Transmitter Circuits - Icom IC-V82 Service Manual

4-1-6 SQUELCH CIRCUIT
Squelch circuit mutes AF output signal when no signals are
received.
A portion of the AF signals from the FM IF IC (IC2, pin 9)
are applied to the D/A converter (IC10, pin 24) to control its
level. The level controlled signals are output from pin 23 and
applied to the active filter (IC2, pins 7, 8; R86–R88, C121,
C122). The filtered signals are applied to the noise amplifier
section to amplify the noise components only.
The amplified noise components are detected at the noise
detector section, and output from pin 13 as the "NOIS" signal
and applied to the CPU (IC8, pin 26). Then the CPU outputs
"AFON" signal from pin 71 according to the "NOIS" signal
level to toggle the AF regulator (Q15, Q16) ON/OFF.

4-2 TRANSMITTER CIRCUITS

4-2-1 ALC AMPLIFIER AND PRE-EMPHASIS CIRCUIT
The ALC (Audio Level Control) amplifier maintains the level
of the audio signals from the microphone constant. The pre-
emphasis circuit amplifies the audio signals within +6 dB/oct
pre-emphasis characteristic.
• When ALC function is ON.
The AF signals from the microphone are passed through
the ALC switch (IC13, pins 3, 4) and applied to the ALC
amplifier (IC17, pin 3) to limit its level.
The level limited signals are output from pin 5 and passed
through the another ALC switch (IC13, pins 8, 9) and
UN_SEC switch (IC13, pins 10, 11). The signals from UN_SEC
switch are then applied to the pre-emphasis circuit (IC3, pins
8, 10) to obtain frequency characteristic of +6 dB/oct.
• When ALC function is OFF.
The audio signals from the microphone are passed
through the ALC switch (IC13, pins 1, 2 and pins 10, 11)
and UN_SEC switch (IC13, pins 10, 11). The signals from
UN_SEC switch are then applied to the pre-emphasis circuit
(IC3, pins 8, 10).
• APC CIRCUITS
T5
Pre-
drive
RF signals
AMP
from TX/RX SW
3
+
T3
APC
1
CTRL
–
IC3A
2
Q3 Q2
Power
Drive
AMP
AMP
The signals from the pre-emphasis circuit are passed
through the analog switch (IC4, pins 3, 4) and LPF (IC3,
pins 13, 14). The signals from the LPF are passed through
another analog switch (IC4, pins 8, 9), digital/analog switch
(IC15, pins 8, 9 and pins 3, 4) and D/A converter (IC10,
pins 21, 22) to adjust its level. The level adjusted signals
from the D/A converter are applied to the modulator circuit
(D61).
4-2-2 MODULATOR CIRCUIT
The modulation circuit modulates the VCO oscillating signal
with the audio signals from the microphone.
AF signals from the D/A converter (IC10, pin 22) are applied
to the modulation circuit (D61) to modulate the oscillated
signal by changing the reactance of D61 at the VCO (Q76,
D59, D60).
4-2-3 TRANSMIT AMPLIFIERS
The VCO output signal is amplified to transmit power level by
the transmit amplifiers .
The VCO output signal is buffer-amplified by the buffer amp-
lifiers (Q74, Q75) and passes through the TX/RX switch (D3).
The signals from the TX/RX switch are applied to the pre-
driver (Q3), driver (Q2) and power (Q1) amplifiers, to be
amplified to the transmit output power level.
The power amplifier output is applied to the antenna conector
(J1: CHASSIS UNIT) via the antenna switching circuit (D1),
power detector (D32, D33), and two-stage LPFs.
4-2-4 APC CIRCUIT
The APC (Automatic Power Control) circuit stabilizes transmit
output power and controls transmit output power High, Middle
and Low.
The RF output signal from the power amplifier (Q1) is detect-
ed at the power detector (D32, D33). The detector converts
the RF signals into DC voltage, and the detected voltage is
applied to the APC amplifier (IC3, pin 2).
The transmit output power setting voltage "T3" is applied to
input terminal of the APC amplifier (IC3, pin 3) as the reference
voltage. The APC amplifier controls the bias of the pre-driver
(Q3), driver (Q2) and power amplifier (Q1) by comparing the
detected voltage and the reference voltage. Thus the APC
circuit maintains a constant output power.
Power detecter
Q1
LPF
D33
4 - 2
VCC
ANT
To ANT
LPF
SW
PS5
D32
TXC
Q37