Motorola MTM700, Detailed Service Manual

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THEORY OF OPERATION (400 MHz) MTM700 Mobile Radio / Detailed Service Manual 4.1 - 7
The analog modulation signal is passed on to the LNODCT where it is up-converted to the transmit-
ter operating frequency. The modulated RF output signal from the LNODCT is converted from differ-
ential to single-ended by a balun, and passed to the attenuator, pre-driver IC, driver transistor,
power amplifier transistor, directional coupler, and isolator (depends on radio model), from where it
is routed to the antenna via the antenna switch and harmonic filter. An RF feedback signal is taken
from the directional coupler and fed back to the LNODCT for transmitter power control and lineariza-
tion.
Frequency Generating Section
The frequency generating section comprises the following components
(see figure below):
• 16.8 MHz TCXO Reference Oscillator.
• Main Synthesizer - consists of the Main Voltage Controlled Oscillator (VCO) and Low Voltage
Fractional-N (LVFRACN) integrated circuit PLL. In receive mode, the main synthesizer
provides the local oscillator injection to the receiver first mixer. In transmit mode, this
synthesizer provides the frequency reference for the transmit frequency translation loop.
• Transmit frequency translation loop - consists of the transmit Voltage Controlled Oscillator
(VCO) and ESCORT Near Unity Divider (NUD) Phase Locked Loop IC.
Figure 4 Frequency Generating - Block Diagram
Buffer
Frequency
Translation
PLL
(ESCORT)
Loop
Filter
TX
VCO
Buffer
Tx Injection
to LNODCT
To Rx
1
st
LO
Buffer
Main
VCO
Loop
Filter
Phase
Detector
Reference
Divider
Buffer
Warp DAC
16.8 MHz
TCXO
Buffer
16.8 MHz Ref. to
Controller
Prescaler
LVFRACN PLL
16.8 MHz Ref.
to IF Digitizing
Subsystem
VCO BIC