Transmitter Circuits - Icom IC-F1721/D Service Manual

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT (MAIN UNIT)
The microphone amplifier circuit amplifies audio signals
within +6 dB/octave pre-emphasis 6characteristics from the
microphone to a level needed for the modulation circuit.
This transceiver employs the base band IC which is com-
posed of microphone amplifier, compressor, scrambler,
limiter, splatter filter, MSK modulator, etc. at the microphone
amplifier section.
The AF signals (MIC) from the microphone connector
(FRONT UNIT; J2, pin 6) are passed through the micro-
phone switch (FRONT UNIT; IC8, pins 1, 6, 7) and are then
applied (or bypass) to the ALC amplifier (FRONT UNIT; IC9)
for digital modulation. The signals are amplified at the micro-
phone amplifier (FRONT UNIT; IC5, pins 3, 4) and then
applied to MAIN UNIT via J1 (pin 10).
The amplified signals are applied to the microphone ampli-
fier section of the base band IC (IC2, pin 3). The amplified
signals are passed through (or bypass) the compressor,
scrambler sections of IC2, and are then passed through the
high-pass, limiter amplifier, splatter filter sections of IC2.
The output signals from the base band IC (IC2, pin 7) are
applied to the FM/PM switch (IC43, pins 12–14) after pass
through the DA switch (IC42, pins 12, 14). The signal are
passed through the low-pass filter (IC1, pins 4, 13) and then
applied to the D/A converter (IC8, pins 3, 4). The output sig-
nal from D/A converter (IC8, pin 3) are applied to the modu-
lation circuit (D12).
4-2-2 MODULATION CIRCUIT (MAIN UNIT)
The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signals.
The AF signals from the D/A converter (IC8, pin 3) change
the reactance of varactor diode (D12) to modulate the oscil-
lated signal at the TX VCO circuit (Q12, D8, D9). The modu-
lated VCO signal is amplified at the buffer amplifiers (Q20,
Q22) and is then applied to the YGR amplifier circuit via the
T/R switch (D19).
The CTCSS/DTCS signals ("CENC0", "CENC1", "CENC2")
from the MAIN CPU (IC23, pins 89–91) are combined at
resistors (R158, R159, R162) and are then pass through
the low-pass filter (IC9, pins 8, 10). The filtered signals are
applied to the D/A converter (IC8, pins 9, 10), and are then
mixed with the filtered microphone audio signals.
• APC CIRCUIT
VCC
T8V
RF signal
from PLL circuit
T2
TMUT
YGR
amp.
–
IC37
APC
amp.
+
APC control circuit
The mixed signals are applied to the D/A converter (IC8,
pin 3, 4) after pass through the low-pass filter (IC1, pins 4,
13). The output signal from D/A converter (IC8, pin 3) are
applied to the D12 in the VCO circuit.
4-2-3 YGR/POWER AMPLIFIER CIRCUITS (MAIN UNIT)
The YGR/power amplifier circuits amplify the VCO oscillating
signal to an output power level.
The signal from the VCO circuit passes through the T/R
switch (D19), and is amplified at the YGR (Q23), power (IC29)
amplifiers to obtain 50 W (IC-F1721/D/F1821/D; 25 W for
IC-F1710/F1810) of RF power.
The amplified signal is passed through the low-pass filter
(L35, C345, C346, C348, C349), antenna switching circuit
(D29, D30), low-pass filters (L42, L43, C375, C388, C389,
C391), power detector (D38, D40), low-pass filter (L44,
C432, C461), and is then applied to the antenna connector
(CHASSIS unit; J1).
The bias voltage of the YGR amplifier (Q23) and power
amplifier (IC29) are controlled by the APC circuit.
4-2-4 APC CIRCUIT (MAIN UNIT)
The APC circuit protects the power amplifier from a mis-
matched output load and stabilizes the output power.
The power detector circuit (D38, D40) detects the forward
signals and reflection signals and converts it into DC volt-
age. The output voltage is at a minimum level when the
antenna impedance is matched with 50 Ω and is increased
when it is mismatched.
The detected voltage is applied to the differential amplifier
(IC37; pins 3, 4), and the "T2" signal from the D/A converter
(IC30, pin 2), controlled by the MAIN CPU (IC23), is applied
to the other input for reference (IC37, pin 1). When antenna
impedance is mismatched, the detected voltage exceeds the
power setting voltage. Then the output voltage of the differ-
ential amplifier (IC37, pin 4) controls the bias voltage of the
YGR amplifier (Q23) and power amplifier (IC29) amplifiers to
reduce the output power.
Q23
IC29
Power
amp.
4 - 3
FOR
to ANT unit
REV
LPF
D40
D38