4 - 2
4-1-5 SQUELCH CIRCUIT (MAIN UNIT)
The noise squelch circuit cuts out AF signals when no RF
signals are received. By detecting noise components in the
AF signals, the squelch circuit switches the AF mute switch.
A portion of the AF signals from the FM IF IC (IC1, pin 9) are
applied to the active filter section (IC1, pin 8). The active fil-
ter section amplifies and filters noise components. The fil-
tered signals are applied to the noise detector section and
output from IC1 (pin 14) as the “NOIS” signal.
The “NOIS” signal from IC1 (pin 14) is applied to the CPU
(IC9, pin 99) to analyze the noise condition. The “SQLV” sig-
nal from R1 (VR unit) is applied to the CPU (IC9, pin 95) to
detect squelch level. The CPU detects the receiving signal
strength and cut the AF signal line.
4-2 TRANSMITTER CIRCUITS
4-2-1 MICROPHONE AMPLIFIER CIRCUIT
(MAIN UNIT)
The microphone amplifier circuit amplifies audio signals with
+6 dB/octave pre-emphasis characteristics from the micro-
phone to a level needed for the modulation circuit.
The AF signals from the microphone are passed through the
pre-emphasis circuit (R31, C36) and are then applied to the
microphone amplifier (IC4d). The amplified AF signals are
applied to the optional voice scrambler unit to scramble the
audio via the “MICOUT” signal, or are bypassed around the
unit via an analog switch (IC5, pin 4).
The AF signals which bypassed or passed through the voice
scrambler unit are amplified again at the limiter-amplifier
(IC4a) and then passed through the low-pass filter (IC4b,
pins 6 and 7). The filtered audio is applied to the RF unit as
the “MOD” signal.
4-2-2 MODULATION CIRCUIT (RF UNIT)
The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signals.
The audio signals “MOD” change the reactance of D4 to
modulate an oscillated signal at the transmitter VCO (Q4,
Q5). The oscillated signal is amplified at the buffer-amplifiers
(Q6, Q8).
4-2-3 DRIVE/POWER AMPLIFIER CIRCUITS
(RF UNIT)
The signal from the VCO circuit passes through the trans-
mit/receive switching circuit (D7, D8) and is applied to the
buffer-amplifier (Q9). The amplified signal is amplified by the
pre-driver (Q10) and the power amplifier (Q11) to obtain 5 W
of RF power (at 7.4 V). The amplified signal passes through
the antenna switching circuit (D9), and low-pass filter
(L15–L17, C72–C77, C79) and is then applied to the anten-
na connector.
The bias current of the power amplifier (Q11) is controlled by
the APC circuit to stabilize the output power.
4-2-4 APC CIRCUIT (RF AND MAIN UNITS)
The APC circuit provides stable output power from the
power amplifier even when the input voltage or temperature
changes, and, selects HIGH, LOW or EXTRA LOW output
power. The APC circuit consists of an APC sensor and APC
control circuits.
• APC SENSOR CIRCUIT (RF UNIT)
The APC sensor circuit (D10, C80, C81, R53, R54) detects
the transmit output power level and converts it to DC voltage
as an “TXDET” signal. The detected signal is applied to the
APC control circuit on the MAIN unit.
• APC CONTROL CIRCUIT (MAIN UNIT)
The “TXDET” signal from the APC sensor circuit is applied
to the CPU (IC9, pin 96) to control the input voltage of the
pre-driver (RF unit; Q10) and the power amplifier (RF unit;
Q11). When the output power changes, the CPU (IC9) out-
puts “POWCON” signal to the D/A converter (RF unit; IC2).
And then “POWCON” signal controls the APC controller (RF
unit; IC3) to provide stable output power.
I. sens
Q10
IC3
APC ctrl
To 1st IF circuit
PWR amp.
Pre-Drive
D10, C80
Q11
D9
D11
Ant SW
Ant
D12
+
"POWCON" signal
from IC2 (pin 6)
"TXDET" signal
to CPU (pin 96)
"HV"
"T4V"
PWR det.
TX
RX
APC CIRCUIT
