4 - 2
If an external speaker-microphone or headset is attached to
the [SP MIC] connector (J250), the de-emphasized AF signals
are passed through the analog switch (IC430, pins 8, 9) and
applied to the AF power amplifier (IC280, pin 4) to obtain 0.2
W of AF output power. The power-amplified AF signals are
output from pin 10, and then applied to the external speaker
via the [SP MIC] connector (J416).
4-1-6 SQUELCH CIRCUIT
The squelch mutes the AF output signals when no RF
signals are received. By detecting noise components in the
demodulated AF signals, the squelch circuit toggles the AF
power amplifier ON and OFF.
A portion of the demodulated AF signals from the FM IF IC
(IC170, pin 9) are applied to the D/A converter (IC190, pin
13) for level adjustment (squelch threshold adjustment). The
level-adjusted AF signals are output from pin 14, and passed
through the noise filter (R174−R176, C177, C179, C180). The
filtered noise signals are then applied to the noise amplifier
in the FM IF IC (IC170, pins 7, 8) to be amplified the noise
components only.
The amplified noise components are converted into the
pulse-type signal at the noise detector section, and output
from pin 14 as the “NOISV” signal. The signal is applied
to the CPU (IC360, pin 32), and the CPU outputs “AFVS”
signal from pin 100 according to the “NOISV” signal level, to
the AF power regulator (Q230, Q231, Q401, Q540) which
toggles the AF power amplifier ON and OFF.
4-2 TRANSMITTER CIRCUITS
4-2-1 MICROPHONE AMPLIFIER CIRCUITS
The AF signals from the microphone (MIC signals) are
fi ltered and level-adjusted at microphone amplifi er circuits.
The AF signals from the microphone are passed through
the AF mute switch (IC430, pins 1, 2).
While an external microphone is connected to the [SP MIC]
connector (J416), the mute switch shuts out the AF signals
from the internal microphone (MC1).
AF signals from the AF mute switch (IC430, pins 1, 2) are
passed through another AF mute switch (IC430, pins 3, 4),
and passed through the pre-emphasis circuit (R253, C254) to
3 dB of characteristic. The pre-emphasized signals are
then applied to the microphone amplifier (IC200, pins 6, 7). The
amplified MIC signals are passed through AF mute switch
(IC260, pins 8, 9), and are applied to the gain controller
(Q450, Q451) whitch adjust the AF signal level (=deviation)
according to the control signals (“MIC1/2/3”) from the CPU
(IC360, pins 106/107/108).
The level adjusted MIC (MOD) signals are applied to
the limiter amplifier (IC200, pins 13, 14) which limits the
amplitude of the MIC signals to prevent over deviation. The
amplitude-limited MIC signals are then passed through the
splatter filter (IC200, pins 1, 3) which suppresses the 3 kHz
and higher audio components.
The filtered MIC signals are applied to the modulation circuit
(D20).
4-2-2 MODULATION CIRCUIT
The modulation circuit modulates the VCO oscillating signal
with the AF signals from the microphone.
The MIC signals from the microphone amplifier circuits
are applied to the D20, and modulate the VCO oscillating
signal by changing the reactance of D20. The modulated
VCO output signal is buffer-amplified by Q23 and Q24, then
applied to transmit amplifiers as a transmit signal via the
TX/RX switch (D50 is ON, D51 is OFF).
4-2-3 TRANSMIT AMPLIFIERS
The VCO output signal is amplifi ed to transmit output power
level by the transmit amplifi ers.
The transmit signal from the TX/RX switch (D50) is applied
to the buffer amplifier (Q50). The amplified transmit signal is
amplified to the transmit output level by the pre-driver (Q53)
and power amplifier (Q54). The power-amplified transmit
signal is passed through the TX power detector of APC
circuit (D91), antenna switch (D52, D90 and D353 are ON),
TX power detector for transmit indicator (D80) and LPF (as a
harmonic filter; L81, L82, C80, C83−C86, C89, C226) before
being applied to the antenna connector (CHASSIS; J1).
4-2-1 APC CIRCUIT
The APC (Automatic Power Control) circuit stabilizes trans-
mit 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 circuits (D91) detects 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 differential amplifi er (IC50, pin 3). The transmit power
setting voltage “PCON” is applied to another input terminal
(pin 1) as the reference voltage.
The differential amplifi er compares the detected voltage and
reference voltage, and the difference of the voltage is output
from pin 4. The output voltage controls the bias of the buffer
amplifi er (Q50), pre-driver (Q53) and power amplifi er (Q54)
to reduce/increase the gain of transmit amplifi ers for stable
transmit output power.
The change of transmit power is carried out by the change
of reference voltage "PCON" and "LOW."
Power
amp.
APC
amp.
Buffer
amp.
+
–
VCC
• APC CIRCUIT
to the anntena
T5V
PCON
LOW
TXMS
from the TX/RX switch (D50)
Q53
Pre-drive
amp.
Q50
IC50
3
1
4
Q351
Q54
LPF
ANT
SW
Q357
D91
D52
T5V
