Icom IC-F4029SDR, Service Manual

Page: 21 / 48

The AF signal from the analog switch (IC3, pin 11) are
applied to the volume buffer amplifi er (IC6, pin 9). The buffer-
amplifi ed AF signals are adjusted its level (=audio level) by
volume control pot (R315), then applied to the AF power
amplifier (IC15, pin 4) and amplified to the 0.5 W of audio
output power (max., at 8
Ω
load).
The power-amplifi ed AF signals are output from pin 10, then
applied to the speaker (CHASSIS UNIT; SP1) or an external
speaker via [SP] jack (J2).
4-1-6 SQUELCH CIRCUITS
4-1-6-1 NOISE SQUELCH
The squelch mutes the AF output signals 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 demodulated AF signals from the FM IF IC
(IC9, pin 9) are applied to the D/A converter (IC12, pin 24)
for level (=squelch threshold) adjustment. The level-adjusted
AF signals are output from pin 23 and passed through
the noise filter (IC9, pins 7, 8, R42, R44−R46, C69, C70,
C413). The filtered noise signals are amplified the noise
components only at the noise amplifi er.
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 converted signal is applied to the
CPU (IC22, pin 75). Then the “RMUTE” signal from the CPU
(IC22, pin 96) to the RX mute switch (Q32, Q33) and analog
switch (IC3, pins 12, 13) becomes “Low” according to the
“NOIS” signal level to cut off the AF line.
At the same time, the “AFON” signal from the CPU (IC22,
pin 70) to the AF amplifier controller (Q41, Q42, D21, D23)
becomes “Low”, and the controller turns the AF power
amplifi er (IC15) OFF.
4-1-6-2 TONE SQUELCH
• CTCSS/DTCS
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 sub audible 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 are passed through
the active LPF (Q39) to filters CTCSS/DTCS signal. The
fi ltered signal is applied to the CPU (IC22, pin 46). The CPU
compares the applied signal and the set CTCSS/DTCS, then
the CPU controls the status (“Low” or “High") of “RMUTE” and
“AFON” signals as same as “NOISE SQUELCH."
• DTMF
DTMF signals in the demodulated AF signals are passed
through the LPF (IC6, pins 5, 7) to remove unwanted compo-
nents (voice signals), then applied to the CPU (IC22, pin 45)
and decoded.
4-2 TRANSMITTER CIRCUITS
4-2-1 MICROPHONE AMPLIFIER CIRCUITS
(MAIN-C UNIT)
The AF signals from the microphone (MIC signals) are
fi ltered and level-adjusted at microphone amplifi er circuits.
• MIC SIGNALS
MIC signals from the microphone are applied to or bypassed
the ALC (Automtic Level Control) circuit (IC24, pins 3, 5)
and the A/D switch (IC25, pins 1, 7), then applied to the D/A
converter (IC12, pin 1).
In digital mode, the MIC signals are applied to the ALC
circuit which keeps the level of MIC signals constant.
The level-adjusted MIC signals are output from pin 2, and
passed through the MIC mute switch (Q31), HPF (IC5, pins
13, 14) and gain switch (Q34, Q35) which controls the gain
of MIC amplifi er (IC5) according to the Analog/Digital mode,
then applied to the MIC amplifi er (IC5, pin 9). The amplifi ed
MIC signals are output from pin 8, and passed through the
analog switch (IC3, pins 3, 4), AF mixer (IC5, pins 6, 7)
where the MIC signals and tone signals are mixed with.
• TONE SIGNALS
The CTCSS/DTCS signals are generated by the CPU (IC22)
and output from pins 19−21. The output signals are passed
through the 3 registers (R222–R224) to change its wave form.
The wave form changed CTCSS/DTCS signals are passed
through the LPF (IC7, pins 8, 10), tone filter switch (Q40) and
D/A converter (IC12, pins 11, 12) for level adjustment. The level
adjusted CTCSS/DTCS signals are then applied to the AF mixer
(IC5, pin 6).
DTMF signals are generated by the CPU (IC22) and output
from pin 43. The output DTMF signals are passed through two
LPF’s (IC6, pins 1, 3 and pins 12, 14), then applied to the AF
mixer (IC5, pin 6).
The mixed AF signals are output from pin 7 of the AF mixer (IC5)
and passed through the analog switch (IC3, pins 8, 9), then
applied to another AF amplifier (IC7, pin 6). The amplified
AF signals are output from pin 7, and applied to the D/A
converter (IC12, pin 9) to be adjusted its level (=deviation).
The level-adjusted MIC signals are then applied to the
modulation circuits as the modulation signals.
4-2-2 MODULATION CIRCUITS
The modulation circuits modulates the VCO oscillating signal
using the modulation signals.
The modulation signals from the D/A converter (IC12, pin
10) are applied to the D12 at the TX VCO (Q16, D10, D13)
to modulate the VCO oscillating signal by changing the
reactance of D12.
The modulation signals are also applied to the reference
frequency oscillator (X2) via D/A converter (IC12, pins
15, 16) and the buffer (IC7, pins 12, 14), to ensure the
modulation of low frequency components of the modulation
signals.
The modulated VCO output is buffer-amplifi ed by Q15 and
Q29, then applied to the transmit amplifi ers as the transmit
signal via TX/RX switch (D16=ON, D17=OFF).
4 - 2