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4-1-5 AF AMPLIFIER CIRCUIT (MAIN unit)
The AF amplifier circuit amplifies the demodulated AF sig-
nals to drive a speaker.
AF signals from the FM IF IC (IC1, pin 9) are amplified at the
AF amplifier (IC7a) and then pass through the high-pass fil-
ter (IC6) whose characteristics are controlled by the
"AFHPF" line. When "AFHPF" is at a high level, the cut off
frequency is shifted higher to remove CTCSS or DTCS sig-
nals.
The filtered signals from IC6 (pin 14) pass through the AF
switching IC ( IC11, pin 8, 9) and are applied to the level con-
troller (IC12, pin 16, 15). The audio level controlled signals
are again passed through the AF switching IC (IC11, pin 2,
1), applied to the de-emphasis circuit (IC4) with frequency
characteristics of –6 dB/octave, and are then power ampli-
fied at the AF amplifier (IC9) to drive a speaker.
4-1-6 RECEIVER MUTE CIRCUITS
(MAIN and FRONT units)
•
NOISE SQUELCH
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 level controller (IC12, pin 24). The level con-
trolled noise components are output from pin 23 and are
applied to the active filter in IC1 (pin 8). Noise components
are amplified and output from pin 7 and are then applied to
the noise detector section (pins 10, 11). The detected noise
signals are rectified and output from pin 13 without smooth-
ing.
The noise signal (NOIS) from IC1 (pin 13) is applied to the
CPU (FRONT unit; IC1, pin 19). The CPU analyzes the
noise condition and outputs the RMUT signal via the I/O
expander IC (IC13) to toggle the AF mute switches (IC11a).
• CTCSS AND DTCS
The tone squelch circuit detects AF signals and opens the
squelch only when receiving a signal containing a matching
subaudible tone (CTCSS or DTCS). When tone squelch is in
use, and a signal with a mismatched or no subaudible tone
is received, the tone squelch circuit mutes the AF signals
even when noise squelch is open.
• AF circuit
AF signal
IC7b
from IC1
Amp.
AF mute
IC6
switch
8
9
HPF
6
IC11c
AF HPF
Volume control signal
TXC
A portion of the AF signals from the AF amplifier (IC7b)
passes through the low-pass filter (FRONT unit; Q5) to
remove AF (voice) signals and is applied to the CTCSS or
DTCS decoder inside the CPU (FRONT unit; IC1, pin 97) via
the "CTCIN" line to control the AF mute switch via the I/O
expander IC (IC13).
4-2 TRANSMITTER CIRCUIT
4-2-1 MICROPHONE AMPLIFIER CIRCUIT
(MAIN unit)
The microphone amplifier circuit amplifies the audio signals
from the microphone, within +6 dB/octave pre-emphasis
characteristics, to a level needed for the modulation circuit.
The AF signals from the microphone are amplified at the AF
(IC8b) and limiter (IC8a) amplifiers and are then passed
through the pre-emphasis circuit (R152, C180) which has +6
dB/octave pre-emphasis characteristics.
The pre-emphasized signals are passed through the splatter
filter (IC7a) and are then applied to the AF switching IC
(IC11, pins 11, 10).
The signals are applied to the level controller (IC12, pins 16,
15). The deviation level controlled signals are passed
through the AF switching IC (IC11, pins 3, 4), and are then
applied to modulation circuit as the "MOD" signal.
The narrow/wide switch (Q35) is connected to the input of
the splatter filter (IC7a) and switched by the "NWC" signal
coming from the I/O expander IC (IC13). When "NWC" is at
a high level, the narrow/wide switch (Q35) shifts the filter
cut-off frequency for narrow deviation selection.
4-2-2 MODULATION CIRCUIT (MAIN unit)
The modulation circuit modulates the VCO oscillating signal
(RF signal) using the microphone AF signals.
The "MOD" signals from the AF switching IC (IC11, pin 4)
change the reactance of D21 to modulate the oscillated sig-
nal at the TX VCO circuit (Q25). The modulated signal is
amplified at the buffer amplifiers (Q28, Q30) and is then
applied to the drive amplifier circuit.
AF mute
IC12
switch
16
15
2
1
13
IC11a
RMUT
4 - 2
AF power
IC4a
amp.
Amp.
IC9
Speaker
AF ATT
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