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

Theoryof Opera0on- 4g4[l4g4Ap Servlce,Vot. 1

Tune Amplilier

.. Thg FET-inputoperationatamptifier(Ui034) takes the output of the phasefrequencyd€tector, amplifies tJregqgr and supplies an appropriate tune vottage to the 1O0MHz voltage controlled oscillator. The tun€ ampliller, with feedback components c1031, c1o3g, Rl028, and Rl029, determinethe toop transfercharac- teristics. The toop dc gain is very high which takes advantaggof the high accuracyof tire internalor exter- nal references.The loop ac aain(determinedby ClOgl) rollsoff very quicklyso any phasenoise,on an external referencesignal,is not amplified.

7-22

Lock Detector

U1012 is used as a tune volts window detector. R1013,R1012,and R1011set the upper threshotdat 11 V6s,and the lowerthresholdat 2 Vo". As longas the tune volts stays within these limits,d trigtroutputtells the processorthat the 3rd Lo loop is locked. A low output from Ul012, indicatesthat the referenceoscilta_ tor frequencyis beyondthe OrdLO'stune range. This REF LOCK status line, along with the othErtwo proc€s_ sor interface lines, is routed through thE Swe€p board for processor interrupt generation. The processor r€ads the lines and displays their status on th€ crt r€adout.

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