Icom IC-F4029SDR, Service Manual

Page: 22 / 48

4-2-3 TRANSMIT AMPLIFIERS (PA-C UNIT)
The transmit signal from the VCO is amplifi ed to the transmit
output level by the transmit amplifi ers.
The transmit signal from the TX/RX switch (D16=ON,
D17=OFF) is amplified by two pre-drive amplifiers (Q703,
Q704), drive amplifi er (Q702) and power amplifi er (Q701) in
sequence to obtain 0.5 W (aprrox.) of transmit output power.
The power-amplified transmit signal is passed through the
antenna switch (D701) and LPF (as a harmonic fi lter; L709,
L710, C742, C744−C748), then applied to the antenna via
the ANT UNIT.
4-2-4 APC CIRCUIT (PA-C UNIT)
The APC (Automatic Power Control) circuit prevents the
transition of the transmit output power level which is caused
by load mismatching or heat effect, etc.
A portion of transmit signal is detected by the transmit power
detector (D702) to produce a DC voltage corresponding to the
transmit output power level. The detected voltage is applied to
the APC amplifi er (IC701, pin 3). The transmit power setting
voltage “APC (T1)” from the D/A converter (MAIN-C UNIT;
IC23, pin 1) is applied to another input terminal (pin 1) as the
reference voltage.
The APC amplifier compares the detected voltage and
reference voltage, and the difference of the voltage “APCV” is
output from pin 4. The voltage “APCV” controls the bias of the
drive (Q702) and power (Q701) amplifi ers to reduce/increase
the gain of these amplifi ers for stable transmit output power.
The transmit power muting is carried out by the TX mute switch
(MAIN-C UNIT; Q46), using the “TMUT” signal from the CPU
(IC22, pin 35).
4 - 3
Power
amp.
APC
amp.
Drive
amp.
+
1
3 4
–
VCC
MAIN-C UNIT
• APC CIRCUIT
to the anntena
“APC (T1)”
“TMUT”
from TX/RX switch
(MAIN-C UNIT; D16, D17)
T5V
Q702
Q703, Q704
IC701
Q701
to the receive circuits
LPF LPF LPF
ANT
SW
D702
D701, D704
Transmit
power detecter
Q46
Pre-drive
amplifiers