Icom IC-F33GT Service Manual page 11

MODULATION CIRCUIT
• VOICE SIGNAL
The modulation circuit FM-modulates the VCO oscillating
signal with the AF signals from the TX AF circuit.
The AF signals from the microphone amplifier circuits are
applied to the variable capacitor to change its reactance for
FM modulation.
The AF signals from the D/A converter (IC12, pin 3) change
the reactance of varactor diode (D12) to modulate the oscil-
lated signal at the TX VCO circuit (Q16, D10, D13, D501).
The modulated VCO signal is amplified at the buffer ampli-
fiers (Q15, Q29) and is then applied to the drive amplifier
circuit via the T/R switch (D16).
• TONE SIGNAL
Tone signals are generated in the CPU and applied to the
both of the VCO and reference frequency oscillator to be
modulated.
The CTCSS/DTCS signals ("CENC0," "CENC1," "CENC2")
from the CPU (IC22, pins 13, 15, 16) are combined at the
resistors (R222–R224) and are then pass through the low-
pass filter (IC6, pins 12, 14). The filtered signals are applied
to the D/A converter (IC12, pin 12) via the "TONC" line. The
output signals from the D/A converter (IC12, pin 11) are
mixed with the filtered Mic audio signals.
The mixed signals are passed through the D/A converter
(IC12, pin 3, 4), and are then applied to the D12 in the TX
VCO circuit.
TRANSMIT AMPLIFIERS
The transmit amplifers consist several RF amplifier
(predriver, driver, power) and amplify the VCO output to the
transmit output level.
• APC CIRCUIT
VCC
T5V
RF signal
from PLL circuit
T2
APC control circuit
TMUT
IC501
Q702
Q704
YGR
Driver
amp. amp.
–
IC2
APC
amp.
+
APC CIRCUIT
The APC (Automatic Power Control) circuit stabilizes
transmit output power to prevent transmit output power level
change which is caused by load mismatching or heat effect,
etc,. The APC circuit also selects transmit output power from
high, middle and low power.
The power detector rectifi es a portion of the transmit output
and converts it into DC voltage which is in proportion to the
transmit output power level. The detected voltage is applied
to the comparator. The transmit power setting voltage is ap-
plied to another input terminal as the reference voltage.
The comparator compares the detected voltage and refer-
ence voltage, and the difference of voltage is output. The
output voltage controls the bias of the RF amplifi ers to re-
duce/increase the gain of these amplifi ers for stable transmit
output power.
The change of transmit power is carried out by the change
of reference voltage.
The power detector circuit (PA unit; D702, D703) detects the
transmit power output level and converts it into DC voltage.
The output voltage is at a minimum level when the antenna
impedance is matched with 50 Ω and is increased when it is
mismatched.
The detected voltage is applied to the differential amplifier
(MAIN unit; IC2; pin 3), and the "T2" signal from the D/A
converter (MAIN unit; IC12, pin 23), controlled by the CPU
(MAIN unit; IC22), is applied to the other input for reference.
When antenna impedance is mismatched, the detected
voltage exceeds the power setting voltage. Then the output
voltage of the differential amplifier (MAIN unit; IC2, pin 4)
controls the input bias voltage of the drive (PA unit; Q702)
and power (PA unit; Q701) amplifiers to reduce the output
power.
PA unit
Q701
Power
LPF
amp.
D702
4 - 3
ANT
LPF
SW
to ANT unit
D701
D703