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

Theory of Operaton - 4g4A/4g4ApService,Vol. 1

to lock the difference trequencyfrom th€ Harmonic Mixerto 25 MHz.

The phase/frequency

detector output is a voltage

that is proportional phasedifierencebetweenthe to the

25 MHz refer€nceand the lF signalfrom the g29MHz HarmonicMixer. This correctionvoltageis thenapplied to the 719MHzVCOto lock it to the reference.

The detectorcircuit consists of two D-typeflip-flops,U2U7A and U2047B, and a differentiatariiptifieistage used as a NAND-gate(U1044A).The 25 MHz referenie signal, from the frequencydivider, is apptiedto the cfock inputof flip-flopU204tA;the nominat25 MHz sig- nal from the 829 MHz HarmonicMixer is appliedto the clock input of flip-flopV204ZB. The rising€dg€ of the input signalto each flip-flopcausesthe e(bar)outputs to return to the low level only after both flip-flopshavebeen clocked.

lf the frequencyout of the g2gMHz HarmonicMixer is below25 MHz,or if its phaselagsthatof the 25 MHz reference,the Q(bar) output of ftip-flop U2047Awitl remainhigh longer than the a (ba0 outputot u20478. lf the frequencyout of the HarmonicMixer is above 25 MHz, or if its phase leads, the oppositewill occur. When the two flip-flopsare clocked at the same fre- quencyand phase,the two outputswill be highfor the sam€ amountof time. The e(bar) outputsare applied to a compensationor differentialamplifierU3059,that determineswhichoutputis highfor the longertime.

CompensationamplifierUg05gprovidespart of the loop gain to ensure that the 719 MHz oscillatorwill track the sweep of the 2192MHz referenceoscillator. The compensation amplifier also limits the loop bandwidthto 100 kHz to make certainthat the loop wilf not oscillate.Note the differentialinputsto the amplifier each include a low-pass RC filter to attenuatethe undesired high frequency clock pulses from the phase/frequencydetector.

7-18

The nominal swing of the U3053 output is from +12V to -12V.Sincethe compensationamptifieris capableof considerablymore output than is neededto controlthe oscillator,a voltagedivideris used to limit the outputand reduceamplifierrelatednoise. This vol-

tage divider, consisting of resistors R2053, R2054, R3051,and R3052,reducesthe possible*12V swing to +5 V to +12 V, as reguired by varactor diode CR1011.Nominalvoltageswingin a lockedconditionis +6.75 to +7.5 V. Thus, dependentupon whether the HarmonicMixer frequencyis above or below 25 MHz, the correctionvoltageswing,appliedto diode CR1011, is more than nominalto correct the oscillatorfrequency.

2nd Local Oscillator Oupuf Circult A portion of each 2nd LO output signal is sent to the front panel2nd LO OUT connector. This output provides signal for externalaccessoryequipment,such as a trackinggen- erator. Each local oseillator(719MHz and 2182MHz) output is applied through power dividers to a power combinerfor applicationto th€ 2nd LO OUTconnector.

The 719MHz oscillatorfrequencyis appliedfrom a power splitter(R3027,R3028,R3029)througha 1 GHz fow-passfilter(C3025,C2024,C1023.C1021, and three printed inductors),to the power combiner (R2024, R2025, R2026),and the front panel 2nd LO OUTPUT. The 2182MHz oscillator signal is applied through a power splitter(R1021,R1022,R1023),a 2.2 GHz band- passfilter(consistingof coupled1/4 wavelengthprinted lines) to the power divider (R2024,R2025,R2026)and the frontpanel2nd LO OUTPUT.

Both 2nd local oscillatorsignals,2182MHz and 719MHz, are present at the front panel when the 829MHz2ndConverteris selected.

719 MHz Output Clrcult The 719MHz 2nd Local Oscillatorsignal is applied is applied through divider resistors R2021, R2023, and R2A24 to isolation amplilier Q2021. 02021 boosts th€ signal level from about 0 dBm to +12 dBm to drive the 829 MHz mixer. The output of the amplifierincludesa 3 dB attenuator (consistingof resistorsR2027,R2028,and R2029),to ensur€ a 50 ohms non-r€flectivesource impedance. The signallevelat test point J2026is typically-6dBm.

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