4 - 2
•
SQUELCH CIRCUIT (RF AND LOGIC UNITS)
- NOISE SQUELCH -
The noise squelch mutes the AF output when no RF signals
are received. By detecting noise components (around 30 kHz
signals) in the demodulated AF signals, the squelch circuit
toggles the mute switch and AF power amplifi er ON and OFF.
A portion of the AF signals from the IF IC (RF UNIT; IC2,
pin 9) are passed through the D/A converter (RF UNIT;
IC10, pins 23, 24) for level adjustment. The level adjusted
AF signals are applied to the noise filter section (RF UNIT;
IC2, pins 7, 8; R86−R88, C121, C122) which allows only
noise components to pass. The filtered noise components
are amplified at the noise amplifier section, and rectified
into the DC voltage which is proportional to the noise signal
level, at the noise detector section, then output from pin 13
as the “NOIS” signal.
The “NOIS” signal is applied to the CPU (LOGIC UNIT; IC8,
pin 17), and the CPU compares the “NOISE” signal and set
reference level (squelch threshold level). When the input
“NOIS” signal is higher than reference level, the CPU cuts
off “AFON” signal (from pin 4) to the AF power amplifier
control circuit (LOGIC UNIT; Q14−Q16) to deactivate the AF
power amplifier (LOGIC UNIT; IC23).
- TONE SQUELCH -
The tone squelch circuit detects tone signals and opens the
squelch only when receiving a signal containing a matched
sub audible 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.
A portion of the demodulated AF signals is passed through
the LPF (LOGIC UNIT; Q53) to filters CTCSS/DTCS signal.
The filtered signal is applied to the CPU (LOGIC UNIT; IC8,
pin 93). The CPU compares the applied signal and the set
CTCSS/DTCS, then the CPU controls the status (“Low” or
“High”) of “AFON” signal as “
• NOISE SQUELCH
.”
HPF
LPF
Pre-
emph.
1
6
10
8
7
IC3
IC3
IC4
“MMUTE”
“RMUTE”
• AF and MIC amp. CIRCUITS
Demodulated AF signals
From IF IC (IC2, pin 9)
From the microphone
AF signals
“MICS”
RF UNIT
LOGIC UNIT
“AFON”
D/A
converter
IC4
IC10
IC3
AMP
4
11
22
10
IC23
SP
2
4
3
10
13
14
11
12
21
9
8
VCO
4-2 TRANSMITTER CIRCUITS
•
MICROPHONE AMPLIFIER CIRCUITS (LOGIC UNIT)
The AF signals from the microphone (MIC signals) are
fi ltered and level-adjusted at microphone amplifi er circuits.
Audio signals from the microphone (MC1) are applied to IC3
(pin 10) which combines a microphone amplifier and pre-
emphasis circuit. The MIC signals are amplified and filtered
to 6 dB/oct of frequency characteristic by IC3.
The amplified and emphasized MIC signals are output
from the pin 8, and passed through the splatter filter (IC3,
pins 13, 14) which suppresses 3 kHz and higher audio
components, via the TX/RX switch (IC4, pins 3, 4; 8, 9).
The filtered MIC signals are then applied to the modulation
circuit.
•
MODULATION CIRCUITS (RF UNIT)
- MIC SIGNALS -
The modulation circuits modulates the VCO oscillating signal
using the modulation signals.
The MIC signals from AF circuits are passed through
the D/A converter (IC10, pins 21, 22) for level (deviation)
adjustment. The level adjusted MIC signals are applied to
the variable capacitor (D61) in the VCO (Q76, D59−D61),
and modulate the VCO oscillating signal by changing the
reactance of D61. The modulated VCO output are buffer-
amplified by Q75 and Q74, then applied to the transmit
amplifier as a transmit signal.
- TONE SIGNALS -
CTCSS/DTCS signals are output from CPU (LOGIC UNIT;
IC8, pin 2) and passed through the LPF (R224, R227,
C219), then applied to the LPF (as the AF mixeRF UNIT;
IC3, pin 13) to be mixed with MIC signals. The mixed
CTCSS/DTCS signals are applied to modulation circuits as
“
• MIC SIGNALS
.”
•
TRANSMIT AMPLIFIERS (RF UNIT)
The transmit signal from the buffer amplifi er (Q74) is amplifi ed
to the transmit output level by the transmit amplifi ers.
The buffer-amplified VCO output signal is amplified by
transmit amplifiers; pre-drive (Q3), drive (Q2) and power
(Q1) amplifiers in sequence, after being passed through the
TX/RX switch (D3).
The power-amplified transmit signal is passed through the
power detector (D32, D33), antenna switch (D1) and LPF
(as a harmonic filter ; L1, L2, C1−C5), before being emitted
from the antenna.
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