Transmitter Circuits - Icom IC-F4TR 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 "NOISE" (pulse type) signals at the noise
comparator section (IC2, pin13). The "NOISE" signal is
applied to the CPU (IC17, pin 32).
The CPU detects the receiving signal strength from the
number of the pulses, and outputs an "RMUTE" signal from
pin 94. This signal controls the analog 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 deviation adjustment pot
(R584), analog switch (IC4, pins 2–4) and low-pass filter cir-
cuit (IC3d, pins 13, 14). The filtered AF signals are applied
to the modulator circuit after being passed through the ana-
log switch (IC4, pins 8–10) and the modulation balance
adjustment pot (R119).
• APC circuit
Current sensor circuit
VCC
T5
Q28
RF signal
from PLL
S5V
TMUT
T4
R161
Q9
APC control circuit
–
Q37
IC3a
+
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 (D404)
to modulate an oscillated signal at the TX-VCO circuit (Q7).
The oscillated signal is amplified at the buffer-amplifiers (Q6,
Q4), then applied to the T/R switching circuit (D3, D4).
4-2-3 DRIVE/POWER AMPLIFIER CIRCUITS
The signal from the TX-VCO circuit passes through the T/R
switching circuit (D3) and is amplified at the buffer (Q403)
pre-drive (Q3), drive (Q2) and power (Q1) amplifiers to
obtain 4 W 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 power (Q1) amplifiers
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 CPU (IC17, pin
41).
4-2-5 POWER DETECTOR CIRCUIT
The power detector circuit (D2) detects the transmit power
output level and converts it into DC voltage. The detected
signal is applied to the APC circuit (IC19).
4-2-6 APC CIRCUIT
The APC circuit (IC19, Q37) protects the drive and the
power amplifiers from excessive current drive, and selects
HIGH or LOW output power.
The signal output from the power detector circuit (D2) is
applied to the differential amplifier (IC19, pin 3), and the "T4"
signal from the expander (IC10, pin 14), controlled by the
CPU (IC17), is applied to the other input for reference.
Q1
Q2
Driver Power
amp.
amp.
4 - 2
to antenna
Power detector
D2