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

Theory of Opera$on- 4944/4g4ApServtce,Vol. I

off. Negativecurrent.suppliedthroughRl014, €nsures that Q1011is turned off. Diode CRt011 protectsthe base of 01011 from excessivereversebias. Voltage across R1017is approximately9.4V when e1011ls turned on and approximately4.4V when it is off. Overall gain is approximately 12.9dB when the amplilieris turnedon.

The 110MHz lF signalis transmittedvia p2g2to the 110MHzlF Amplifiershownon diagram17.

LO Section (Diagram'15)

The 829MHz 2nd Converter LO generates the 719 MHz frequencythat is mixed with the 929 MHz tF to produceth€ 110MHz lF signat. ln the following description,the circuitsare referredto as the 71g MHz LO. The 719MHz LO consistsof a phaselock loop,a 719 MHz outputcircuit,and a 2nd LO frontpaneloutput circuit.

PhaseLock Circult The phaselock circuitreceives referencefrequencyinputs and uses phase/frequency detectiontechniquesto use those signalsin controlling the outputfrequencyof the 719 MHz oscillator.Thecir- cuit consists of a voltage controlled oscillator (VCO),a phase/frequencydetector,a harmonicmixer, and vari- ous amplificationstag€sand power splitters. Whenthe 719 MHz LO is enabled,the 2182MHz LO outputfr€- quency is used as a swept r€f€rence to d€rive the 719 MHz frequency. The VCO is controlled so that the third harmonic of its output frequency is a constant differencefrom the 2182MHz reference.This controlis accomplishedby the phaselock loop. Refer to Figure 7-6for a simplifiedblockdiagram.

In the phase lock loop, the harmonicmixer gen- erates a frequencythat is the difierencebetweenthe swept 2182 MHz input referenceand the third harmonic of the VGOoutput frequency.ldeally,this differenceis 25 MH4 which in tum. is comparedwith the 25 MHz that is divideddown from the 100MHz oscillatoroutput suppliedfrom the 3rd Converter. This comparisonis done by the phase/frequencydet€ctorwhose outputis

acorrectionvoltagethat drivesthe VCO and shiftsthe oscillatorfrequencyin the directionto hold the nominal output frequencyat 719 MHz. This completesthe loop that causesthe vco to trackthe 2182MHz reference.

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PHASE/FREOUENCY	COMPENSATION
DETECTOR
	AMPLIFIER	OSCILLATOR

25 MHz DIFFERENCE FREOUENCY

829MHz

HARMONICMIXER

Figure7$. Blockdiagramof the phasetock loop in the 829 MHe2ndConverter.

7-16

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