2Nd If Circuits; Weather Alert (Wx) Signals; Agc (Automatic Gain Control) Circuit; Rx Af Circuits - Icom IC-A210 Service Manual

• 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.

• 2ND IF CIRCUITS

IC81
13 14
AGC 3
IF
AGC 3
IC81
IF
AMP
10 9
IF
Q24
AGC 2
AGC 2
Q21
AM
DET

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.
• RX AF CIRCUITS
D29 IC32 IC38
1 7
1 4
AM/FM
ANL LPF
SW
WX DET
MOD/AF
IC26
Headset
<2nd IF=450 kHz>
IC45 FI 5
CERAMIC
IF IF F lter
AMP AMP SW
Q25 Q26
FI 6
CERAMIC
2ndIF for WX
IC 9
7
IC8 IC8
4 8 5 9 12
LPF HPF BUFF
1
SW
IC7
T4
IC4
AF
5
AF
POWER
TRANS
AMP
Impedance
conversion

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.
BW 9 kHz IC43 IC39
9 16
F lter
FM IF IC
BPF SW
5 9 2
BW 20 kHz
BPF
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.
To RX AF circuits
From IF IC (Pin16)
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.
IC14
1
1 Electric
volume
12
7
3 - 2
IF IC (TA31136FNG) BLOCK DIAGRAM
8
7 5 3
2nd
Noise
Noise
Mixer
Filter
AMP
2nd IF
FM
AMP
Detector
Noise Det.
RSSI
9 12 14
10 11
NOISE SQUELCH DIAGRAM
Fc=30 kHz
Rectify
Noise
"NOISE"
AMP
Active filter
Noise detector
2
16