Tektronix 494AP service manual O a O o a o, MHz 2nd Converter Diagrams15

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divideand offset lhe output of U1031so the tune vol- tagerangesbetween0 and -12.5 V.

The outputof divid-erRl0gg/R1Og4is apptiedto the varactorof the 2192MHz microstriposcillaior(2ndLO). This closesthe phase-locktoop, tuningif," ZnOLO so !l"l-j! 9t9JrVtracksthe IB'MHZvto. whenthe 18 MHz VCO is tuned,Ul0gt simuttaneoustytun€s the microstriposcillatoran equal amount. Withinthe toop bandwidth,the 2nd LO performanceis OeiermineOUy the 18 MHz VCO insteadof the microstriposciilator, giving a significantimprwement in frequencystabitity andreductionof phasenoise.

829 MHz 2nd CONVERTER (Diagrams15 and 16)

The 829 MHz 2nd ConvErterassembty(A23)down- convertsthe lst Converterband 2-4 g2girtHzlF signal to 110 MHz to drivethe 3rd Gonverter.lt also provides the switchingto select either the 2Ot2 MAz 2nd Con- verteror the 829 MHz2nd Converter.The lF circuitsin the signal path are shown on diagram16, tF Section. The local oscillatorcircuits are sh-ownon diagram15, LO Section.

lF Section (Diagram16)

The829 MHzlF circuitsincludean inputdiplexer,an amplifier,a band-passfilter, a mixer, and a diode switch.

829 MHz Diptexer_The.- 829 MHz Diptexer(A23A4) passes signats at 929MHz with approximateiyf Ofi rninimumattenuationand 200 MHz'pass-band. Fre- quencies outside the pass-band but betvyeenabout 50 kHz to 2 GHz are terminatedin 50O loads with a matchof at least 10dB. FigureZ-3 showsa simplified schematicof the diplexer.

At 829 MHz, the series resonatorsprovide a low irnpedancepathfrom inputto output. fnl inputis from the 1st Converterthrough low_passfilter FL.5l and P231. Signal loss across the SOO resistors is insignificantbecauseof the low impedancepatharound theser€sistors. The paraltetresonantcircu'ltto ground appearsas an opencircuitat 929 MHz.

At frequenciesabove or below the pass-band,thes.eriesr.€sonatorsapp€ar as large reaciances,shifting the primarysignalflow throughthe 50O r€sistors. The out-of-bandimpedanceof the parallelresonantcircuitis now small comparedto S0O. Thus,the 50O resistors are.essentiallygroundedat their junction,terminating boththe inputand outputports of the diplexer.

Theoryof Operation- 4g4[l4g4Ap Servtce,Vot. 1

SERIESSERIES RESONATORfo : 829MHz RESONATORlo = 829 MHz

5 0 0 OUTPUT

PARALLEL

RESONATORfo : 829 MHz

44.t695

Flgure7-3. Stmpllfieddlplexerdiagram.

Awide bandwidthis used to minimiz€loss in the resonantcircuits and eliminateadjustments.Relative bandwidthsof the series and parailelresonantcircuits are optimizedto providereasonablematchat the band edgEs.

As shown in the schematicdiagram,the diplexer contains componentsnot shown in Figure 7-3. The 50o terminationsare actuallytwo pairsbt i00O resis_ tors, Rl014-R1015and Ri011-Ri0t2, connect€din parallelto reduce load inductance.Small capacitors, C1010and C1013,are connectedacrosseach load to improve impedancematch at frequenciesabove the pass-band.The inductorin the parallelresonatoris a printed l€ngth of transmissionline that is tapped to establishthe correctbandwidth.one end of this induc- tor is groundedthroughfour capacitorsso that dc bias from the lst Local OscillatorDrivercan be introduced to the lst Mixer (A12)or the EXTERNALMIXERinput through this diplexer. Severatcapacitorsare used in paraflelto minimizeinductanceand circuit e degrada- tion. A low-passfilter is includedin the bias line to minimizeanynoisefrom the 1st LO driver.

The diplexerdrivesthe 829 MHzAmptifierthrougha 1.2GHz Low-PassFilter that consistsof three shunt capacitorsand two series inductors. Cutofffrequency for thisfilteris 1.2GHz.

829 MHz Amplitier. The 829 MHz Amptifier (A2SA5) provides about 18 dB of signal gain at g2g MHz. The amplifier consists of two similar cascaded amplifler stages, Q1017 and Q1025, and a 3 dB pad. The overall noise figure is approximately 2.8 dB. The gain stages are stable amplifiers that are designed for use in a 50 ohm system.

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