Icom IC-RP1620 Service Manual page 7

3 . 2
TRANSMITTER CIRCUITS
3.2-1 MICROPHONE AMPLIFIER CIRCUIT
(TX AND T-VCO UNITS)
The AF signal from the microphone is applied to a
microphone amplifier (ICl0, pin 6). The amplified signal
is output from IC10 (pin 7) and applied to lClO (pin 2)
to mix with the "MOD" signal from IC15 on the LOGIC
UNIT. The signal is output from lClO (pin 1) and
applied to a limiter (IC7, pin 3). The AF signal obtains
+6 dBIoctave pre-emphasis at C53 and R51 which
are connected to pin 2.
The output signal from lC7 (pin 1) is applied to the
splatter filter circuit (IC7, R55, R59, R60, C56, C61, C62)
which attenuates components of frequencies of 3 kHz
and above. IC7 (pin 7) outputs an "MOD" signal. The
"MOD" signal is applied to the T-VCO UNIT.
3-2-2 MODULATION CIRCUIT (T-VCO UNIT)
The modulation circuit modulates the VCO oscillating
signal (RF signal) using the microphone audio signal.
The "MOD" signal from IC7 (pin 7) changes the reactance
of varactor diode (D5) to modulate the oscillated signal
at T-VCO ((21). The oscillated signal is buffer-amplified
at Q2 and is then applied to the drive amplifier circuit.
3-2-3 DRIVE AMPLIFIER CIRCUIT
(TX AND PA UNITS)
The VCO output is buffer-amplified at IC1 and Q2. The
signal is applied to the drive amplifier (Ql) on the PA
UNIT. The drive amplifier (Ql) amplifies the signal to
obtain 200 mW.
The control voltage from the APC circuit is applied to the
collector of Q l for stable RF output power from a power
amplifier (ICl).
3-2.4 RF POWER AMPLIFIER (PA UNIT)
lC1 is a power module which provides a stable 25 W
(DC 13.8
V) of output power.
An RF signal from the drive amplifier (Ql) is applied
to IC1 (pin 1). The amplified signal is output from IC1
(pin 4) and applied to the antenna connector through
the RF detector circuit and low-pass filter circuit.
3-2-5 APC CIRCUIT (PA UNIT)
This circuit controls a current of the power module's first
stage and a current of the driver amplifier to obtain stable
RF output power.
The APC mismatch detector circuit consists of C14-C17,
02, 03, L3, R8 and R9. When the antenna impedance
is matched at 50 i2, D2 and D3 detect the minimum
voltage. The detected voltage is applied to a differential
amplifier (IC2, pin 6). The APC reference voltage is
applied to lC2 (pin 5).
When the antenna impedance is mismatched, the
voltage in pin 6 of IC2 exceeds the reference voltage in
pin 5 of IC2. The output level from lC2 (pin 7) decreases.
Q2 amplifies the current from the differential amplifier
(IC2) which controls Q3. 0 3 changes the supply voltage
to Q1 and ICl.
This decreases the output power from the drive amplifier
(Ql) and power amplifier (IC1) until the input voltage in
pin 6 of 1C2 reaches the same level as pin 5 of IC2.
APC
Fig. 2
3-2