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

Theory of Operation - 4g4ful4g4Ap Servlce, Vol. 1

afray in the feedback loop of inverting amplifier

ul073A.

All of the amptifier'sfeedback is through R1072 when the output swings to the negativelimit. As th€ output voltage swings less negative,it sequentially passes the tap-point voltages of a series of voltage dividersconnectedb€tween0 V (the summingnode at pin 12) and a negativereferences€t by e1047. lf the ot tput becomespositivewith respectto a givendivider tap, a correspondingdiode in U2059forward biases and connectsthe outputto the tap, which createsaddi- tional feedbackthrough one leg of the dividerto the summingnode. This causesR2051,then R2052,then R2053 (as so on through R20S6)to be connectedin parallelwith R1072 as the amptitieroutput becomes le$s negativs. This progressivelyincreasesthe feed- back,whichcausesthe gainof Ul073Ato decrease.

Another series of dividers connectedbetweenthe amplifier'soutput and a negative vottage reference. causesthe diodes in U1059to sequ€ntlaltyconductas the output becomes more positive. ResistorsR2O6O, then R2061,then R2062(as so on throughR206S)are sequentiallyadded in parallelwith the existing feed-

back. Soft diode turn-on characteristicsand a large numberof breakpointsresult in smoothgain changes. The nonlinearamplifier'svoltage-gaincharacteristiciscontrolledby the shaperreferencevoltage,whichis set by R2049. Altering R2049 wiil make the breakpoints eitherclosertogetheror furtherapart; in practice,this resistoris sel€ctedto correctthe tolerancevariationsof th€ 18 MHzVCOvaractor.

The fomrard drop of the shaper diodes gives Ul073A an offset voltage. Temperaturecorrection diodes CR1086,CR1087,and CR1OBBcorrect this ofiset over a wide temperaturerange by summinga co.rectionvoltage through R1074. These diodes also compensatefor the lack of s€ries diode drop across R1072 and eliminateoffsets at the summinginput of Ul0738. SelectingR1070 providesfine adjustmentof the VCO'scenterfrequency. lC U107gBis an inverting amplifierthat increasesthe shaperoutputvoltageswing to a levelthat can control the varactorof the 1g MHz

vco.

Adifferential amplifier with weil-definedtimiting characteristicsis used for the 1g MHz VCo. Emitter degenerationis Lsed to controlloop gain. Transistors 02096 and Q2087form the differentiatpair of transis- tors, with the emitters coupled through C2091. Transformer T2092 provides ac feedback for the collector-basejunctionof 02096 and also createsthe maiorityof the resonatorinductance.The total resona- tor inductancemay b€ adjustedby trying differentcom- binations of connections between taps on inductor Tl 091andtransformerT2092. Thesetaps allowcoarse adiustmentof the VCOcenterfrequency.The capacitor of the resonatoris varactorCR1089. CapacitorC10gg

7-12

complet€s the resonator ac path and acts as a dc bloc( which allowsa bias voltageto be impressedon the varactor. Resistor R2092 and capacitor C2090 damp the 02096 collector, which prevents high- trequencyinstabilityin the oscillator. TranslstorQ2087 providesa bufieredoscillatoroutput.

Adiscrete two-stage amplifier provides an unsa- turated voltage gain of approximat€ly 43 dB for thg 18MHz signal lrom the 22A0MHz ReferenceMixer board. Transistor Q1041 ls the common-emitterfirst stage while Q1042 and Q1043 form the difierential second stage. Th€ differential stage limits the output swing to 0.8 V to prev€nt over- driving the following ECL circuitry. Dc bias is malntainedby Q1041,which has dc collsctor-basefeedback via R1O46and the Rl043/R1048 vdtage divider. Transistor Q1043 receivesits base bias throughR1042. Eachtransistor operateswith 5 mA of quiescentcurrent.

ECL line receiversU2041Dand U20418amplifyand bufierthe 18 MHz signalsfrom the ReferenceMixerand the VCO, respectively. These two signals are then applied to the phase/frequencydetector for com- parison.

Apair of ECL D-type flip-flops, U2031A and U20318, comprisethe phase/frequencydetector. The flip-flops drive a common reset lins with a wired-AND output. The clock input of u20318 is drivenwith the signal from the 18MHz VCO, and the clock inputof U20314is drivenwiththe signalfromthe 18 MHzsignal from the RefereneeMixer.

Both flip-flops are configured to r€set together wheneverboth are set. lf they are clockedwith signals that exactly match in trequency and phase, then both flip-flops set simultaneouslyand then almost immedi- ately reset. lf the R€ferenceMixer signalhas a slight phase lead, U2031A will remain set longer than U20318. lf the ReferenceMixer signal has a slight phaselag, U20318will set first and remainset the long- est. The signalthat has the phaselead will causethe associated flipflop to be set a grgater p€rcentag€of time than the laggingflip-flop. lf there is a frequency differencebetweenthe two inputs,the flip-flopwith th€ higherinputfrequencywill be set more of the timethan the other flip-flop, The ratio betweenthe filteredoutput signalsof the two flip-flopsindicateswhetherthe Refer- ence Mixer signal leads, lags; or differs in frequency from the 18 MHzVCOsignal.

The outputs of the flip-flopsare low-passfilteredby Cl031 and C1028and appliedto differentialamplifier U1031. U1031comparesthe outputs of the flip-flops and producesan output that controlsthe tuningof the 2182MHz microstrip oscillator. The phase-lockloop

bandwidthis controlledby R1026,C1029,Rl027,and C1026. The gain slope breaksto -l2dB/octavefor

frequenciesbetow16 kHz. ResistorsR1033andR1034

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