Icom IC-F50V Service Manual - page 10
4 - 4
CTCSS/DTCS signals ("CEN0"–"CEN2") are generated by
the CPU (F: IC401, pins 79–81) and converted its wave
form by R461–R463 (F), then passed through the LPF (F:
IC403, pins 3, 1). The fi ltered CTCSS/DTCS signals are
then level-adjusted by DAC (M: IC6, pins 9, 10) and applied
to the AF mixer (M: IC5, pin 2) to be mixed with MIC signals,
then applied to the TX VCO as the modulation signals. The
CTCSS/DTCS signals are also applied to the reference
signal oscillator (M; X2) too, via the buffer (M: IC16, pins 2, 1).
2/5 tone and DTMF signals are ("SEN0"–"SEN3") are
generated by the CPU (F: IC401, pins 72–75) and converted
its wave form by R471–R474 (F), then passed through
the two-staged LPF (F: IC403, pins 5, 7 and pins 10, 8).
The filtered 2/5 tone and DTMF signals are applied to the
AF mixer (M: IC5, pin 2) to be mixed with MIC signals.
The mixed signals are then applied to the TX VCO as the
modulation signals). The CTCSS/DTCS signals are also
applied to the reference signal oscillator (M: X2, pin 1) too,
via the buffer (M: IC16, pins 2, 1).
The modulated VCO output is buffer-amplifi ed by two buffers
(Q12, Q10), and applied to the pre-driver via TX/RX SW.
C126
R65
C107
R87
C99
0.001
C93
TX
C129
C190
C122
L21
C195
C110
L41
C103
L42
L27
Q12
R82
from VCO
C191
C127
R72
C113
D1
6
C194
D21
L38
C193
C118
C148
C95
1stLO
To PLL
R84
C102
R71
C124
C108
0.01
C104
R69
C114
R81
R98
L43
R88
L26
R67
D17
EP1
R76
Q15
1
2
3
4
5
Q11
C137
C205
L25
R7
5
C105
C117
D15
C111
C120
R86
C121
Q14
D1
9
Q10
C100
LV
C109
R68
L39
R77
L24
Q17
D18
Q16
D14
D2
0
C133
C203
L37
R80
R85
C116
L28
R79
C139
C192
C119
D22
C123
R83
C134
C115
R78
Q13
R70
LV
A
RX VCO
TX VCO
R5V
VCC
T5V
MOD
BUFFER
BUFFER
BUFFER
RIPPLE
FILTER
VCO
SW
TX/RX
SW
Freq.
adjust
Freq.
adjust
Modulation
L14
C324
C183
C323
C49
LPF
• MODULATION CIRCUIT AND TX/RX VCOs
The applied MIC signals are amplified at the amplifier
(TXA1), and level adjusted at the volume controller (VR1).
The level adjusted MIC signals are applied (bypassed) the
compressor section, pre-emphasis section, TX/RX HPF,
de-scrambler, limiter, splatter, in sequence, then applied to
another volume controller.
The compressor compresses the MIC signals to provide
high S/N ratio for receive side, and the pre-emphasis obtains
+6 dB/oct audio characteristics. The TX/RX HPF fi lters out
250 Hz and lower audio signals, the limiter limits its level and
the splatter fi lters out 3 kHz and higher audio signals. The
filtered MIC signals are level adjusted at another volume
controller (VR2), and then output from pin 7 via smoothing
fi lter (SMF).
The output MIC signals are passed through the FM/PM
SW (IC11, pins 6, 1) and LPF (IC5, pins 2, 1) then applied
to the DAC (D/A Converter; IC6, pin 4) and level-adjusted
(deviation adjustment). The level-adjusted MIC siganls are
output from pin 3, then applied to the modulation circuit as
the modulation signals "MOD” via buffer (IC5, pins 6, 7).
MODULATION CIRCUIT
The modulation circuit FM-modulates the VCO oscillating
signal with the modulation signals from the TX AF circuits.
• VOICE SIGNAL
The buffer-amplified AF signals are applied to the variable
capacitor (VD) to change its reactance for FM modulation.
• TONE SIGNALS
Tone signals are generated in the CPU and applied to the
both of the VCO and reference frequency oscillator to be
modulated.