HYT TC-618, Service Manual

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TC-618/626 Service Manual
15
2 ） Block diagram of Rx audio & signaling processing circuit
U201
AF
AMP
16 Ω IW
RX
1
sp Amp
SF
AMP(TX)
V
O
X
T
o
n
e
AGC
tone-LPF
DET
B
G
R
EN
RSSI
DATA
Mic amp
VCO
0
U200
AN29160AA
PROCESSER
IFAMP2
5th-LPF
CLK
AF
HPF
PE
5th-HPF
signalling output
IFAmp1
SW
To MCU
IC
CF302
/4.5K
OUT IN
VREF
TX
De
Emphasis
LATCH
AF
mute
SPVCC
volume
To MCU
CDC/CTC
MCU
POWERREG
mix2
mic mute
LIM
(TX)
VCC
SQ
AUDIO OUT
Figure 7
After demodulating the received signal, U200 outputs initial audio signal containing noise, signaling and
voice components. Therefore, audio signal is processed via three flows.
1) Rx Voice Process: One flow of audio signals from U200 goes back to U200 after passing through the
RC low-pass filter and de-emphasis circuit. This flow of audio signals are amplified at U200, and
restored after frequencies below 300Hz are removed. Then the restored audio signals are adjusted
by the potentiometer and feed to the audio power amplifier U201, which performs power
amplification to drive the speaker directly. To obtain larger power, the BTL bridge dual-end output is
adopted.
2) Rx Signaling Process: One flow of audio signals from U200 goes to the 300Hz low-pass filter circuit
(U640). After audio signals over 300Hz are removed, signaling signals (CTCSS or CDCSS) go to the
QTIN pin of CPU, and then are decoded by CPU.
3) Noise Signal Process: One flow of audio signals from U200 goes back to U200. After they are
filtered, amplified and rectified at U200, a DC voltage signal (SQ) corresponding to the noise
component is derived and is sent to the BUSY pin of MCU via the ND pin of U200. Then MCU
processes the input signal.