1INTRODUCTION RT2048
1.4. PRINCIPLE OF OPERATION
FREQUENCYGENERATION
All the internal frequencies are referenced to a crystal oscillator, running on 14.85 MHz.
The 14.85 MHz is divided by 4 in the REFERENCE DIVIDER, to generate a 3.7125 MHz signal, which is the input
tothePLL-referencedividerand clock-signal for the microcomputer.
Thelocal oscillator signal for the first mixerin the receiverand the transmit signal are generated ina phase-locked
loop(PLL). To generate the needed frequencies, which are specified asfollows:
139.1 MHZ < fLO1 < 148.45 MHz; 154.5 MHz < fTX NORMAL < 159.15 MHz
159.0 MHz < fTX SPECIAL << 163.75 MHz
a bandshift is performed in the VCO.
The loop reference frequency - and so the resolution - is 12.5 kHz, derived by dividing the reference divider output
with 297.
TheVCOoutputfrequencyisdivideddownto12.5kHzafterthedual-modulusprinciplewithaPRESCALERdividing
with32/33.
The phase detector output controls the CHARGE PUMPE feeding the loop filter integrator.
Ifthereisadifferenceinphase/frequencybetweentheinputstothephasedetector.Anerrorcurrentfromthecharge
pumpe will be integrated in the loop filter, producing the needed voltage for the VCO.
RECEIVER
The antenna signal is fed to the RX AMPLIFIER through the RX/TX relay.
ThebandpassfiltersaroundtheRXamplifieraretunedbymeansofcapacitordiodes,whicharecontrolledbyaDC-
voltage derived from the VCO control voltage in the PLL.
Thereceivedsignalisconvertedtothefirstintermediatefrequencyon15.3MHzintheFIRSTMIXER,usingtheVCO
signal from the RX BUFFER AMPLIFIER as local oscillator signal.
Thesignalisfilteredandamplifiedbeforedown-conversionto450kHzintheSECONDMIXER.Thecrystaloscillator
signal is used as local oscillator signal.
After filtering in the SECOND IF FILTER, the signal is amplified and detected.
The AF signal is passed through a mute switch before undergoing appropriate amplification and filtering to get the
rightfrequencyresponse.
Besides,thedetectedoutputisfilteredintheSQUELCHFILTERbeforeitisamplified,detected,andcomparedwith
a reference level to get a logical level for the microcomputer, which controls the mute circuit.
The telephone amplifier and the AF power amplifier produce the wanted output levels for the earpiece and the
loudspeaker.
TRANSMITTER
ThesignalfromthemicrophoneispassedthroughaPRE-EMPHASISnetworkbeforeappropriateamplificationand
compression in the AF AMPLIFIER COMPRESSOR.
This signal is filtered before it is fed to the VCO, where the modulation of the transmitter signal takes place.
As the VCO oscillates direct on the transmitting frequency in TX-mode, the signal only has to be amplified. This is
done in the TX BUFFER AMPLIFIER and the TX POWER AMPLIFIER.
The power supply for the TX power amplifier is regulated by a feed-back loop via the POWER SENSE circuit to
maintainconstantoutputpower level. Switching between full and reduced outputpowerlevelismade by means of
thePA-REGULATOR.
Toreducethelevelofharmoniccomponentsinthe outputsignalitispassedthroughaHARMONICFILTERbefore
it is led to the antenna via the RX-TX-RELAY.
MICROCOMPUTER
The microcomputer on the interface unit is taking care of various functions, among these the user interface, which
meansreading of the keyboard and readout to the LED-display via theDISPLAY-LATCHES.
Moreoverthecomputercalculatestheappropriatedivision figuresforthePLL,controlsthetransmitterpowerlevel,
the AF mute circuits, and reads and writes to the EEPROM’s.
When a selcall unit is installed, the microcomputer also controls the selcall switch capacitor filter.
The serial interface is only for factory production use.
9403
Page1-6
