Icom IC-F14 Service Manual Addendum page 29

4-2-2 MODULATION CIRCUIT
The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signals.
The AF signals from the D/A converter (IC8, pin 3) change
the reactance of varactor diode (D18) to modulate the oscil-
lated signal at the TX VCO circuit (Q13, D16, D17). The
modulated VCO signal is amplified at the buffer amplifiers
(Q12, Q10) and is then applied to the drive amplifier circuit
via the T/R switch (D14).
The CTCSS/DTCS signals ("CENC0", "CENC1", "CENC2")
from the CPU (IC13, pins 23–25) pass through the low-pass
filter (IC5, pins 12 and 14) via 3 registers (R191–R193) to
change its wave form. Then the signals are applied to the
D/A converter (IC8, pin 9). The output signals from the D/A
converter (IC8, pin 10) pass through the low-pass filter (IC6,
pins 6 and 7) to be mixed with "MOD" signal at the filter (IC6),
and are then applied to the D/A converter again (IC8, pin 4).
4-2-3 DRIVE/POWER AMPLIFIER CIRCUITS
The drive/power amplifier circuits amplify the VCO oscillat-
ing signal to an output power level.
The modulated RF signal from the TX VCO circuit passes
through the T/R switch (D14), and is amplified at the pre-
drive (Q9), drive (Q8) and power (Q7) amplifiers to obtain
5 W of RF power (at 7.2 V DC).
The amplified signal passes through the power detector (D1),
antenna switch (D2) and low-pass filter (L1–L3, C2–C5,
C175, C176), and is then applied to the antenna connector
(CHASSIS unit; J1).
The bias current of the pre-drive (Q9), drive (Q8) and power
(Q7) amplifiers are controlled by the APC circuit.
APC CIRCUITS
VCC
T5V
RF signal
from Buffer AMP
+
T2
Differential
AMP
TMUT
–
Q8
Q9
Driver
YGR
AMP
AMP
IC2
4-2-4 APC CIRCUITS
The APC circuit (IC2, D1) protects drive and power ampli-
fiers from the reflected signal, and selects output power of
HIGH, LOW2 or LOW1.
The power detector (D1) detects transmit output power and
converts it into DC voltage. The DC voltage is at a minimum
level when the antenna impedance is matched to 50 Ω, and
increased when mismatched.
The detected voltage is applied to the differential ampli-
fier (IC2, pin 3), and the "T2" signal from the D/A converter
(IC8, pin 23), controlled by the CPU (IC13), is applied to the
other input for reference. When antenna impedance is mis-
matched, the detected voltage exceeds the power setting
voltage. Then the output voltage of the differential amplifier
(IC2, pin 4) controls the input current of the pre-drive (Q9),
drive (Q8) and power (Q7) amplifiers to reduce the output
power.
Q7
Power
LPF
AMP
D1
4 - 3
ANT
LPF
SW
to ANT unit
D2