4 - 2
4-1-5 AF AMPLIFIER CIRCUIT
(MAIN-E AND FRONT UNITS)
The AF amplifier circuit amplifies the detected signals to
drive a speaker. The AF circuit includes an AF mute circuit
for the squelch.
AF signals from IC2 (pin 9) are applied to the de-emphasis
circuit (R231, C232). The de-emphasis circuit is an integrat-
ed circuit with frequency characteristic of –6 dB/octave.
The integrated signals pass through the bandpass filter
(Q31, R232, R234–R237, C233, C234) to suppress unwant-
ed noise signals.
The filtered signals are applied to the AF mute circuit (IC4),
and then passes through the [VOLUME] control (VR board;
R1) via the “VOL1” signal. The signals are applied to the AF
power amplifier (IC9, pin 1). The output signal from IC9 (pin
4) drives the internal (external) speaker.
4-1-6 SQUELCH CIRCUIT (MAIN-E UNIT)
A 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 (IC2, pin 9)
pass through the squelch adjustment pot (R214), and are
then applied to the active filter section (IC2, pin 8). The
active filter section filters and amplifies noise components.
The amplified signals are converted into the pulse-type sig-
nals at the noise detector section. The detected signals out-
puts from pin 14 as the “SQL” signal.
The “SQL” signal from the FM IF IC is applied to the CPU
(LOGIC-E board; IC1, pin 90). The CPU analyzes the noise
condition and outputs as the “RMUTM” signal to the AF mute
switch (IC4).
4-2 TRANSMITTER CIRCUITS
4-2-1 MICROPHONE AMPLIFIER CIRCUIT
(MAIN-E UNIT)
The microphone amplifier circuit amplifies audio signals with
+6 dB/octave pre-emphasis from the microphone to a level
needed at the modulation circuit.
The AF signals from the microphone are amplified at the
microphone amplifier (IC7, pins6, 7) via the analog switch
(IC4, pins 5, 6). A capacitor (C302) and resistor (R303) are
connected to the amplifier to obtain the pre-emphasis char-
acteristics.
The amplified signals are passed through the MIX circuit
(IC7, pins 1, 2) to do amplitude limiting, and are then applied
to the splatter filter (IC8, pins 1, 3) to suppress unwanted 3
kHz or higher signals. The filtered signals are then applied
to the modulation circuit.
4-2-2 MODULATION CIRCUIT (MAIN-E UNIT)
The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone audio signals.
Audio signals from the splatter filter (IC8, pins 1, 3) pass
through the frequency deviation adjustment pot (R327) and
are then applied to the modulation circuit (D3, D4) to change
the reactance of D3, D4 and modulate the oscillated signal
at the VCO (Q4, Q5).
4-2-3 DRIVE AMPLIFIER CIRCUIT (MAIN-E UNIT)
The drive amplifier circuit amplifies the VCO oscillating sig-
nal to a level needed at the power amplifier.
The VCO output is buffer-amplified by Q6 and Q7, and is
then applied to the Tx/Rx switch (D7). The transmit signal
from the Tx/Rx switch is amplified to the pre-drive (Q10) and
YGR (Q12) amplifiers to obtain an approximate 50 mW sig-
nal level. The amplified signal is then applied to the RF
power amplifier (IC3).
AF
MUTE
BPF
IC9
R1
IC4
External SP
Internal SP
(FRONT unit)
Vcc
PWR
AMP
"DET" signal from the
FM IF IC (IC2, pin 9)
de-emphasis
circuit
R231
C232
(VR board)
• AF AMPLIFIER CIRCUIT