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3-1-10 SQUELCH CIRCUIT (MAIN UNIT)
The transceiver has two squelch circuits, voice activated
squelch for J3E/H3E and S-meter squelch for A1A/FSK/J2B.
(1) AF ACTIVATED SQUELCH
A portion of the AF signal from the active low-pass filter
(IC20b) is amplified at the limiter amplifier (IC20a) and is
then applied to the one-shot multi-vibrator (IC22c, IC22d).
The one shot multi-vibrator functions as an F-V converter
which generates a signal only when audio signals are
received.
The output signals pass through the NOR gate (IC22b) and
then the 3 Hz low-pass filter (IC21a) to remove the remain-
ing noise components. The filtered signal is applied to the
window comparator (IC21b). The NOR gate (IC22b) deacti-
vates the audio activated squelch during A1A/FSK/J2B
mode operation.
The comparator outputs "High" when the integrated signals
exceed the reference voltage. C269, R310 and R780 are
used as a time constant circuit. The resulting signal output
from IC22a is inverted at Q46 and is then applied to the CPU
as the "SQLS" signal. The CPU controls the squelch gate
(IC12a) when the "SQLS" signal is received.
(2) S-METER SQUELCH
The S-meter signal from IC19b is applied to the squelch
comparator (IC19a) to close or open the squelch circuit. The
reference voltage is adjusted by R257 and then applied to
the (–) terminal of the comparator (IC19a). When the S-
meter signal exceeds the reference voltage, the comparator
outputs "High" to the CPU via IC22a and Q46 in the same
manner as the voice activated squelch circuit.
3-2 TRANSMITTER CIRCUITS
3-2-1 MICROPHONE AMPLIFIER CIRCUIT
(LOGIC BOARD IN THE RC-21)
The AF signals from the [MICROPHONE] connector pass
through the microphone gain controller (R81) and AF ampli-
fier (IC12a), and are applied to the CTRL1 board. The
microphone AGC circuit (D9, D10, Q13, Q14) controls the
amplifier gain to prevent signal distortion.
• MODULATOR CIRCUIT
Microphone
2-tone alarm
ACC (1) socket
NBDP socket
IC8a
5
6
IC38b
2
1
IC38a
2
1
IC39a
5
6
IC39b
The AF signals are converted into PWM signal at the PWM
modulation circuit (IC3, Q1, D1) via the AF amplifier (CTRL1
board; IC4), and are applied to the serial interface IC (IC1).
The serial interface IC outputs PWM signal to the CTRL2
board (IC-M710 RT) via a coaxial cable.
The PWM signal from the RC-21 (controller) is applied to the
MAIN unit after being converted into an analog signal by
passing though the serial interface IC on the CTRL2 board.
The AF signals are applied to the balanced modulator (MAIN
unit; IC9, pin 1) via the AF switch (IC38b).
External modulation inputs from the ACC socket or a 2-tone
emergency signal from the CPU are applied to the balanced
modulator directly via AF switches (IC38, IC39).
3-2-2 MODULATION CIRCUIT (MAIN UNIT)
(1) J3E AND J2B MODES
The balanced modulator is used for J3E and J2B modes to
add the audio signal to the BFO frequency and outputs the
IF signal while suppressing the BFO signal.
The AF signals from the microphone amplifier or external
audio from the modulation terminals are applied to the bal-
anced modulator (IC9, pin 1). The BFO signal from the PLL
unit is applied to IC9 (pin 10) as a carrier signal. A double
sideband signal is output from IC9 (pin 6) and is then applied
to the 9 MHz filter (FI2) to create an SSB signal.
R238 adjusts the balanced level of IC9 for maximum carrier
suppression. In J2B mode, the BFO frequency is shifted 1.7
kHz to set the transmit frequency the same as the displayed
frequency.
The SSB signal from FI2 is amplified at the 9 MHz amplifiers
(Q17–Q19) and is then applied to the mixer circuit (D52).
The switching diode (D19) is turned ON when R8 voltage
disappears.
(2) H3E AND R3E MODES
An SSB signal is applied to the IF amplifier (Q18) in the
same manner as with J3E/J2B mode. The BFO signal from
the PLL unit is amplified at the buffer amplifier (Q30) and is
then applied to the IF amplifier (Q18) as a carrier signal to
be added to an SSB signal. R211 and R212 adjust the car-
rier levels in H3E and R3E modes, respectively.
Balance upset during CW, FSK
Modulator
Fl2
1
IC9
Crystal
filter
6
10
BFO from the PLL unit (MHz)
J3E, R3E, H3E, J2B:
A1A:
FSK:
FSK narrow:
J2B:
During tuning:
3 - 3
CW keying control
(A1A only)
D9
Q19
Q18
9.013 MHz carrier
(R3E, H3E only)
9.013
9.0116
9.0113 (center)
9.0106 (center)
9.1023
9.0115
Q17
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