Transmitter Circuits - Icom IC-F50V Service Manual

SQUELCH CIRCUITS
• NOISE SQUELCH
The noise squelch cuts off the AF output signals when
no RF signals are received. Extracting noise components
(approx. 30 kHz signals) in the demodulated AF signals,
the squelch circuit turns the AF power amplifier and AF
switches ON and OFF.
A portion of FM-demodulated AF signal from the IF IC is
adjusted its level (=squelch threshold level) by DAC (D/A
converter; IC6, pins 1, 2), then passed through the noise
filter (M: IC1, pins 7, 8 and R42, 44–46, C68–70) to extract
the noise components (approx. 30 kHz signals) only. The
noise components are rectified to be converted into the
pulse-type signal by noise detector to produce DC voltage
corresponding to the noise level "NOIS". Then the DC
voltage is applied to the CPU (IC401, pin 41) and compared
with the reference level preset in the CPU.
If the CPU interpretes that the noise level is higher than
preset one, the CPU sets the "AFON" signal to "High" to
turn the AF power AMP controller OFF, and the AF mute
SW is turned OFF simultaneously. Thus closing the squelch
is accomplished.
NOISE SQUELCH DIAGRAM
To RX AF circuits
Fc=30 kHz
1 2
DAC
Active filter
From IF IC (Pin16)

4-2 TRANSMITTER CIRCUITS

TX AF CIRCUITS
The TX AF circuits consist of microphone amplifier (MIC
AMP) and AF fi lters. The AF fi lter cuts off the signals except
voice signals (300 Hz or lower and 3 kHz or higher).
The audio signals from the internal microphone (MIC
signals) are passed through MIC mute SW and amplifi ed by
two MIC AMPs (#1 and #2).
• TX AF CIRCUITS
AUDIO
LEVEL
ADJUST
J402
W601
1
4
2
3
3
2
4
1
VR UNIT
SP401 W401
Rectify
Noise
AMP
Noise detector
MC401
R541
R434
C437 C425
R432
Q413
14
IC403
LPF
C496
C435
W402 R517
C502 R521
R518
C503 R522
• TONE SQUELCH
The tone squelch connects the RX AF line and activates
the AF power amplifier to emit the AF signals only when
receiving a signal which contains the tone frequency
matched with preset in the CPU. Detecting signal in the
demodulated AF signals, the tone squelch circuit turns the
AF power amplifier and AF switches ON and OFF.
<CTCSS/DTCS>
A portion of FM-demodulated AF signals from the FM IF
IC are passed through the tone filter (M: Q6, pins 5, 6) to
remove unwanted voice siganls. The filtered tone signals
are applied to the CPU (IC401, pin 44).
<2/5 TONE, DTMF>
A portion of FM-demodulated AF signals are output from
the baseband IC (M: IC10, pin 21), and passed through
the tone filter (M: R136, C220) to remove unwanted voice
siganls. The filtered tone signals are applied to the CPU
(IC401, pin 43).
The CPU compars the tone frequency/code preset in the
CPU, and if the applied tone frequency/code is matched to
preset one, the CPU controls the AF power AMP and AF
mute SW as same as "NOISE SQUELCH."
"NOIS"
The MIC signals from the external microphone "EXMIC" are
applied to the FRONT UNIT via [SP MIC] jack, and amplifi ed
by MIC AMP (#2).
The amplified MIC signals "MIC" are applied to the MAIN
UNIT via J401(FRONT UNIT) and J1 (MAIN UNIT).
The MIC signals from the FRONT UNIT are applied to the
baseband IC (IC10, pin 3) and processed.
MIC/AF
MUTE SW
MIC AMP
#1
R513
C497 C498
R511
IC407
2
R408
3
13
12
R426
R433
R535
C513 C512
R537
Q415
R428
AF POWER AMP
I C405
20 1
VCC1 GND1
19 2
NC NC
18 3
OUT1- OUT1 +
17 4
IN1- MODE
16 5
I N1+ SVRR
15 6
I N2+ SELECT
14 7
IN2- NC
13 8
OUT2- OUT2 +
12 9
NC NC
11 10
VCC2 GND2
R523
R539 Q406
4 - 3
AFON
MIC AMP
R406
C406
#2
C407
R407
R405
I C407
C405
6
1 7
5
R512
DUSE
BEEP
MIC
SIGNAL
C421
AFON
VCC
Q401, Q402
AF POWER AMP
CONTROLLER
SPON