Transmitter Circuits - Icom IC-A14 Service Manual

4-2 TRANSMITTER CIRCUITS

TX AF CIRCUITS
The TX AF circuit consists of microphone amplifier (MIC
AMP), ALC and AF fi lters. ALC (Automatic Level Control) is
an amplifi er which reduces its gain automatically to prevent
over deviation.
The audio signals from the connected headset's microphone
(MIC signals) are passed through the MIC mute SW (Q51)
and the DAC (IC17_6) which adjusts the MIC signal level.
The level-adjusted MIC signals are amplified by the MIC
AMP (Q86), then applied to the ALC AMP (IC15).
• AF CIRCUITS (for RX and TX)
I C18c
LPF
Control lines
from the CPU (IC26)
AM MODULATION CIRCUITS
The AM modulation circuits mudulate the carrier with the
MIC signals (=modulation signals).
The level-adjusted modulation signals from the MAIN UNIT
are applied to the APC AMP. The output voltege of APC
AMP which controls the gain of drive (Q3) and power (Q2)
AMPs swings corresponding to the amplitude of modulation
signals, thus the AM modulation is obtained.
TX AMPLIFIERS
The TX amplifers consist pre-driver, driver and power
amplifi ers, and amplify the VCO output to the transmit output
level.
The TX VCO (Q59, D45, 46) output is applied to the pre-
driver via buffers (Q60 and Q28), LO switch (D6) and ATT,
and amplified by the pre-driver (Q4) to the driver (Q3) input
level. The amplified TX signal is amplified by driver (Q3) to
the power AMP (Q2) input level, then power-amplified by the
power AMP (Q2) to the TX output level.
The power-amplified TX signal is passed through the LPF,
ANT SW and LPF and entered to the ANT UNIT, then
applied to the antenna via the LPF.
• AM MODULATION, TX AMPLIFIERS AND APC CIRCUITS
From the AM/FM SW (IC2)
IC17 _5
I C18b
I C18a
DAC
LPF BUFF
(MIC VR)
IC18d
IC17 _7
DAC
BUFF
(VOLUME)
Q10
AGC
AM P
D13,50
L18,69,
,326 L52,C235
C60,327,780,781
Q11
RF
BPF
AMP
MAIN UNIT
TXV
R36-38
Q4
signal
PRE
ATT
DRIVE
The ALC (Automatic Level Control) AMP is an amplifier
which controls its gain automatically according to the MIC
signals level. The ALC provides stable modulation signals
by limiting the amplitude of MIC signals to prevent the over
deviation.
The output signals of ALC are applied to another DAC
(IC17_5) for level (deviation) adjustment. The level-adjusted
MIC signals are passed through the buffer (IC18a) and
5-pole LPF (IC18b/c) to filter audio band signals only. The
filtered MIC signals are entered to the PA UNIT, and applied
to the APC AMP (PA: IC27) as the modulation signals.
IC15 IC17 _6
Q86
MIC
ALC
AM P
AM P (MIC VR)
ALC
Q52
CONTROL
Q82,Q83,Q36
AFC AF AMP SPCON
CONTROL
IC31
IC20
PRE PWR
AMP
AMP
APC CIRCUIT (PA UNIT)
The APC (Automatic Power Control) circuit stabilizes transmit
output power to prevent transmit output power level change,
which is caused by load mismatching or heat effect, etc.
The power detector rectifi es a portion of the TX signal and
converts it into DC voltage which is in proportion to the
transmit output power level. The detected voltage is applied
to the input terminal (pin 3) of APC AMP (IC27; as a com-
parator). The TX power setting voltage "POW" is applied to
another input terminal (pin 1) as the reference voltage.
The comparator compares the detected voltage and refer-
ence voltage, and the difference of voltage is output from
output terminal. The output voltage controls the bias of the
driver and power amplifiers to reduce/increase the gain of
these amplifi ers for stable TX output power.
AM P
D1,D8
ANT
BEF ATT
SW
D75
HV
L6,46
L5
C19,212,297,
Q3
Q2
C12,14
791,792,797
PWR
LPF
DRIVE LPF
AM P AMP
IC27
3
4
APC 1
AMP 1
MOD PCON
4 - 3
From the ext. microphone
Q51
DAC
MIC
MUT E
Q 79
D67
MI C
CONTROL
To the int. speaker
To the ext. speaker
ANTENNA
ANT UNIT
L3 L1,45
C5-7 C210-212
LPF LPF
PA UNIT
POWER D2,D3,D74
D75,D76,D77
DET
Microphone
MC1