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The AF signals are applied to the AF power amplifier circuit
(IC9, pin 1) to obtain the specified audio level. The amplified
AF signals, output from pin 4, are applied to the internal
speaker (CHASSIS UNIT; SP1) via the speaker jack (J6)
when no plug is connected to the external speaker jack (J1).
4-1-6 SQUELCH CIRCUIT
(MAIN AND LOGIC UNITS)
A squelch circuit cuts out AF signals when no RF signals are
received. By detecting noise components in the AF signals,
the squelch switches the analog swtich.
A portion of the AF signals from the FM IF IC (IC4, pin 9) are
applied to the D/A converter (IC5, pin 13) as the DETO sig-
nal. The signals from the D/A converter (IC5, pin 14) are
applied to the FM IF IC active filter section (IC4, pin 8) where
noise components are amplified and detected with an inter-
nal noise detector via the SQLIN line.
The trigger circuit converts the detected signals to a HIGH
or LOW signal and applies this (from pin 13) to the CPU
(LOGIC UNIT; IC7, pin 19) as the NOIS signal.The CPU
controls the analog swtich IC (LOGIC UNIT; IC6) via the
expander IC (LOGIC UNIT; IC8). When the CPU receives a
HIGH level NOIS signal, the CPU controls the RMUT line to
cut the AF signals at the analog swtich IC (LOGIC UNIT;
IC6). At the same time, the AFON line controls the AF mute
circuit (Q29) to cut out the VOLOUT signal for the AF power
amplifier (IC9).
4-2 TRANSMITTER CIRCUITS
4-2-1 MICROPHONE AMPLIFIER CIRCUIT
(LOGIC AND MAIN UNITS)
The microphone amplifier circuit amplifies audio signals with
+6 dB/octave pre-emphasis from the microphone to a level
needed for the modulation circuit.
The AF signals from the microphone pass through the MIC
switch (IC2, D4) and high-pass filter (IC3a, pin 2), and are
then applied to the microphone amplifier circuit (IC3d, pin
12) via the R39 and C47 for +6 dB/octave pre-emphasis.
The amplified AF signals are applied to the analog swtich
(IC6, pin 4), and are then applied to the D/A converter (MAIN
UNIT; IC5, pin 1) via the MODIN signal. The AF signals are
applied to the modulator circuit via the MOD signal.
4-2-2 MODULATION CIRCUIT (MAIN UNIT)
The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signals.
The audio signals (SHIFT) change the reactance of D2 to
modulate an oscillated signal at the VCO (Q6, D4). The
oscillated signal is amplified at the LO (Q9) and buffer (Q11)
amplifiers, then applied to the TX/RX switch circuit (D6, D7).
4-2-3 DRIVE/POWER AMPLIFIER CIRCUITS
(MAIN UNIT)
The signal from the VCO circuit passes through the TX/RX
swtiching circuit (D6) and is amplified at the pre-drive (Q17),
drive (Q18, Q20) and power (Q23) amplifiers to obtain 75 W
of RF power (at 13.8 V DC/typical). The amplified signal
passes through the low-pass filter (L37, L39, C151, C156,
C159, C167, C176, C227–C229), and then applied to the
antenna swtiching circuit (D12). The signal is applied to the
antenna connector (CHASSIS UNIT; J1) after being passed
through the low-pass filter (L44, L47, L48, L51, C190, C197,
C203, C208, C210, C217, C218).
The bias current of the drive (Q18, Q20) and power (Q23)
amplifiers is controlled by the APC circuit to stabilize the out-
put power.
4-2-4 APC CIRCUIT (MAIN UNIT)
The APC (Automatic Power Control) circuit (IC6a, Q26) pro-
tects drive and power amplifiers from excessive currents
and selects HIGH or LOW output power.
The output voltage from the power detector circuit (D14,
D17) is applied to the differential amplifier (IC6a, pin 2), and
the “T3” signal from the D/A converter (IC5, pin 23) is
applied to the other input for reference.
When the driving current increases, the input voltage of the
differential amplifier (IC6a, pin 2) will be increased. In such
cases, the differential amplifier output voltage (pin 1) is
decreased to reduce the drive current.
Q26 is controlled by the TXC signal from the expander IC
(IC2, pin 14) to select HIGH or LOW output power.
Q23
Power
amp.
Q20
Driver
amp.
Q18
Driver
amp.
IC6a
+
HV
RF signal from
PLL IC (IC1)
to antenna
T1
TXC
Q26
+5V
APC control circuit
Power detector
circuit (D12, D14)
D14 D12
L44, C190, C191,
C196, C197
LPF
ANT
SW
SWHV
2
3
1
• APC CIRCUIT