Tektronix 20k, Theory ol Operaton 4g4A/494ApService,Vot, Calibrationcontrolselectioncodes

Theory ol Operaton - 4g4A/494ApService,Vot. 1

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Flgure 7-23. Simplified digital-to-analog convertGr,

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Feedbackfor the outputstageis providedby R1056, plus an internalresistor in U1042. The internalfeed- back resistorensuresbettertemperaturetracking. The internalresistorprovidesa gain slightlyless than unity; R1056increasesthe stage gain and permitsgain caii- bration,as describedbelow.

One-of-fourdecoder, U4025,uses data bits DBg and DB4 linesfrom U2015,to controlthree sectionsof

aquad FET switch,U3025. (RC circuit inputs of each FETcontrolline filter out noisefrom the digitalcircuits.) The code is exclusive;.e.,only one FETis switchedon at a time. See Table 7-14 lor a listing of the codes. When a FET is switchedon, it connectsa calibration

adjustmentpotentiometer summing

to the node of the operationalamplifier.AdiustmentRl065 sets the 1st LO tunecoil swE€p,R1071setsth€ 1st LO FM coil sw€€p, and R't067 setsthe 2nd LO span.

Table 7-14

CALIBRATIONCONTROLSELECTIONCODES

R1065(maincoil)

Rl071 (FMcoil)

R1067(2ndLO)

Decade Attenuator

Since accuracyof the digital-to-analogconverteris specifiedas a p€rcentageof full scale, the accuracy decreasesas the attenuationis increased.To maintain accuracyat'l./o,it is neverusedat an att€nuationfactor of more than ten. lf more attenuationis required,the decade attenuator.consistingol K4072,K3075,K3065 and the connecteddivider network, provides further swe€p attenuationof X0.01, X0.1,and Xl . See Figure 7-24tor a simplifiedcircuitdiagram.

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