LA1781M
No.6038-34/48
The wide band AGC circuit in this IC has the frequency characteristics shown above. The pin 46 input frequency
characteristics are identical to those of the RF amplifier gate. This AGC circuit serves to prevent distortion at the FET
input when a strong signal is applied to the antenna circuit. The level at which the AGC circuit turns on can be
adjusted to an arbitrary level with the wide band AGC adjustment resistor. A delayed AGC on level can be handled by
reducing the value of the adjustment resistor.
Fig. 35
AM AGC f characteristics
60
90
100
70
80
800
900
1100
1000
1200
Frequency -- Hz
AGC
on
level
Wide AGC
Operates for
wide band
interference
Middle AGC
Operates for
interference within
±70kHz of the
received frequency.
Middle AGC
Operates for
interference within
±70kHz of the
received frequency.
Narrow AGC
Operates at the
received frequency.
Wide AGC
Operates for wide
band interference
Fig. 33
62
57
+
VCC
0.022
μF
30
Ω
620
Ω
1MH
100
μH
100
μH
30MH
15pF
47
μ
F
100k
Ω
0.022
μ
F
Wide band AGC adjustment resistor
FC18
30
Ω
0.022
μF
0.022
μF
0.022
μF
46
50
Ω
510
Ω
50
Ω
--6dB
SG
ANTD
Wide Band AGC Circuit
70
80
90
100
110
120
1.0
23
57
10
5
Pin 46 input -- MHz
Anternna
damping
on
input
level
-
dB
μ
Received frequency :
1MHz
Fig. 34