FMSECTION:Thisisdividedintotwoseparate
modularsections:
A.The100MHzRadioFrequency(RF)Section.
ThissectionhousesthecompleteFM-RFfront-
endandpartoftheAM-RFcircuits.A special,
foursection,variabletuningcapacitorprovides
ahighdegreeofRFselectivityandexcellent
spuriousresponserejection.Theproblemof
imagerejectionhasbeengreatlyreducedin
theRFsectionoftheMR73.
Thetwostageparallel-fed-cascodejunction
fieldeffecttransistor(JFET)RFamplifiergives
bettersensitivityandhighergainthanconven-
tionalone-stageamplifiers.TheuseofJFET's
helpstofurtherreducetheproblemoffrontend
overloadinstrongsignalareas.
AmixerusingJFEThasbeendesignedfor
highsensitivityandfreedomfromoverload.
Lowtemperaturecoefficientcomponentshave
beendesignedintotheFMlocaloscillatorto
preventfrequencydrift.Thefrequencystability
inherentinthelocaloscillatormakesautomatic
frequencycontrol(AFC)unnecessary.Therate
ofdriftofthelocaloscillatorislessthanten
partspermillionperdegreecentigrade.
BoththeFMandAM-RFfrontendshave
beendesignedinthesamecompletelyencased
metalmodules.Thisdesigngivesprotection
againstradiationorinterference.TheRFcir-
cuitsoftheMR73exceedtheFCCrequire-
mentsforsuppressionofoscillatorradiation.
TheMR73hasantennaconnectionsfor
either300ohmtwinleadtransmissionlineor
75ohmcoaxialcable.Thenormalinputimped-
anceofthefirstRFamplifieris75ohms.Im-
pedancematchto300ohmsisprovidedbya
Mclntoshdesignedbaluntransformerwhich
hasnegligiblelosses.Connectionsfora 300
ohmlinearemadewithnewpushtypetermi-
nals.Notoolsarerequired.A typeF malecon-
nectorisfurnishedfor75ohmcoaxialcable.
Forgreatersignaltransferandlowerdistor-
tion,a specialmatchingtransformerhasbeen
designedtointerfacetheFM-RFtotheFM-IF
amplifier.Thismatchingtransformerconsider-
ablyenhancesthelinearphasecharacteristics
oftheIFamplifier.
B.TheFM-IFandDetectorSection.TheFM-IFand
Detectordesignrequiredextendedengineering
timeandtestingandusesthelatestconcepts
inintegratedcircuitsandcrystalfilters.The
FM-IFconsistsoftwointegratedcircuitsand
twophaselinearcrystalfilters.Theycombine
togivea totalgainofover120dB(thesignal
isamplifiedtoover1,000,000timesitsoriginal
level).Theresponsecurvehasa nearlyflattop
withlinearphasecharacteristics.Theskirtsof
theresponsecurveareverysteep.Themaxi-
mumwidthis240kHzat-3.0dBand400kHz
at-60dB.Theresponsecurveissymmetrical
eachsideofthecenterfrequency.Thecrystal
filtersarepermanentlysealedanddonotre-
quireadjusting.TheIFcannotdriftnorvibrate
outofadjustment.
Eachofthetwointegratedcircuitsusedin
theFM-IFoftheMR73contain16transistors,
3zenerdiodes,5 diodesand23resistors,all
ona singlemonolithicsiliconchip.
Theexceptionallyhighgainofthetwointe-
gratedcircuitsassumes"hardlimiting"atvery
lowlevelsofinputsignals.
A"phase"or"FosterSeeley"discriminator
hasbeendesignedtocomplementtheinte-
gratedcircuitIFsection.TheIFsectionhas
exceptionallyhighgainwithhardlimitingchar-
acteristics.Itdevelopsa captureratiothatis
verylow.Thedetectedoutputsignalofthe
discriminatorisextremelylowindistortioncon-
tent.De-emphasisofthediscriminatoroutput
restoresthefrequencyamplitudecharacter-
isticstothesameleveltheywerebeforetrans-
mission.
C.FMStereoMultiplexSection.MclntoshLabora-
toryhasdevelopeda specialdetectingcircuit
inthemultiplexsection.A particularadvantage
ofthiscircuitistheeliminationofthecritical
adjustmentsnecessarywithcommonlyused
matrixingcircuits.ThecircuitdetectstheL-R
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