OMB EM 1000 HE DIG PLUS COMPACT - Page 71

FMTRANSMITTER
TREM1000HEDIGPLUSCOMPACT
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TechnicalManual1v2November2019  66
presetbytheresistivenetworkR49/R50.The thirdandfourthsectionoftheintegratedIC4
areusedtofilterandisolatethesignalfromtheMicrocontrollerandwhichsetsthereference
thresholdlevelfortheRFoutputpowercontrolloop.
TheTR4transistorisresponsibleforcuttingtheRFoutputwhenthechanneloscillatorPLLis
notengagedinfrequencyforthecorrecttransmissionfrequency.Intherightcentralpartof
thediagramwecanseetheP1connectorofthe"controlline"orcontrolinformationcollector
of the Microcontroller, which carries the digital control lines on their lower pins, and the
analog control lineson their upper pins. From this connector also comesthepower of the
unit,only+13VDCand13VDCareusedthroughouttheunit.
LetusnowgotoElectricalDiagramBofthisunit.Startingfromthebottomleft,wefindIC3,
whichisanelectronicattenuatorwiththreeinputchannels,withdigitalcontrol.Thisdevice
separately handles the left and right audio channels (L and R) and the SCA / RDS auxiliary
channel,whilethestereomultiplexedbasebandinputsignalisprocessedthroughthesame
input corresponding to the right channel. Three separating amplifiers follow the output of
eachchannel;IC7(a),IC8(a)andIC12(a).Theoutputofthefirsttwoseparatingamplifiers
feed the preemphasis stages, whose time constants can be digitally set to 0, 50 or 75s
throughtheanaloggatesofIC9.
Followingisthelimitingstage(ormoreproperlytrimmer)consistingofdiodesD8andD9that
actasdoubleleveltrimmers.Thesignalcutoffthresholdinthesetwodiodesispresetbythe
Microcontrollerthrough IC13(a)byvarying thereferencevoltage andtherefore thecutoff
(orconduction)threshold+VIandVIofbothdiodes.
ThepotentiometerRT4,setsalimitstoptothevalueofthereferencevoltageofthelimiter.
Once limited in amplitude, the signal is sent to the input sections of the stereo encoder
circuit,ifitispresent.Atthesametime,therightchannelsignal(whichisalsothemonaural
signal,ifused,sincetheyusethesameinputconnector)issenttoafilter.
The electronic selector switch IC14 selects the signal from the nonpreemphasized input
through R124, or the filtered and preemphasized signal through R128, or the output
multiplexsignalof thestereo encoder throughR131.The integratedIC12 (b) separatesthe
selectedsignal andaddsit tothatcoming fromtheSCA/RDSauxiliarychannelinput.When
required,diode D17limitsin levelthe totalsignalresulting fromthissum, whichformsthe
totalbasebandofmodulation.Thebasebandthusformedissenttothefrequencymodulator
circuitofthechanneloscillatorthroughIC12(c)thatactsasabasebandamplifierseparator
also extracting an output through the <LF MONITOR> connector for the base band for
purposesofmonitoringorretransmission.
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