Transmitter Circuits - Icom IC-V8 Service Manual

The AF signals are applied to the AF power amplifier 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 speaker jack (CHASSIS unit; J3)
when no plug is connected 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 analog swtich.
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 trigger circuit converts the detected signals to a HIGH
or LOW signal and applies this (from pin 13) to the CPU
(IC8, pin 19) as the NOIS signal. When the CPU receives a
HIGH level NOIS signal, the CPU controls the RMUT line to
cut the AF signals at the analog swtich IC (IC4). At the same
time, the AFON line controls the AF regulator circuit (Q15,
Q16) to cut out the VCC power source for the AF power
amplifier (IC5).
4-2

TRANSMITTER CIRCUITS

4-2-1 MICROPHONE AMPLIFIER CIRCUIT
The microphone amplifier circuit amplifies audio signals with
+6 dB/octave pre-emphasis from the microphone 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 analog swtich (IC4, pins 4, 3). The fil-
tered AF signals are applied to the modulator circuit after
passing through the analog swtich (IC4, pins 8, 9).
4-2-2 MODULATION CIRCUIT
The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signals.
• APC CIRCUIT
VCC
T5
RF signal from
PLL IC (IC1)
PS5
TXC
T3
Q3
Pre drive
2
Q37
IC3a
3
+
APC control circuit
The audio signals (SHIFT) change the reactance of D37 to
modulate an oscillated signal at the VCO (Q50, D38). The
oscillated signal is amplified at the LO (Q6) and buffer (Q4)
amplifiers, then applied to the TX/RX switch circuit (D3, D4).
4-2-3 DRIVE/POWER AMPLIFIER CIRCUITS
The signal from the VCO circuit passes through the TX/RX
swtiching circuit (D3) and is amplified at the pre-drive (Q3),
drive (Q2) and power (Q1) amplifiers to obtain 5.5 W of RF
power (at 7.2 V DC/typical). The amplified signal passes
through the low-pass filter (L4, C278, C287), and then
applied to the antenna swtiching circuit (D1). The signal is
applied to the antenna connector (J1) after being passed
through the low-pass filter (L1, L2, C1–C5).
The bias current of the drive (Q2) and power (Q1) amplifiers
is controlled by the APC circuit to stabilize the output power.
4-2-4 APC CIRCUIT
The APC (Automatic Power Control) circuit (IC3a, Q37) pro-
tects drive and power amplifiers from excessive currents
and selects HIGH or LOW output power.
The output voltage from the power detector circuit (D32,
D33) is applied to the differential amplifier (IC3a, pin 2), and
the "T3" signal from the D/A convertor (IC10, pin 10) is
applied to the other input for reference.
When the driving current increases, the input voltage of the
differential amplifier (IC3a, pin 2) will be increased. In such
cases, the differential amplifier output voltage (pin 1) is
decreased to reduce the drive current.
Q37 is controlled by the TXC signal from the CPU (IC8, pin
55) to select HIGH or LOW output power.
Q2 Q1
Power
Driver
amp. amp.
1
4 - 2
L4, C278, C287
to antenna
LPF
D33 D32
Power detector
circuit (D32, D33)