Transmitter Circuits - Icom IC-208H Service Manual

The signal from the AM detector circuit (Q13) is detected at
the AGC detector circuit (D19–D21). When receiving strong
signals, the detected voltage increases and the AGC voltage
decreases via the AGC circuit. The AGC voltage is used for
the bias voltage of the receive switching PIN diodes to atten-
uate the received signals. Therefore, this transceiver keeps
the audio output at a constant level.
4-1-11 S-METER CIRCUIT (MAIN UNIT)
The S-meter circuit indicates the relative received signal
strength while receiving by utilizing the AGC voltage which
changes depending on the received signal strength.
The FM S-meter signal from the FM IF IC (IC8, pin 12) is
applied to the main CPU (IC505, pin 40) as an S-meter sig-
nal via the "RSSI" signal line.
The S-meter signal is applied to the sub CPU and is then
displayed on the LCD.

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT
(MAIN UNIT)
The microphone amplifier circuit amplifies audio signals
from the microphone to a level needed at the modulation cir-
cuit. The microphone amplifier circuit is commonly used for
both the VHF and UHF bands.
• AUDIO SIGNALS
The AF signals from the microphone are amplified at the
microphone amplifier (Q505, IC506), and are then applied to
the analog switch (IC509, pin 2). The signals from analog
switch (pin 1) are applied to the IDC amplifier (IC3A, pin 3)
via the "MODIN" line, and then pass through the low-pass fil-
ter (IC3D, pins 13, 14) to suppress unwanted components.
IC3A composes limiter amplifier and pre-emphasis circuits.
• TRANSMITTER CIRCUITS
IC506,
IC509
Q505
Analog
Amp
MIC
SW
Antenna
X1
15.2 MHz
Buff
PLL IC
IC3A IC3D IC3C
IDC LPF
Buff
TX/RX
LPF HPF
SW
TX/RX
LPF
SW
The signals are amplified at the buffer amplifier (IC3C, pins
9, 8), and are then applied to the D/A converter IC (IC4, pin
13) to adjust modulation level. The signals from the D/A con-
vertor (IC4, pin 23) pass through the reference controller
(D1, X1), and are then applied to the VHF or UHF modula-
tion circuits separately.
• DATA SIGNALS
The data signals from the 6-pins jack (J504, pin 1) pass
through the limiter circuit (D524) to limit the level.
When setting to 9600 bps mode, The signals pass through
the analog switch (IC509, pins 4, 3), and then pass through
the switch again (pins 9, 8). The signals are amplified at the
buffer amplifier (IC3c, pins 9, 8) via the "DATAMOD" line,
and are then applied to the same line as audio signals
(applied to the D/A converter IC's pin 13).
When setting to 1200 bps mode, The signals pass through
the analog switch (IC509, pins 4, 3), and then pass through
the switch again (pins 10, 11). The signals are then applied
to the same line as "MODIN" (applied to the IDC amplifier;
IC3A, pin 3).
4-2-2 VHF MODULATION CIRCUIT
(MAIN UNIT AND VCO BOARD)
The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signals.
The audio or data signals from the reference controller (D1,
X1) are amplified at the buffer amplifier (Q2), and then pass
through the VHF PLL IC (IC1, pins 1,15) and VHF loop filter
(IC6, Q9, Q10, D4). The filtered signals are applied to the V-
VCO circuit (VCO board).
The signals change the reactance of D1 (VCO board) to
modulate the oscillated signal at the V-VCO circuit (VCO
board; Q3, D1, D2). The modulated signals are amplified at
the buffer amplifiers (VCO board; Q4, Q5), and then passed
through the Tx/Rx switching circuit (VCO board; D5) and
low-pass filter (L6, C32, C33). The filtered signals are
applied to the drive amplifier circuit on the MAIN unit.
IC1
Q2
Loop
PLL IC
filter
IC2
Loop
filter
IC4
D/A
Buffs
IC14
SWR
PWR
LPF
DET
IC13
SWR
PWR
LPF
DET
4 - 4
VCO
Buffs
LPF
SW
Q102,
Q103
VCO
LPF
SW
Q34 Q31
Pre
Drive
drive
UHF TX CIRCUIT
Q33 Q30
Pre
Drive
drive
HPF
ATT
ATT
VHF TX CIRCUIT