4.1 - 6 MTM700 Mobile Radio / Detailed Service Manual THEORY OF OPERATION (400 MHz)
The receive pre-selector filter performs the following functions:
• passes frequencies in the 380 – 430 MHz receive frequency range, and
• blocks unwanted spurious frequencies like half IF and image frequencies.
The signal is mixed with the low side injection local oscillator signal to create the first IF at 73.35
MHz. The IF signal is filtered by the crystal filter, amplified by the IF amplifier, and sent to the IF Dig-
itizing IC.
The IF Digitizing Subsystem performs the following functions:
• down conversion to the second IF at 2.25 MHz,
• perform a bandpass sigma-delta analog-to-digital conversion of the second IF signal into in-
phase (I) and quadrature (Q) digital signals.
• Formats the I and Q data streams into a serial bit stream, which is sent for further processing
to the Digital Signal Processor (DSP) (part of the micro controller) over the Synchronous
Serial Interface (SSI) data link.
The DSP performs the following functions:
• Base band filtering,
• Synchronisation
• Demodulation
• Forward Error Correction (FEC)
• other correction algorithms for overcoming channel errors, and
• the decoder procedure for digital speech data decompression.
Transmitter Path
When the radio is transmitting (see figure below), microphone audio is sent to the CODEC, which
performs analog-to-digital conversion and routes the digital signal to the DSP. The DSP performs
the following functions:
•coding,
• error correction, and
• generation of the baseband modulation waveform.
From the DSP, baseband digital ‘I’ and ‘Q’ modulation waveform samples are sent to the ADDAG IC,
where the digital data is converted into baseband analog ‘I’ and ‘Q’ waveforms and filtered.
Figure 3 Transmit Path Block Diagram
ADDAG
LNODCT
Balun and
Attenuator
DSP
CODEC
Attenuator
Coupler
Antenna
Switch
To
Receiver
Harmonic
Filter
Tx
Rx
Isolator
(Depends
on Model)
RF Feedback
Pre-driver
PA
Mic
Driver
90˚ Phase
Splitter
Power
Splitter
Buffer
Tx LO
Injection
from FGU
Down-
converter
LO
Up-converter LO
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