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The TX/RX HPF filters out 250 Hz and lower audio signals,
and the de-emphasis circuit obtains –6 dB/oct of audio
characteristics. The expander expands the compressed audio
signals and also noise reduction function is provided.
The AF signals are then level adjusted at the volume controller
(VR4) and amplifi ed at the amplifi er (RXA2). The amplifi ed AF
signals are output from pin 20 and passed through another de-
emphasis circuit (IC13, pins 2, 15), and then applied to the D/A
converter (IC4, pin 16) for level adjustment via the AF mute
switch (IC14, pins 3, 4).
The level-adjusted AF signals are applied to the AF amplifi er
(FRONT UNIT; IC509, pin 2). The amplified AF signals are
output from pin 1, and applied to the AF power amplifier
(IC508, pin 17) to obtain 0.5 W (typical) of AF output power.
The power-amplified AF signals are output from pin 18, and
then applied to the internal speaker.
When an external speaker-microphone or headset is attached
to the multi-connector (JACK UNIT; MP801), the AF signals
from the AF amplifier (IC509, pin 1) are applied to the AF
power amplifier (IC508, pin 14). The power-amplified AF
signals are then output from pin 13, and applied to the multi-
connector (JACK UNIT; MP801, pin 7).
5-1-5 SQUELCH CIRCUIT
• NOISE SQUELCH
The squelch mutes the AF output signals when no RF signals
are received. By detecting noise components (30 kHz and
higher signals) in the demodulated AF signals, the squelch
circuit toggles the AF power amplifi er ON and OFF.
A portion of the demodulated AF signals from the FM IF IC
(IC3, pin 9) are applied to the D/A converter (IC4, pin 1) for
level adjustment (squelch threshold adjustment). The level-
adjusted AF signals are output from pin 2 and passed through
the noise filter (IC3, pins 7, 8, R121−R124, C216−C218). The
fi ltered noise signals are 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 13 as the
“NOIS” signal. The signal is applied to the CPU (IC18, pin 37).
Then the CPU outputs serial data to the expand IC (FRON UNIT;
IC505, pin 3), and the expand IC outputs “AFON” signal from pin
4 according to the “NOIS” signal level, to the AF power amplifi er
controller (FRONT UNIT; Q501, Q502, D508). The AF power
amplifier controller toggles AF power amplifier (FRONT UNIT;
IC508, IC509) ON and OFF according to the “AFON” signal.
• TONE SQUELCH
The tone squelch circuit detects tone signals and opens the
squelch only when receiving a signal containing a matched
subaudible tone. When the 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 the
noise squelch is open.
• CTCSS/DTCS
A portion of the demodulated AF signals are passed through
the active LPF (Q28) to filters CTCSS/DTCS signal. The filtered
signal is applied to the CPU (IC18, pin 46). The CPU compares
the applied signal and the set CTCSS/DTCS, then output the
serial data to the expand IC (FRON UNIT; IC505, pin 3), and
the expand IC outputs “AFON” signal from pin 4 to the AF
power amplifi er controller (Q501, Q502, D508).
• 2/5 TONE
2/5 tone signals in the demodulated AF signals are passed
through the LPF in the base band IC (IC5) and output from
pin 21, then applied to the CPU (IC18, pin 45) and decoded.
• DTMF
DTMF signals in the demodulated AF signals are passed through
the LPF in the base band IC (IC5) and output from pin 21, then
applied to the DTMF decoder (IC10, pin 1) and decoded.
5-2 TRANSMITTER CIRCUITS
5-2-1 MICROPHONE AMPLIFIER CIRCUITS (MAIN UNIT)
The AF signals from the microphone (MIC signals) are fi ltered
and level-adjusted at microphone amplifi er circuits.
MIC signals from the microphone are passed through the
microphone switch (FRON UNIT; Q515). The microphone
switch selects the AF signals from the internal microphone
(FRON UNIT; MC501) or from an external microphone.
MIC signals from the microphone switch (FRON UNIT; Q515)
are applied to the microphone amplifi er (FRON UNIT; IC509,
pin 6), and amplified AF signals are output from pin 7, and
passed through the pre-emphasis circuit (IC13, pins 4, 5)
to obtain +6 dB/oct of frequency characteristic. The pre-
emphasized MIC signals are then applied to the microphone
amplifi er (IC9, pin 9). And the amplifi ed MIC signals are output
from pin 8, and applied to the D/A converter (IC4, pin 9) for
level adjustment (=microphone sensitivity adjustment). The
level-adjusted MIC signals are output from pin 10, and applied
to the ALC (Automatic Level Control) circuit (IC15, pin 3)
which limits the amplitude of the MIC signals to prevent over
deviation. The amplitude-limited MIC signals are output from
pin 5, then applied to the base band IC (IC5, pin 3).
The applied MIC signals are amplifi ed at the amplifi er (TXA1),
and level adjusted at the volume controller (VR1). The level
adjusted MIC signals are applied or bypassed the compressor
section, pre-emphasis section, TX/RX HPF, de-scrambler, limiter,
splatter, in sequence, then applied to another volume controller.
The compressor compresses the MIC signals to provide high S/N
ratio for receive side, and the pre-emphasis obtains +6 dB/oct
audio characteristics. The TX/RX HPF filters out 250 Hz and
lower audio signals, the limiter limits its level and the splatter
fi lters out 3 kHz and higher audio signals.
The fi ltered MIC signals are level adjusted at another volume
controller (VR2), and then output from pin 7 via smoothing
fi lter (SMF).
5 - 2
Scrambler/
De-scrambler
TX/RX
HPF
Pre-
emphasis
Limiter Splatter VR2
Expander
VR4
RXA2
SMF
De-
emphasis
Com-
pressor
VR1
(HPF)
RX
LPF
VR3
(HPF)
7 MOD
18
19
20 SIGNAL
3TXIN
• BASE BAND IC BLOCK DIAGRAM
23RXIN
21SDEC
TXA1
RXA1
BASE BAND IC (IC5)