Transmitter Circuits - Icom IC-F3 Service Manual

The applied AF signals are amplified at the AF power ampli-
fier circuit (IC5, pin 4) to obtain the specified audio level. The
amplified AF signals, output from pin 10, are applied to the
internal speaker (SP1) via the [SP] jack when no plug is con-
nected to the jack.
4-1-6 SQUELCH CIRCUIT
A squelch circuit cuts out AF signals when no RF signals are
received. By detecting noise components in the AF signals,
the squelch switches the AF mute switch.
A portion of the AF signals from the FM IF IC (IC2, pin 9) are
applied to the active filter section (IC2, pin 8) where noise
components are amplified and detected with an internal
noise detector. The squelch level adjustment pot (R92) is
connected in parallel to the active filter input (pin 8) to con-
trol the input noise level.
The active filter section amplifies noise components. The fil-
tered signals are rectified at the noise detector section and
converted into "NOIS" (pulse type) signals at the noise com-
parator section. The "NOIS" signal is applied to the CPU
(IC8, pin 19).
The CPU detects the receiving signal strength from the
number of the pulses, and outputs an "RM" signal from pin
43. This signal controls the mute switch (IC4) to cut the AF
signal line.

4-2 TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT
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 are applied to the
microphone amplifier circuit (IC3c, pin 10). The amplified AF
signals are passed through the low-pass filter circuit (IC3d,
pins 13, 14) via the mute switch (IC4, pins 2–4). The filtered
AF signals are applied to the modulator circuit after being
passed through the mute switch (IC4, pins 8–10) and the
deviation adjustment pot (R119; IC-F4/S only).
• APC circuit
Current sensor circuit
VCC
T5
Q28
RF signal
from PLL
IC-F3/S only
"ISENS" signal
S5
to the CPU
TXC
Q37
T4
APC control circuit
4-2-2 MODULATION CIRCUIT
The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signal.
The audio signals change the reactance of a diode (D6 for
IC-F3/S, D404 for IC-F4/S) to modulate an oscillated signal
at the VCO circuit (Q7, Q8). The oscillated signal is ampli-
fied at the buffer-amplifiers (Q4, Q6), then applied to the T/R
switching circuit (D3, D4).
4-2-3 DRIVE/POWER AMPLIFIER CIRCUITS
The signal from the VCO circuit passes through the T/R
switching circuit (D3) and is amplified at the buffer(s) (Q3 for
IC-F3/S, Q3, Q403 for IC-F4/S), drive (Q2) and power ampli-
fier (Q1) to obtain 5 W (IC-F3/S)/4 W (IC-F4/S) of RF power
(at 9.6 V DC). The amplified signal passes through the
antenna switching circuit (D1), and low-pass filter and is
then applied to the antenna connector.
The bias current of the drive (Q2) and the power amplifier
(Q1) is controlled by the APC circuit.
4-2-4 CURRENT DETECTOR CIRCUIT
The current detector circuit (Q9, Q28) detects the total dri-
ving current of the drive and the power amplifiers, using the
current sensor (R161). The differential amplifier (Q9) detects
the voltage difference of the current sensor input and output
voltages, then outputs control voltage to the APC circuit (IC-
F3/S only) and the CPU (IC8, pin 97).
4-2-5 POWER DETECTOR CIRCUIT (IC-F4/S ONLY)
The power detector circuit (D2) detects the transmit power
output level and converts it to DC voltage. The detected sig-
nal is applied to the APC circuit.
4-2-6 APC CIRCUIT
The APC circuit (IC3a, Q37) protects the drive and the
power amplifiers from excessive current drive, and selects
HIGH or LOW output power.
R161
Q9
Q2
Driver
amp.
–
IC3a
+
4 - 2
Q1
Power
to antenna
amp.
Power detector
circuit (D2; IC-F4/S only)