Tektronix 494AP service manual O o a o o o o o O a a o a O o o o o o o o o o a, DB4, DB7

Theory of Operafon - 494ful4g4ApServtce,Vot.1

rThe mixer bias circuit producesand outputs the requiredbiasvoltagesfor the 1st Mixer.

. The programmablebias circuit providespeaking voltagefor the Preselectoror externalmixersbias voltagebased on data suppliedby the microcom- puter.

Digital Control

The digitatcontrolcircuitcontrolsthe osciilatorspan volts, the 1st Mixer bias, and programmabtebias. DecoderU4034output 1 (pin 14) goes low when the input addressis 72 and output Z {gin T) goes tow for address7E. Whenoutput 1 goes high,data is clocked or latchedinto U4017,and when output 7 goes high data is latchedinto U4024or tJ4022,dependingon the statusof databits DB6and DB7.

Data for U4017 eonsists of control codes for the oscillator drive circuits and the switches in U1016, whichselectl st Mixer bias or the bias set by the front- panel MANUAL PEAKING control. The codes are describedin Table7-16.

Oata at address 7E tor U4022 and U4024 drives DAC U3022and is convertedto an analogsignalwhich providesthe ProgrammableBias.

Table 7-16

U4017OUTPUTLINES

lnput		State
iiqnal
DBO	Bias1 on	Bias 1 off
DB1	Bias2 on	Bias2 off
DB2	Bias3 on	Bias3 off
DB3	MANUALPEAKon	MANUALPEAKoff
DB4	Not Used	Not Used
DB5	Driverinputon	Driverinputoff
DB6	SpanVoltsline off,	SpanVoltsline on,
	oscillatorfilter on	oscillatorfilt€roff
DB7	Not Used	Not Used

Input Switching

lf the maincoil of the oscillatoris not to be swept, DBOgoes low at address72. This cuts off e3029, de- energizesK3034,and disconnectsthe SPAN VOLTS signal to the summingamptitier. Diode CR303t pro- tects 03028 from the inductiveleedbacksurges that occur at turn-off.

7-68

Oscillator Filter Switch Driver

WhenDBOis low, relay K3034is de-energizedand 02029 is biasedon which drives a capacitorswitching relayon the 1st LO Interfaceboard. The capacitorsare switched acroEs the main coil, when it is not being swept,to lilter noiseridingon the tuningcurrent. capa- citor C2025provides a gradualdecay of currentthrough the r€lay aft€r power is turnedoff.

Summing Amplifler

Amplifier U2032 and the compl€mentarypair of translstors, 02035 and 02039, plus related com- ponents, comprise an operational amplifier. The COURSETUNE VOLTS ANd thE SPAN VOLTS ArE summedat the input to u2032. The feedbackresistor, for the operationalamplifier,is Rl038. The inputresis- tance is R2O27for the COARSETUNE VOLTS signal and R20{}1for the SPAN VOLTS signals. (R2030is switched across R2031, as mentionedpreviously,to increasestage gain for maximumspan operation.)The output of the summingamplifier,which can swingfrom -10V to +10 V, is appliedto the PreselectorDrivercir- cuitsandto the VideoProcessorboard.

Oscillator Driver

The output of the summingamplifieralso drivesthe inputto the oscillatordriver stage when FET Q2040is switched on. The oscillator drivEr stage consists of active componentsQ2045, U2043,Q3047, and Q352. The inputresistanceconsistsof R2041,the 1st LO Sen- sitivity adjustmentR1031,plus R2043. The feedback resistanceis R2042.

The arnplifierbonvertsa voltageinput into a cunent drivefor the l st LO tuningcoil by controllingth€ voltage acrosscurrentsens€ resistorR1040,which is in s€ries with the oscillatortune coil. 03047 assuresthat Q352 basecurrentremainswithinthe oscillatortuningcoil cir- cuit. Q2040is biasedon except when the oscillatoris degaussed. The summing amplifieroutput is applied through the 1st LO SensitivityadjustmentR1031and summedwith an offsetvoltageset by the 1st LO Offset adjustmentR1032at the inputto the preamplifierstage 02045. AdjustmentsRl031 and Rl032 matchthe oscil- lator driver stage to the oscillator characteristics, R1032 adds offset to the input of the preamplifierto placethe oscillatorat centeroperatingfrequencywhen the amplifierinputis at 0 V.

02045 is a low-noise,matched,dual transistor.The feedbackpath through R3040and R2042sets the vol- tage across a four-terminalresistor R1040. This vol- tage sets the currentthroughthe resistorwhichis also emittercurrent for driver transistorQ352. The 1st LO SensitivityadjustmentFll031, sets the voltagegain of

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