FM540 Service Manual
3.6 Audio Processing Circuit
Figure 3-5 Frequency Synthesis Diagram
•
MIC Signal Processing
The audio signal from MIC is sent to U6 for A/D audio switch and sent to U1 SCT3252 for internal adio
encoding/decoding, communication protocol processing, channel coding modulation. Afterwards, MOD2,
MOD1 are output through two separate paths from U6 and sent separately to TCXO and VCO for two-point
modulation. 4FSK modulation signal is output in digital status and sine wave signal is output in analog
status.
•
RX Audio Signal Processing
The audio signal demodulated from IC700 is sent to U6 for A/D audio switch and then be processed by U1
SCT3252. After audio processing, communication protocol processing and DSP, the signal is sent to U6 for
D/A switch and output to audio power deviceU800(TDA2822) for amplification, motivating the speaker to
sound.
Squelch Circuit: demodulated and output byIC700; it passes filter circuit and the noise will be removed from
demodulated signal and amplified by Q700; after detection by D701, it will be sent to MCU. MCU identifies
the noise volume and controls the squelch.
Speaker Impedance: 16
Ω.
Notes:
•
Any terminal of the speaker must not be attached to the ground!
•
The emergency alert tone is not controlled by volume.
3.7 Power Supply
This radio applies 13.8V battery. The transmitter power amplifier circuit (IC1) and the receiver audio
processor (IC7) directly use the battery for power supply with other circuits using 5V for power supply.
Q38: 8T switch; controlled by MCU. 8T supplies power for the transmitter front end.
Q40: 8R switch; controlled by MCU. 8R supplies power for receiver RF amplifier, mixer, IF processor, and
other units.
FM receiver
FM modulator
PA
TR
SW
Host MCU
4FSK
Protocol
stack
Voice
coder
U6 CODECL
U6 CODECR
ANT
SCT3252
U1
IC2
SPK
MIC
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Summary of Contents for FM540
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