Icom IC-A210 Service Manual - page 22
3 - 2
RX AF CIRCUITS
The RX AF circuits consist of AF filters, AF amplifier, AF
power amplifier, etc., and amplify, filter the AF signals
demodulated by the demodulator circuits.
The demodulated AF signals are passed through the AM/
AF SW and LPF. The filtered AF signals are passed through
the MOD/AF SW and the switched capacitor filter (IC7)
which removes unwanted signals. The filtered AF signals
are passed through the HPF, and amplified by buffer, then
applied to the electric volume IC for audio level adjustment.
The level-adjusted AF signals are power-amplified by AF
power AMP to obtain AF output level. The power-amplified
AF signals are converted its impedance into 500 ohms by
the AF transformer, then applied to the connected headset
via the K-CONNECT UNIT or MB-113.
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; IC70 is OFF), demodulated AF
signals are applied to the cathode of D29 (switching diode)
only. If a pulse-type noise is inclued in the demodulated
AF signals, the cathode voltage of D29 becomes higher
than anode voltage, and D29 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 RX AF
circuits.
IF
AMP
IF
AMP
IF
AMP
BPF
CERAMIC
F lter
F lter
SW
BPF
CERAMIC
SW
AM
DET
IF
AGC 3
AGC 3
FM IF IC
16
9
14
13
9
10
5
9
2
IC39
IC43
FI 5
FI 6
IC45
Q26
Q25
Q24
Q21
IC81
FIL1
2nd LO
WX_DET
2ndIF for WX
IC81
IF
AGC 2
AGC 2
AGC voltage
BW 20 kHz
BW 9 kHz
HPF
LPF
LPF
BUFF
ANL
SW
MOD/AF
Electric
volume
Impedance
conversion
Headset
AM/FM
SW
7
1
4
7
1
4
8
5
9 12
1
1
12
5
7
1
D29
IC32
IC38
IC26
IC8
IC14
IC4
T4
AF
TRANS
AF
POWER
AMP
WX DET
IC7
IC 9
IC8
• 2ND IF CIRCUITS
SQUELCH CIRCUIT
The squelch circuit cuts off the AF output signals when
no RF signals are received. Extracting noise components
(approx. 30 kHz signals) in the demodulated AF signals, the
squelch circuit sets attenation level to infinity.
A portion of FM-demodulated AF signal from the FM IF
IC is passed through the noise filter to extract the noise
components (approx. 30 kHz signals) only. The noise
components are rectified to be converted into the pulse-
type signal by noise detector to produce DC voltage
corresponding to the noise level "NOISE" signal. The
"NOISE" signal is applied to the CPU (F; IC4, pin 90) and
compared with the reference level preset in the CPU.
If the CPU interprets that the noise level is higher than
preset one, the CPU sends the serial data "SDA_VOL2" to
the electric volume IC (IC14), via the expander (IC49) and
dual gate logic IC (IC46). The serial data sets the attenation
level to infinity to close the squelch.
Noise
AMP
Fc=30 kHz
Active filter
From IF IC (Pin16)
To RX AF circuits
Noise detector
Rectify
“NOISE”
NOISE SQUELCH DIAGRAM
• RX AF CIRCUITS
2nd
Mixer
FM
Detector
16
Noise Det.
Noise
AMP
RSSI
11
10
Noise
Filter
2nd IF
AMP
IF IC (TA31136FNG) BLOCK DIAGRAM
9
14
12
2
8
7
3
5
• Weather Alert (WX) signals
FM IF demodulator IC “TA31136FNG” contains whole of the
2nd IF circuits and FM demodulator circuit too.
The amplified 1st IF signal is applied to the FM IF IC, and
converted into the 2nd IF signal, by being mixed with the
2nd LO from the X2, at internal 2nd mixer. The converted
2nd IF signal is filtered by external 2nd IF filter (FI6), and
amplified by external 2nd IF AMP. The amplified 2nd
IF signal is applied to the FM IF IC again (pin 5), and
saturation-amplified by internal 2nd IF AMP. The amplified
2nd IF signal is FM-demodulated by the quadrature
detector.
The demodulated AF signals are applied to the RX AF
circuits.
AGC (Automatic Gain Control) CIRCUIT
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 the signal and
automatically adjusting the gain of the 2nd IF AMPs to
maintain an approximately constant average level of the
received signals.
A portion of 2nd IF signal is converted into DC voltage (AGC
voltage) which is in proportion to the RX signal level, at the
AM detector (Q21). The AGC voltages “AGC_2” and “AGC_3”
are applied to the emitter terminals of 2nd IF AMPs, to
control the gain of these AMPs for stable 2nd IF signal level.