Service Manual

14
Block Diagram for Rx Audio&Signalling Process
RX
sp Amp
2ndIF BPF
SF
AMP(TX)
VOX
Tone
tone-LPF
DET
BGR
EN
RSSI
Mic amp
VCO
DATA
0
5th-LPF
IFAMP2
AF
HPF
PE
信令输出
5th-HPF
IFAmp1
SW
To MCU
IC
CF300
450KHz+/4.5K
OUT IN
VREF
TX
De
Emphasis
LATCH
AF
mute
SPVCC
volume
To MCU
CDC/CTC
MCU
mix2
mic mute
POWERREG
VCC
LIM
(TX)
SQ
AUDIO OUT
16Ω IW
U201
AF
AMP
U200
AN29160AA
PROCESSER
Figure 8
U200 outputs initial audio signals (including noise, signaling, audio, etc) after performing
frequency-discrimination and demodulation on the received signals. Therefore, the audio process
is divided into three parts:
Rx Audio Process: One part of the audio signal output by U200 is fed to the RC low-pass filter and
de-emphasis circuit and then amplified by U200. The audio signal is recovered after frequencies
below 300Hz are removed. The recovered audio signal is adjusted by the potentiometer and then
goes to audio power amplifier IC (U201), which amplifies power for the input audio signals to drive
the speaker directly. To obtain higher power, the BTL bridge dual output is utilized.
Rx Signalling Process: One part of the audio signal output by U200 is fed to the 300Hz low-pass
filter circuit (U640). After audio signals above 300Hz are removed, CTCSS/CDCSS goes to the
QTIN pin of CPU. CPU decodes the input signallings.
Noise Signalling Process: One part of the audio signal output by U200 goes to U200 again. After
the signal is filtered, amplified and rectified inside U200, a DC voltage signal (SQ) corresponding to
the noise component is derived. The DC signal is fed to the BUSY pin of MCU via the ND pin of
U200. Then MCU processes the input signals.