Motorola MTM700

4.2 - 6 MTM700 Mobile Radio / Detailed Service Manual THEORY OF OPERATION (800 MHz)
The receive pre-selector filter performs the following functions:
• passes frequencies in the 851 – 870 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,
• bandpass sigma-delta analog-to-digital conversion of the second IF signal into in-phase (I)
and quadrature (Q) digital signals.
• formatting of 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 microcontroller) over the
Synchronous Serial Interface (SSI) data link.
The DSP performs the following functions:
• 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 Transmiter Path Block Diagram
ADDAG
LNODCT
Attenuator
DSP
CODE
Attenuator
Coupler
Tx LO
Injection
from FGU
Antenna
Switch
To
Receiver
Har-
monic Fil-
Tx
Rx
Isolator
RF Feedback
Pre-
driver
Driver PA
Mic
Buffer
Power
Splitter
90°
Phase
Down-
convert-
Up-converter LO