Icom IC-2820H Service Manual page 11

<OPERATION ON THE RIGHT BAND>
The modulation signals are applied to the variable capacitor
D87 of the left band VCO (Q73, D87, 91, 92) via the
modulation selector (IC63) and modulation mute switch
(Q64), and modulated. The modulated VCO output are
amplifi ed by the buffer (Q76) and LO amplifi er (IC45), and
applied to the transmit amplifi ers as the TX signal, via the
VCO switch (D102). The amplifi ed LO signals are applied to
the transmit amplifi ers via the LO switch (D103), two HPFs
(L124, C527, 532; L159, C533, 535) and attenuator (R43,
47, 57).
TRANSMIT POWER AMPLIFIERS
TX signal from the attenuator (R43, 47, 57) is amplifi ed by
pre-drive (Q22) and drive (Q26) amplifi ers to obtain RF level
for power module (IC2). The amplifi ed TX signal is applied
to the power amplifier which is a UHF band PA module
composed by two power MOS-FETs. The power-amplified
TX signal is passed through the LPF, power detector,
antenna switch (D62) and LPF, before being applied to the
antenna connector (CHASSIS; J1).
• APC CIRCUITS
VT8 VT8
D29
Q25
Q27
PRE
DRIVE
LIMIT
DRIVE
AMP
TX signal from
LO switch (D155, D157)
TX signal from
LO switch (D103)
UT8 UT8
D25
Q22 Q26
PRE
DRIVE
LIMIT
DRIVE
AMP
All manuals and user guides at all-guides.com
HV
LPF
RF POWER DETECTOR (VHF)
IC3
PWR
AMP
D39
C164
IC4,Q31,Q36
APC
TX_MUTE
CTRL
TX_MUTE
PCON_U PCON_V
IC2
L50
PWR
AMP
D42
R166
C176
HV
LPF RF POWER DETECTOR (UHF)
4 - 5
APC CIRCUITS
A portion of the TX signal from IC2 is rectifi ed at the power
detector (D42, 52), and converted into the DC voltage
which is in proportion to the RF power, and applied to the
operational amplifier (IC4, pin 2). IC4 is an APC amplifier
for both of V/UHF bands. The TX power setting voltage
"PCON_U" from the D/A converter (IC1, pin 6) is applied
to the pin 3 as a reference. IC4 is rolled as a differential
amplifier which outputs voltage in inverse proportion to
rectified one. When the TX power increased, the rectified
voltage also increased, that causes the decrease of output
voltage of differential amplifier. The decrease of output
voltage of differential amplifi er causes the drop of the gate
voltage of IC2, Thus the TX power maintained to keep stable
level.
TX muting is carried out by TX mute SW (Q36) controlled
by "TX_mute" signal. Applying "TX_mute" signal to the base
terminal of Q36 to turn it ON, 8 V DC appears on the pin 6 of
IC4 and its output voltage downs to 0 V DC to inactivate IC3.
D59,D75,D77
C227
TX/RX
SW
C212
D53,D61,D62
L61
C275
TX/RX C296
SW
D52
R178
C265
LPF
L92 L96
LPF
L108
L101 L104
C308
HPF