Tektronix 494AP service manual A o o

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_^ wIT the foop is unlocked,RF teakagefrom the 16-20 MHz osciilatorbufferis present.ip224 with a level of --35dBm. The-amplifiedOmeiencefrequencycan be monitoredat Tp20gS.

Another check of phaselock operationcan be done by measuring the dc voltage on the 21g2MHz Tune Line at.feedthroughcapaciior C22Og.-ttominally

itris voltageis approximately-5 V whenphase lock€d. Use a FREQ SPAN/DV of S00kHz or greater before measuring. lf there is no differencefreq-uency,the vol- tagewilt be about 0 V.- l vottageof _13 V mayindicate lossof signalfrom the VCO.

Narrow-bandnoise on the 2nd LO signal may be dueto noisemodulationof ths 16_20 MiivCO. trton_

itor the stgnatat the tg MHz portto ii ttreoscittator signal is noisy. Noise on this line """

is often caused by

noise on the *12V lines. Use a differentiatoscillo_ scope with 1 Hz to 300 Hz bandwidthlimits to check supplynoise. Measurethe ac OifferentiatLetweenthe supplyand the 2nd LO housing.Less than 5 pV peak- to-peak of noise will cause noticeableperformance jeOradatign.Output noisefrom the shapel is typicaily lessthan5 pV peak-to-peak.

Maintenance_ 4g4N4g4Apservice vot. I

. When making power m€asurem€nts of microwave circuitry,at circuit board interfaces,use a coaxialprobe with v9ry litfle stray. inductance (see figure'O-ee1Ground the outer conductor of the pioUe tolhe circuit housingas closeas possibleto the measurementport. Disconnectotherloadsfrom ths meaEurementpoint.

100 MHz Oscillator in the 3rd Converter

Avariable capacitor, C10gg, inside the cover (Figure6-25),shouldonly need actjustingafter reptacing a.componentor componentsin the 100MHz oscillatoi circuit.

1.Wth the cover removed,monitorthe CAL OUT connectorwith a frequencycounter and adiust Cgoiil , with a non-metallictunning tool, for a readingof 100 MHz rlkHz.

2.The Cat Amplitude adjustment, Rl041, is describedin the Adjustmentproceduresection.

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Flgure 6-25-3rd Converter test poinb and adjustments.

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