The AF OUT signals from the VOL1 line are also amplified
at the AF amplifier (LOGIC unit 02) to output AF signal
from the microphone connector.
3-1-5 SQUELCH CIRCUIT
(MAIN and LOGIC UNITS)
Asquelch circuit cuts out AF signals when no RF signals
are being received. By detecting noise components
in the AF signals, the squelch circuit turns the AF mute
switch OFF.
Aportion of the AF signals from the FM IF 1C (IC4 pin 9)
are applied to the active filter (104 pin 10) through the
noise filter (0145—0147, R128). The active filter section
in 104 amplifies noise components of frequency of 20 kHz
and above, and are rectified at the noise detector (D26)
and then triggers the noise switch (Q45).
3-2-3 DRIVE/POWER AMPLIFIER CIRCUITS
(MAIN UNIT)
The signal from the modulation circuit is passed through
the transmit/receive switching circuit (D2) and amplified
at the pre-driver (Oil), driver (012), and the power module
(102) in sequence to obtain 50 W* (at 13.8 VDC) of RF
power.
*10 Wfor the IC-2000 Thailand version.
The amplified signal is passed through the antenna
switching circuit (D7), ARC detector circuit (L10, D8, D9),
and low-pass filter (L11, LI 2, 061, 062) and is then
applied to the antenna connector.
The collector current of the driver (012) are controlled
by the ARC circuit to protect the power module from a
mismatched condition as well as to stabilize the output
power.
The noise switch (045) converts the rectified signals to
a“High” or “Low" signal and applied this to the CRU
(LOGIC unit 102 pin 5) as the busy signal. When the
CRU receives “High,” the CRU outputs the mute signal
through the Data expander 1C (MAIN unit IC9, pin 11)
to cut the AF signals at the detector mute switch (037).
3-2-4 APC CIRCUIT (MAIN UNIT)
The ARC circuit protects the power module (IC2) from a
mismatched output load and selects High or Low output
power.
Even when the squelch is closed, the AF mute switch
(040) opens at the moment of emitting been tones.
3-2 TRANSMITTER CIRCUITS
3-2-1 MICROPHONE AMPLIFIER
(LOGIC UNIT)
The ARC detector circuit (L10, D8, D9) detects forward
signals and rectified signals at D8 and D9 respectively.
The combined voltage is at aminimum level when the
antenna is matched at 50 Oand is increased when it is
mismatched.
The detected voltage is applied to the inverting amplifier
(IC3a) to control the base of 016 and input current of
IC2 (pin 2) and 012 using 013.
The microphone amplifier circuit amplifies audio signals
with -F6 dB/octave pre-emphasis characteristics from the
microphone to alevel needed for the modulation circuit.
The AF signals from the microphone through the mic mute
switch (04), and are amplified at the microphone amplifier
(05) and the limiter amplifier (IC1 a) which has anegative
feedback circuit for -1-6 dB/octave pre-emphasis.
The amplified signals are applied to the low-pass filter
(ICIb) to filter out RF components and then applied to
the MAIN unit as the “MOD” signal.
3-2-2 MODULATION CIRCUIT (MAIN UNIT)
The modulation circuit modulates the VCO oscillating
signal (RF signal) using the microphone audio signals.
The audio signals (MOD) change the reactance of D19 on
the VCO circuit (022, 023, D19) to modulate the oscillated
signal at the VCO (022, 023). The oscillated signal is
amplified at the buffer amplifier (024) and LO amplifier
(025, 026), then applied to the drive amplifiers.
When the antenna impedance is mismatched, negative
input voltage of the inverting amplifier (IC3a) increases,
and the base voltage of 016 and the collector current of
013 decreases to reduce the output power.
•APC CIRCUIT
3-2
