Pa Unit; Receive Signal Processing; Transmit Signal Processing; Section 3 Circuit Description - Icom IC-7300 Service Manual

SECTION 3

3-1 PA UNIT

The PA UNIT is composed of the transmit power amplifier,
harmonic filters, and sensing circuits for the transmit power
and SWR.

(1) RECEIVE SIGNAL PROCESSING

The received signal from the TUNER UNIT is passed through
the TX/RX SW (RL801), and applied to the RF UNIT through
the LPF whose cut-off frequency is 76 MHz, and the TX mute
switch (Q811, D811 and D812).
To prevent abnormal oscillations caused by feedback of the
transmit signal, the TX mute switch connects the transmit line
to ground while receiving.

(2) TRANSMIT SIGNAL PROCESSING

• DRIVE AMPLIFIER
The transmit signal from the RF UNIT is sequentially ampli-
fied by the RF amplifier (Q101), pre-drive amplifier (Q111)
and drive amplifier (Q121). These RF amplifiers are the high
gain source-grounded amplifiers. The idling current of drive
amplifier (Q121) is appropriately adjusted to compensate for
variation in gain.
• PA AND TUNER UNITS (Receive circuits)
From the TX circuits
Q811
D811/D812 fc=76 MHz
MUTE LPF
TX signal
RX signal
• PA AND TUNER UNITS (Transmit circuits)
LPF A
0.03-1.59 MHz
LPF
Q101 Q111 Q121
PRE
RF
DRIVE DRIVE
AMP
AMP
1.60-30.00 MHz 1.60-1.99 MHz
LPF BPF
2.00-2.99 MHz
BPF
3.00-4.49 MHz
BPF
4.50-6.49 MHz
BPF
6.50-7.99 MHz
CIRCUIT DESCRIPTION
D961,D962
RL801
PWR/SWR
DET
PA UNIT
0.03-2.0 MHz
LPF
2.0-4.0 MHz
LPF
4.0-7.3 MHz
Q131
Q132
LPF
PWR
AMP
7.3-14.35 MHz
15.00-21.99 MHz
LPF
BPF
14.35-21.45 MHz
22.00-29.99 MHz
LPF
BPF
21.45-33.0 MHz
RF UNIT
30.00-49.99 MHz
LPF
54.01-69.99 MHz
33.0-76 MHz
BPF
LPF
50.00-54.00 MHz
To the RX circuit
BPF
70.00-74.80 MHz
•

POWER AMPLIFIER

The transmit signal from the drive amplifier (Q121) is applied
to the push-pull power amplifier (Q131 and Q132) to be ampli-
fied up to 100 watts (for the HF/50 MHz band) or 50 watts (for
the 70 MHz band). The idling current of the power amplifier
is appropriately adjusted to compensate for variation in gain.
• LPF
The transmit signal from the power amplifier is filtered by
one of the LPFs that is selected according to the transmit
frequency. These LPFs are the Chebyshev filters with sharp
cut-off characteristics, to reduce the harmonic components in
the transmit signal.
There are 7 BPFs with different cut-off frequencies. The LPFs
are selected by the relays on both sides of each LPF, accord-
ing the transmit frequency.
• FORWARD AND REFLECT WAVES DETECTION CIRCUIT
The transmit signal is passed through the SWR detection cir-
cuit, which is composed of the CM coupler and rectifier diodes
(D961 and D962). The forward and reflect waves are detected,
resulting in the forward wave and reflect wave sensing volt-
ages. The sensing voltages are amplified by the buffer (IC981),
and then applied to the CPU in the MAIN UNIT for monitoring.
The amplified transmit signal is applied to the antenna,
through the TUNER UNIT.
D1311 D1312 D1505
SWR
ATT
DET
IC1701
PHASE
DET
D1201
POWER
DET
IC1701
D1271
IMP
DET
TUNER UNIT
PA UNIT
Q1211
Q1212/Q1213
PRE
RF
AMP
D961,D962
RL801 SW11
PWR/SWR
DET
3-1
TUNE
NET
SWRL
PHASEI
IC1901
BUFF TPWRL
IMPI
TUNER UNIT
SW11
D1311 D1312 D1505
SWR
ATT
DET
IC1701
SWRL
PHASE
DET
PHASEI
D1201
IC1901
POWER
BUFF TPWRL
DET
IC1701 D1271
IMP
IMPI
DET
HF/50 MHz/70 MHz
HF/
50 MHz/
70 MHz
TUNE
NET