Tx Af Circuits; Squelch Circuit; Modulation Circuit; Apc Circuit - Icom IC-2300H Service Manual Addendum

RX AF CIRCUITS
The demodulated AF signal from the MAIN UNIT is passed
through the HPF (IC3c), AF line SW (IC6), LPF (IC3d), AF
line SW (IC6), another AF line SW (IC28) and the electronic
volume (IC25) for adjustment in level. The level-adjusted AF
signal is applied to the MAIN UNIT.
The AF signal is applied to the AF AMP (IC7), through the
speaker mute SW (Q20). The amplifi ed AF signal is output
to the internal speaker through the speaker mute SWs (Q33,
Q35–Q37), or external speaker through the speaker mute
SW (Q36, Q37) and the [SP] connector.
• RX AF CIRCUITS
LOGIC UNIT
VOLOUT
MICAF/
IC25
IC28
(VOUT2)
RXAF
ANALOG ELEC.
SW VOL
TXAF/
RXAF
IC6
LPF
ANALOG
SW
TXAF/
IC3b
RXAF
IC3c
-> 2nd IF circuits
HPF

SQUELCH CIRCUIT

The squelch circuit cuts off the AF output signal while no RF
signal is received. Detecting noise components in the demod-
ulated AF signal, the squelch circuit stops audio signal from
being emitted.
A portion of the demodulated AF signal from the IF IC (IC3) is
passed through the D/A converter (IC5) for level (=threshold)
adjustment. The level-adjusted AF signal is passed through
the noise fi lter (IC3, pins 7, 8 and R37, R39, R40, R191, C52,
C53, C308) to fi lter only the noise components (approximate-
ly 30 kHz signals). The noise components are rectifi ed by the
noise detector, resulting in a DC voltage corresponding to the
noise level.
When the noise level is higher than the preset one, the inter-
nal comparator sets the "NOIS" signal to the CPU to "High,"
and then the CPU controls the speaker mute SW (Q20) to
stop audio signal from being emitted.
MAIN UNIT
Q20 Q36-Q38
IC7
J3
AF
SP SP AF OUT
AMP
MUTE SW SP
SP1

4-2 TX AF CIRCUITS

The AF signal from the microphone (MIC signal) is applied to
the MIC AMP (IC3a), through the [MIC] jack. The amplifi ed
MIC signal is passed through the AF line SW (IC28) and the
electronic volume (IC25) for adjustment in level. The level-
adjusted AF signal is applied to the IDC (IC3d) which func-
tions as the splatter fi lter.
The fi ltered MIC signal is passed through another AF line SW
(IC6) and LPF (IC3b), then fed back to the AF line SW (IC6),
and then applied to the modulation circuit on the MAIN UNIT.
• TX AF CIRCUITS
EXT MIC
MIC MICAF MIC
AMP
JACK
LOGIC UNIT

MODULATION CIRCUIT

The MIC signal from the LOGIC UNIT is applied to the D/A
convertor (IC5) for adjustment in level (=deviation). The level-
adjusted MIC signal is applied to the TX VCO (Q13, D7–D9)
as a modulation signal.
The modulated TX VCO output signal is amplifi ed by the LO
AMP (Q17), and then applied to the TX AMP circuit as the TX
signal, through the buffer (IC10), LO SW (D13) and another
buffer (Q21).
• MODULATION CIRCUITS
IC1
X1
PLL
REF
IC
OSC
(15.3MHz)
MAIN UNIT
TX AMP CIRCUITS
The TX signal is sequentially amplifi ed by the pre-drive (Q22),
drive (Q26) and power AMP (Q29). The amplifi ed TX signal is
applied to the antenna, through the ANT SW (D24, D25) and
LPF (L36, L39, L41, L42, C291, C295, C296, C298, C300,
C303, C304, C306, C307).

APC CIRCUIT

The voltage produced on both sides of L39 is rectifi ed by D27
and D33, and used as the TX power sensing voltage. The
voltage is applied to the APC AMP (IC9b), and the output
voltage controls the gate voltages of Q29, Q26 and Q22.
The voltage difference between both sides of R157 is detect-
ed by IC9d, to control the TX power when abnormal current
is detected.
• TX AMPLIFIER AND APC CIRCUITS
D12-D13
Q21
-> TX VCO
TX/RX
BUFF
SW
MAIN UNIT
4 - 2
IC25
MICAF
MICAF/
IC28
ELECTRONIC
AF SW RXAF
VOLUME
TXAF
(VOUT1)
MODIN
TXAF/RXAF
TXAF IC6
IDC
TXAF/RXAF
AF SW
IC3d
Q17 IC10
Q13,D7-D9
LOOP LO
BUFF
FIL AMP
MOD
Q11
W/N
MOD
CTRL.
IC5
MODIN
REFMOD
D/A
D24-D25
D29-D32
Q26
Q22
PRE PWR
ANT
DRIVE
DRIVE
AMP
SW
AMP
Q29
RX circuits
T2_APC(D/A)
APC
TXMUTE(EXP)
CTRL
IC9b,Q31
IC3b
LPF
ANTENNA
LPF
PWR
DET
D27,D33