Transmitter Circuits - Icom IC-2200H Service Manual

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT
(LOGIC unit)
The microphone amplifier circuit amplifies audio signals with
+6 dB/octave pre-emphasis characteristics from the micro-
phone to a level needed for the modulation circuit.
The AF signals from the microphone pass through the
microphone switch (IC2, D4), and are then amplified at the
microphone amplifier (IC3a ,pin 2). The amplified signals
pass through (or bypass) the ALC circuit.
4-2-2 ALC CIRCUIT (LOGIC UNIT)
The ALC (Automatic Level Control) circuit reduces the gain
of IF amplifiers in order for the transceiver to output a con-
stant RF power set by the RF power setting even when the
supplied voltage shifts, etc.
• IN CASE OF PASSING THROUGH THE ALC CIRCUIT
The amplified signals are applied to the ALC switch (IC520,
pin 3), and are then output from pin 4. The signals are
applied to the ALC amplifier (IC514, pin 3), and are then
reapplied to the ALC switch (IC520, pin 8). The signals pass
through the high-pass filter (IC3d, pins 12, 14), and are then
applied to the analog switch (IC6, pin 4).
• IN CASE OF BYPASSING THE ALC CIRCUIT
The amplified signals are applied to the ALC switch (IC520,
pin 2), and are then output from pin 1. The signals are
applied to the ALC switch (IC520, pin 9) again, and then
pass through the high-pass filter (IC3d, pins 12, 14) via the
IC520, pin 10. The signals are applied to the analog switch
(IC6, pin 4).
4-2-3 DIGITAL SWITCH CIRCUIT (LOGIC UNIT)
The signals from the analog switch (IC6, pin 3) pass through
the high-pass filter (IC3, pins 6, 7), and are then reapplied to
the analog switch (IC6, pin 9). The signals are applied to the
DI/AN switch via the "MOD_I" line.
• IN CASE OF DIGITAL AF SIGNALS
The AF signals are applied to the DI/AN switch (IC522, pin
10), and are then applied to the optional digital voice unit
after being passed through the J4 (pin 4) via the "ANA-
LOGUE_MOD_IN" line. The A/D converted AF signals are
applied to the DI/AN switch (IC522, pin 1) from the J4 (pin 3)
via the "DIGITAL_MOD_OUT" line. The digital AF signals
from DI/AN switch (IC522, pin 2) are applied to the modula-
tion circuit after being passed through the J3 via the
"MODIN" line.
• ALC CIRCUIT
MIC
switch
Microphone
IC2, D4
"ALC" from the CPU
(MAIN unit; IC1, pin 81)
"THROUGH" from the CPU
(MAIN unit; IC1, pin 80)
"THROUGH" from the CPU
(MAIN unit; IC1, pin 80)
MIC
IC520
ALC
IC3A
switch
Power
supply
• IN CASE OF ANALOG AF SIGNALS
The AF signals are applied to the DI/AN switch (IC522, pin
9), and are then reapplied to the switch (pin 4) from pin
8.The signals output from the switch (pin 3) as analog AF
signals , and are then applied to the modulation circuit after
being passed through the J3 via the "MODIN" line.
4-2-4 MODULATION CIRCUIT (MAIN UNIT)
The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signals.
The digital or analog audio signals (MODIN) change the
reactance of D2 and D4 to modulate the oscillated signal at
the TX-VCO circuit (Q6). The modulated signal is amplified
at the LO amplifier (Q9) and buffer amplifier (Q11), then
applied to the drive amplifiers.
4-2-5 DRIVE AMPLIFIER CIRCUIT (MAIN UNIT)
The drive amplifier circuit amplifies the VCO oscillating sig-
nal to the level needed at the power amplifier.
The RF signal from the buffer amplifier (Q11) passes
through the T/R switch (D6) and is amplified at the pre-drive
(Q17) and drive (Q35) amplifiers. The amplified signal is
applied to the power amplifier circuit.
4-2-6 POWER AMPLIFIER CIRCUIT (MAIN UNIT)
The power amplifier circuit amplifies the driver signal to an
output power level.
The RF signal from the drive amplifier (Q35) is applied to the
power module (IC10) to obtain 65 W (50 W for Korea ver-
sion, 25 W for Taiwan version) of RF power.
The amplified signals is passed through the antenna switch-
ing circuit (D12), APC detector circuit (D14, D17, L39), and
low-pass filter (L47, L48, C208, C210, C217) and is then
applied to the antenna connector.
Control voltage for the power amplifier (IC10, pin 2) are con-
trolled by the APC circuit to protect the power module from
a mismatched condition as well as to stabilize the output
power.
IC520
Through
switch
IC520
IC514
ALC
ALC
switch
Q524
4 - 3
to the modulator circuit
Switch
IC525