Transmitter Circuits - Icom IC-E2820 Service Manual

SQUELCH CIRCUITS
• NOISE SQUELCH
A portion of FM-demodulated AF signals from the IF IC
(IC18, pin 9) are level-adjusted by D/A converter (IC8), and
passed throuhgh the noise fl tier (IC18 and some R and C) to
be fi ltered noise components (30 kHz and above signals) in
the AF signals. The fi ltered noise components are detected
in the IC18 and output from pin 13, then applied to the CPU
as "R_SQL" signal.
Then the CPU outputs "R_AF_MUTE" signal from pin 51 to the
speaker mute switch (Q102), according to the "R_SQL" signal
level. Thus the AF line is connected to the GND to turn the AF
output OFF.
• CTCSS/DTCS
CTCSS/DTCS signals in the demodulated AF signals from
the AF switch (IC16) are passed through the tone fi lter (Q42)
. The fi ltered CTCSS/DTCS signals are applied to the CPU
IC12) as "R_DTCS" signal.
The CPU (IC25) compares the applied signal and the set
CTCSS/DTCS, then outputs control signal as same as "NOISE
SQUELCH."
• DTMF
DTMF signals in the demodulated AF signals from the AF
switch (IC16) are passed through two AF switches (IC57
and IC58), then applied to the DTMF decoder (IC56) to be
decoded.
AF CIRCUITS
The AM/FM-demodulated AF signals from the AF switch
(IC12) are passed through the AF filter (Q48). The filtered
AF signals are applied to the electric volume (IC33) to be
adjusted its level. The level-adjusted AF signals are applied
to the dual AF power amplifiier (IC38) to obtain AF output
power level, then applied to the an external speaker via
external speaker jack (J8).
• MICROPHONE AMPLIFIER CIRCUITS
5VS
Q87
MIC signals from
C619 C620
the microphone
C623 C624
R527
HPF
R566
C636 C637
R560
Q88
MIC_SENC
IC29
R562
7
1
R545 5
3 6
4
1
AF
2
switch
IC28
Microphone
amplifier

4-2 TRANSMITTER CIRCUITS

MICROPHONE AMPLIFIER CIRCUITS
The audio signals from the microphone (MIC signals) are
applied to the microphone amplifi er (IC28) via J2 and HPF
(Q87). The amplified MIC signals are passed through the
microphone gain switch (Q88) and MIC mute switch (IC30),
then passed through or by-passed ALC amplifi er (IC32) via
AF switches (IC29 and IC52).
The MIC signals from the AF switch (IC52) are passed
though the HPF (IC48), LPF (IC48) and AF switch (IC51),
and then applied to the AF amplifier (IC48). The amplified
MIC signals are applied to the D/A converter (IC8) for level
(deviation) adjustment. The level adjusted MIC signals are
applied to the VCO as the modulation signals via modulation
signal selector.
<OPERATION ON THE LEFT BAND>
The modulation signals are applied to the variable capacitor
D147 of the left band VCO (Q111, D145–147) via the
modulation selector (IC9) and modulation mute switch
(Q109), and modulated. The modulated VCO output are
amplifi ed by the buffer (Q113) and LO amplifi er (IC44), and
applied to the transmit amplifi ers as the TX signal, via the LO
switches (D155, 157), LPF(L157, C818, 820) and attenuator
(R33, 37, 46).
TRANSMIT POWER AMPLIFIERS
TX signal from the attenuator (R33, 37, 46) is amplifi ed by
pre-drive (Q25) and drive (Q27) amplifi ers to obtain RF level
for power module (IC3). The amplifi ed TX signal is applied
to the power amplifier which is a VHF band PA module
composed by two power MOS-FETs. The power-amplified
TX signal is passed through the LPF, power detector,
antenna switch (D59) and LPF, before being applied to the
antenna connector (CHASSIS; J1).
APC CIRCUITS
A portion of the TX signal from IC3 is rectifi ed at the power
detector (D39, D47), and converted into the DC voltage
which is in proportion to the RF power, and applied to the
operational amplifier (IC4, pin 6). IC4 is an APC amplifier
for both of V/UHF bands. The TX power setting voltage
"PCON_V" from the D/A converter (IC1, pin 7) is applied
to the pin 5 as a reference. IC4 is rolled as a differential
amplifier which outputs voltage in inverse proportion to
rectified one. When the TX power increased, the rectified
voltage also increased, that causes the decrease of output
voltage of differential amplifier. The decrease of output
voltage of differential amplifi er causes the drop of the gate
voltage of IC3, Thus the TX power maintained to keep stable
level.
TX muting is carried out by TX mute SW (Q36) controlled
by "TX_mute" signal. Applying "TX_mute" signal to the base
terminal of Q36 to turn it ON, 8 V DC appears on the pin 6 of
IC4 and its output voltage downs to 0 V DC to inactivate IC3.
IC30
1
MIC
mute
switch
2
IC52
7
C862
1
6
C648 AF
R802
switch
AN6123MS
C645
INPUT DET
3 2
GND
4
OUTPUT VCC
5 1
IC32
ALC
4 - 4
HPF
LPF
R765
IC48
1 14
R785
2 13
3 12
C855
5 10
6 9
7 8
HPF
Modulation signals
Amplifier
to the modulation circuits
IC51
7
1
6
R767
AF switch