OMB EM 1000 HE DIG PLUS COMPACT - Main Board

FMTRANSMITTER
TREM1000HEDIGPLUSCOMPACT
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TechnicalManual1v2November2019  65
3.3MAINBOARD
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Throughthisboardtheinputaudiosignals,theirleveladjustment,filteringandlimitationare
processed;italsocontrolsthelevelofRFpowerintheloadortotheantenna;andcontains
thecouplinginterfaceforexternalsupervision,controlandmonitoring.Byactingasthemain
card,itisresponsibleforintercouplingthedifferentmodulesbymeansofflatcables.
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Figure:Mainboard
OntheupperleftsideoftheElectricalDiagramA,apeartheamplificationandisolationstages
performedwiththesixsectionsofthemultipleoperationalamplifiersIC1andIC2.Thissection
alsoincludestwoimpedanceselectorjumpersfortheinputsoftheaudiochannels,aswellas
the protective networks consisting of resistors and diodes that protect the inputs against
eventualstaticdischarges,accordingtotheCEstandard.
Itisfollowedbyfourunitgainisolationamplifiersandtwoactiveconvertersthattransform
thesymmetricinputsintoasymmetric,whichinturnfeedtheelectronicattenuatorincluded
in Electrical Diagram B. The last operational IC2 (d) amplifies and filters the input of the
RDS/SCAauxiliarychanneleliminatingthelowfrequenciesofthisinputandfeedingthethird
inputchanneltotheelectronicattenuatorincludedinElectricaldiagramB.
The RS232 serial interface (IC5) and the active interface of the parallel I/O port with its
protection network, consisting of TR1, TR2 and TR3, are located in the lower left part of
ElectricalDiagramA.Throughthem,thelogicinputoroutputlevelslessthan1Vareshownas
zero,andthelevelsgreaterthan10V,thatis,10to12V,asone.
InthelowerrightpartofthisdiagramistheoutputRFpowercontrolcircuit.Theinputsignals
proportional to the transmitted or direct RF power, and reflected, from the directional
coupler thatacts as the output power sensor, enter theunit and are amplified byIC3in a
symmetricalcircuit.
Thecontrolofthetransmittedpower,constitutedbyIC4(a)continuouslycontrolsthegainof
the power RF amplifier stage, varying the bias voltage or byas of the RF output MOSFET
transistor.
Thereflectedpowerlimitingcircuit,constitutedbyIC4(b)onlycomesintooperationinthe
same loop when the voltage at the output of IC3 (b) is greater than the threshold voltage