Icom IC-A14 Service Manual page 8

AGC (Automatic Gain Control) CIRCUITS
(
For receiving AM signals
The AGC circuit effectively reduces the RX signal level if
the signal is strong, and raises it when it is weaker. The
AGC circuit detects the overall strength of RX signals and
automatically adjusts the gain of RF and IF AMPs, and
attenuation level of the ATT to maintain an approximately
constant average level of the received signals (=the AF
output level).
• 2nd IF AMP gain control
The DC component in the AM-demodulated signals whose
signal level is in proportion to the RX signal (RF signal)
level, is fedback to the 2nd IF AMP in the IF IC. The DC
voltage controls the gain of 2nd IF AMP to keep the 2nd IF
signal level constant.
• RF AMP and 1st IF AMP gain control
The AGC voltage "RF AGC" which is in proportion to the RX
signal (RF signal) level is generated by the internal 2nd IF
AMP (similar as RSSI voltage). The AGC voltage "RF AGC"
is output pin 17 and applied to the AGC line drivers (Q10
and Q80) via AGC SW (Q85). Q10 controls the gain of RF
AMP (Q11), and Q80 controls the attenuation level of ATT
(D75–77) for stable 2nd IF signal level.
• RF AMP AND 1ST IF AMP GAIN CONTROL
RF AGC from IF IC
Q10
AGC
AM P
D14,D15
D13,50
L21,22
C67,71,73,326
Q11
RF
BPF
AMP
IC17
DAC
RX AF CIRCUITS
The RX AF circuits consist of AF fi lters, AF amplifi ers, etc.,
and amplify, fi lter the demodulated AF signals.
The demodulated AF signals from the IF IC are passed
through the AM/FM SW (IC2), and applied to the buffer
(IC18a). The AF bufffer-amplified AF signals are passed
through the 5-pole LPF (IC18b/c) to filter audio band signals
only, then buffer-amplified by another AF bufffer (IC18d).
The buffer-amplified AF signals are level-adjusted (audio
output level) by DAC (IC17_7), then applied to the AF power
AMP (IC31) via AF Pre-AMP (IC20). The AF signals are
power-amplified to obtain AF output level, then applied to
the internal speaker or external speaker via [SP] jack (J5).
• AF CIRCUITS (for RX and TX)
I C18c
LPF
Control lines
from the CPU (IC26)
)
Q80
RF AGC AGC
AM P
L18,69,
L52,C235
C60,327,780,781
BPF BEF ATT
D75
From the AM/FM SW (IC2)
IC17 _5
I C18b
I C18a
DAC
LPF
BUFF
(MIC VR)
IC18d
IC17 _7
DAC
BUFF
(VOLUME)
SQUELCH CIRCUITS
The squelch circuit cuts off the AF output signals when no
RF signals are received.
• For receiving AM signals (RSSI SQUELCH)
The "RSSI" signal which is in proportion to the RX signal
level is applied to the CPU (IC26, pin 30). The CPU
interprets that RX signals are present or absent according
to the voltage of "RSSI" signal level.
• For recieving FM (WX) signals (NOISE SQUELCH)
A portion of FM demodulated AF signals is filtered by active
BPF (IC1; pins 12, 13, R790, 792, 793, C736, 737, 744)
which filters and amplifies only noise components (around
10 kHz signals) in the demodulated signals. The amplified
noise components are detected into pulse-type signal by
D79, then applied to the CPU (IC26) via the noise AMP
(IC28). The CPU interprets that RX signals are present or
absent by counting the pulse of noise signal.
If the CPU interprets that the RX signals are absent, the
CPU sends the "DETMUTE" signal to the AM/FM mute SW
(IC2). Then the AM/FM SW cuts off the AF line to close the
squelch.
To RX AF circuits
From Pin16
ANL (Automatic Noise Limiter) CIRCUIT
The ANL circuit is a limiter which reduces pulse-type noises
by cutting off the AF line temporarily. When the ANL function
is activated (ANL SW (Q21) is ON), demodulated AF signals
are applied to the cathode of D82 (switching diode) only. If a
pulse-type noise is included in the demodulated AF signals,
the cathode voltage of D82 becomes higher than that of
anode, and D82 turns OFF to cut-off the AF line. Thus, if the
RX signals contains pulse-type noise, the demodulated AF
signals are not applied to the AM/FM SW (IC2).
IC15 IC17 _6
Q86
DAC
MIC
ALC
AM P AM P
(MIC VR)
ALC
Q52
CONTROL
Q82,Q83,Q36
SPCON
AFC AF AMP
CONTROL
IC31
IC20
PRE PWR
AMP
AMP
4 - 2
NOISE SQUELCH DIAGRAM
Fc=10 kHz
Rectify
"NOISE"
Pin18 Pin13
Active filter
D79
From the ext. microphone
Q51
Microphone
MIC
MC1
MUT E
Q 79
D67
MI C
CONTROL
To the int. speaker
To the ext. speaker