Transmit Circuits - Icom IC-M400BB Service Manual

4-2 TRANSMIT CIRCUITS

TX AF CIRCUIT (MAIN UNIT)
The AF signal from the COMMANDMICIV™ is passed
through the MIC mute SWs (IC502C, pins 9, 8 and IC502B,
pins 4, 3), and then applied to the MIC AMP (IC513D).
• MIC AMP CIRCUITS
IC502C
AF
MUTE
COMMANDMIC
TX AF CIRCUIT (MAIN UNIT)
The amplifi ed AF signal is passed through the AF SW (IC901,
pins 6, 1) and applied to the noise canceller IC (IC902,
pin 36).
The processed AF signal is output from pin 26, and then
passed through the LPF (IC905, pins 1, 4) and AF SW
(IC904, pins 1, 6), and then applied to the D/A converter
(IC514, pins 13, 14) to be adjusted in level.
The level-adjusted AF signal is applied to the IDC AMP
(IC513B) which limits the amplitude of MIC signal to prevent
over deviation. The amplitude-limited signal is passed through
the LPF (IC513C), and then applied to the modulation circuits
on the VCO UNIT.
• TX AF CIRCUIT
IC902
IC901 IC905
From the
MIC AMP circuits
AF NOISE
LPF
CANCELLER
SW
From the
demodulation
MODULATION CIRCUIT (VCO UNIT)
The MIC signal from the TX AF circuits is applied to the VCO
(Q3, Q4 and D1–D3). The modulation signal is applied to D2,
to obtain Frequency Modulation.
The modulated VCO output signal is passed through the buf-
fers (Q5 and Q7), and then applied to the TX AMP circuits as
a TX signal, through the LO SW (MAIN UNIT: D7).
• MODULATION CIRCUIT
VCO UNIT
Q2
Q8
Q3,Q4
D1,D2
D 3 Q5
From the
TX AF circuits
VCO
SHIFT BUFF
IC513D
IC502B
MIC To the TX AF circuits
AF
AMP
MUTE
MAIN UNIT
IC514
IC904 To the
IC513B IC513C
modulation
D/A
AF
circuits
IDC
LPF
SW
To the
RX AF circuits
D7
Q7
To the
TX AMP circuits
LO
BUFF
SW
TX AMP CIRCUIT (MAIN UNIT)
The TX signal is amplifi ed by the YGR (Q13) and power AMP
(IC1), and then passed through the LPF (L20, C111 and
C112), TX output power detector (D12 and D13), ANT SW
(D14), and two LPF (L21, L22, and C114–C116), before be-
ing sent to the antenna.
APC CIRCUIT (MAIN UNIT)
The voltage produced at the LPF (L20, C111, and C112) is
rectifi ed by D12 and D13, and is used as the TX power sens-
ing voltage.
The voltage is applied to the APC AMP (IC2, pin 3), and the
output voltage controls the gate bias voltage of power AMP
(IC1) to keep the TX output power constant.
• TX AMP CIRCUIT
Q13
From the modulation
circuits
YGR
AMP
4-3
ANT
IC1
D14,D16
PWR ANT
LPF LPF
AMP SW
D12,
PWR
D13
DET
IC2
RX circuits
APC
CTRL