FP520 Service Manual
Page 9 of 64
comparison with reference frequency after being divided by the programmable frequency divider, and the error
signal is then generated to change the oscillation frequency of VCO after filtered by the low pass filter. VCO is
locked when the frequency of VCO reaches a certain set value.
Loss of Lock Detection: When PLL loses its lock, IC300 pin14 outputs the electric level signal to MCU, and MCU
control transmitter forbids transmitting and issues an alert tone. The output voltage of IC300 pin14 is high electric
level when it is locked.
3.6 Audio Processing Circuit
FM receiver
FM modulator
PA
TR
SW
Host MCU
4FSK
Protocol
stack
Voice
coder
U
6
C
O
D
E
C
L
U
6
C
O
D
E
C
R
ANT
SCT3252
U1
IC2
SPK
MIC
MIC Signal Processing:
The speech signal from MIC is sent to U6 for A/D switch, and sent to U1 SCT3252 for audio coding/decoding,
communication protocol processing, channel coding modulation. Afterwards, the signal outputs MOD2 and MOD1
separately from U6, and then sends them to TCXO and VCO for two-point modulation. The signal outputs 4FSK
modulating signal in digital state, and sine wave signal in analog state.
Receiving Audio Signal Processing:
The audio signal demodulated from IC700 is sent to U1 SCT3252 for processing after finishing A/D switch by U6.
After the audio encoding/decoding, communication protocol processing and DSP, the audio signal is sent to U6 for
D/A switch, and it is then output to the U800 (TDA2822) audio power amplifier for amplification so as to sound the
speaker.
Squelch Circuit
：
It is output from IC700 after demodulation, and it filters out the noise from the demodulated signal
after going through the filter circuit. It is sent to MCU after being detected by D701 and amplified by Q700. The MCU
identifies the noise level and controls the squelch.
Speaker Impedance: 16
Ω
Note: 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 7.4V battery. The transmitter power amplifier circuit (Q601, Q600) and receiver audio processor
(U800) directly use the battery for power supply with other circuits using the voltage stabilized 5V for power supply.
Q102
：
5T switch
，
controlled by MCU.
5T: Supplies power for the transmitter front end.
Q100: 5R switch
，
controlled by MCU.
5R: Supplies power for the receiver RF amplifier, frequency mixer, IF processor, audio signal processor and the like.
Summary of Contents for FP520
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Page 44: ...FP520 Service Manual Page 43 of 64 Figure 1 FP520 01 Mainboard Schematic Diagram 136 174MHz ...
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Page 54: ...FP520 Service Manual Page 53 of 64 Figure 4 FP520 02 Mainboard Schematic Diagram 400 470MHz ...
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Page 64: ...FP520 Service Manual Page 63 of 64 Figure 7 KBC 51 7 4V Electrical Schematic Diagram ...
