,R1067,and R1066,is near OV ana Q1065conducts. When e1061 is off, voltageto pinch off Q'l065is apptiedthroughR1062and CR.064t.

v2070 tracksthe outputof U1065whenthe circuitis in the track mode and serves as the invertingamplifier in ths fe€dbacksystemshown in Figure7-26. Normally the incoming signal is routed through R2067. T; improvetl.e slewingrate of the integrator,CRI067 and CR1069 conduct and connect R1O6gacross R2067 wheninputsignalsover1 V are present.

7-76

Write-Back Circuit

This circuitconsistsof comparatorUl055 and ena- bling transistorQ1058. When it is necessaryto do a carry betweenthe low- and high-orderDACs,the circuit is put into the hold mode by turningofr Q1065. U2060 is incrementedone bit and U2055is resetto all zeroEs_ The output of u1065 is now at somethingother than 0 V. The purpose of th6 following approximationrou- tine is to get output of U1065as closeto 0 V as possi- ble before switchingthe circuit back into track rnodeby tuming on Ql065. ComparatorUl055 det€ctswhether

the output of U1065 is above or below coarse tune ground. The instrumentrnicrocomputerbeginsto exer- cise th€ low-orderDAC bits one at a time from MSB to LSB. After each bit is turnedon, U1055ls enabtedby turningoff Q1058. lf U1055detectsthat the outputof u1065 has crossed 0v, that bit is turned ofr and the next lower bit is turned on. This continuesthroughall

12bits and when completed,th€ output of U1065 shouldbe close to 0V. TransistorQ1065can now be turned back on without causingexcessivejumpingof the signalon the screen.

-10V Reterence Buffer

The circuituses the voltagereferencedevelopedon the 1st LO Driver board as a referencefor the DACs. Differentialamplifier U20/5 receivesthe -10V refer- ence and -10 V referenceretum, and removes eny common-mode signals present. Resistor pairs Rl048/R1049and Rl050/R1051are matchedfor tem- perature coefficientto minimizereferencevoltage drift overtemperature.

a o

Io

a o o o o

t

I

o o o o a o o o a o

I

I

o o c o a o

t

o o

to

a o o o o o o o o o