4-1-6 SQUELCH CIRCUITS (MAIN UNIT)
Noise squelch circuit mutes AF output signals when no RF sig-
nals are received. By detecting noise components (30 kHz and
higher signals) in the demodulated AF signals, the squelch
circuit switches the AF mute switch ON and OFF.
A portion of the demodulated AF signals from the FM IF IC
(IC2, pin 9) are passed through the squelch threshold adjust-
ment pot (R214) to be adjusted its level. The level adjusted
AF signals are passed through the noise fi lter (IC2, pins 7, 8;
R211–R214, R220, C211, C212, C217–C219). The fi ltered sig-
nals are then applied to the noise amplifi er section in the IC2
to be amplifi ed the noise components only.
The amplifi ed noise components are converted into the pulse-
type signal at the noise detector section, and output from pin
14 as the “SQL” signal. The “SQL” signal is applied to the CPU
(LOGIC BOARD; IC1, pin 91), and the CPU outputs “EXSTB,”
“EXTOE” signal to the expand IC (AF UNIT; IC27, pins 1, 15)
according to the “SQL” signal level. Then the expand IC out-
puts “RMMUTEM” signal from pin 7 to the AF mute switch (AF
UNIT; IC4, pin 6), and outputs "SPMUTE" signal from pin 12
to the AF mute switch (Q65) to cut off the AF line.
4-2 TRANSMITTER CIRCUITS
4-2-1 MICROPHONE AMPLIFIER CIRCUITS (AF UNIT)
The AF signals from the microphone (MIC signals) are filtered
and level-adjusted at microphone amplifier circuits.
The MIC signals from the microphone are passed through the
MIC mute switch (IC5, pins 10, 11) and the electric volume
(IC13, pins 7, 8) which adjusts the MIC signal level (=microphone
sensitivity). The level-adjusted MIC signals are then applied to the
MIC amplifier (IC7, pin 2). The amplified MIC signals are passed
through the MIC line switch (IC3, pins 1, 2) and pre-emphasis
circuit (R307, C305) to 3 dB of characteristic. The pre-
emphasized signals are then applied to the IDC (Instantaneous
Deviation Control; IC8, pins 6, 7) circuit which limits the amplitude
of MIC signals to prevent over deviation. The amplitude-limited
MIC signals are passed through the LPF (IC8, pins 1, 3) which
suppresses 3 kHz and higher audio components.
The filtered MIC signals are passed through the deviation
adjustment pot (MAIN UNIT; R327), then applied to the
modulation circuit (MAIN UNIT; D2).
4-2-2 MODULATION CIRCUIT (MAIN UNIT)
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 D2 of TX/CH70-RX VCO (Q4, Q5, D1, D3),
and modulate the VCO oscillating signal by changing the
reactance of D2. The modulated VCO output signal is buffer-
amplified by Q6 and Q7, then applied to transmit amplifiers as
a transmit signal via the TX/RX switch (D7 is ON, D8 is OFF).
4-2-3 TRANSMIT AMPLIFIERS
(MAIN UNIT)
The output signal of TX/CH70-RX VCO is amplified to transmit
output power level by the transmit amplifiers.
The transmit signal from the TX/RX switch (D7) is applied
to the YGR amplifier (Q10). The amplified transmit signal is
applied to the power module (IC3, pin 1) and amplified to the
transmit output level. The power-amplified transmit signal is
output from pin 4, and passed through the LPF (L19, C121,
C124) to filter out the harmonic components. The filtered
transmit signal is passed through the antenna switch (D14 and
D21 are ON) and another LPF (L21, L22, C126−C129) before
being applied to the antenna connector (CHASSIS; J1).
4-2-4 APC CIRCUIT
(MAIN UNIT)
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 and
low power.
A portion of the transmit signal is detected at the power
detector circuits (D12, D13), and converted it into DC voltage
which is in proportion to the transmit output power level. The
detected voltage is applied to the base terminal of differential
amplifier (Q15). The divided voltage of the T5V line is applied
to another base terminal as the reference voltage.
The differential amplifier compares the detected voltage and
reference voltage, and the difference of the voltage controls
the bias of the power amplifier module (IC3) via Q14, to
reduce/increase the gain of transmit power amplifiers for
stable transmit output power.
The change of transmit output power is carried out by the
power controller (Q17) using “H/L” signal from the expander (AF
UNIT; IC27, pin 4). The transmit power muting is carried out by
the TX mute switch (Q16), using the “TMUTE” signal from the
expander (AF UNIT; IC27, pin 5).
4 - 2
HV
• APC CIRCUIT
to the anntena
“TMUTE”
T5V
T5V
YGR
amp.
Q10
LPF
ANT
SW
D12
D14
TX/RX
SW
D7
From the buffer (Q7)
Power detecter
D13
Q16
Q14
Q15
R112
“H/L”
Q17
T5V
1
2
3 4
Power module (IC3)
S-AV35