FMTRANSMITTER
TR
EM1000HEDIGPLUSCOMPACT
TechnicalManual1v2November2019
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3.3MAINBOARD
Throughthisboardtheinputaudiosignals,theirleveladjustment,filteringandlimitationare
processed;italsocontrolsthelevelofRFpowerintheloadortotheantenna;andcontains
thecouplinginterfaceforexternalsupervision,controlandmonitoring.Byactingasthemain
card,itisresponsibleforintercouplingthedifferentmodulesbymeansofflatcables.
Figure:Mainboard
OntheupperleftsideoftheElectricalDiagramA,apeartheamplificationandisolationstages
performedwiththesixsectionsofthemultipleoperationalamplifiersIC1andIC2.Thissection
alsoincludestwoimpedanceselectorjumpersfortheinputsoftheaudiochannels,aswellas
the protective networks consisting of resistors and diodes that protect the inputs against
eventualstaticdischarges,accordingtotheCEstandard.
It is followed by four unit gain isolation amplifiers and two active converters that transform
thesymmetricinputsintoasymmetric,whichinturnfeedtheelectronicattenuatorincluded
in Electrical Diagram B. The last operational IC2 (d) amplifies and filters the input of the
RDS/SCAauxiliarychanneleliminatingthelowfrequenciesofthisinputandfeedingthethird
inputchanneltotheelectronicattenuatorincludedinElectricaldiagramB.
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
ElectricalDiagramA.Throughthem,thelogicinputoroutputlevelslessthan1Vareshownas
zero,andthelevelsgreaterthan10V,thatis,10to12V,asone.
InthelowerrightpartofthisdiagramistheoutputRFpowercontrolcircuit.Theinputsignals
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
symmetricalcircuit.
Thecontrolofthetransmittedpower,constitutedbyIC4(a)continuouslycontrolsthegainof
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