
LA1787M
AM AGC f characteristics
Fig.33
Wide AGC
Operates for wide
100
band interference
90
Wide AGC
Operates for
wide band
interference
Middle AGC
Operates for
interference within
± 70 kHz of the
received frequency.
80
Middle AGC
Operates for
interference within
± 70 kHz of the
received frequency.
70
Narrow AGC
Operates at the
received frequency.
60
800
900
1000
1100
1200
Frequency — Hz
120
Wide Band AGC Circuit
Fig.34
30 ?
0.022 μ F
Received frequency:
1 MHz
110
–6dB
SG
50 ?
50 ?
46
0.022 μ F
100
90
80
ANTD
0.022 μ F
510 ?
70
1.0
2
3
5
7
10
2
3
5
Pin 46 input — MHz
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.
Wide band AGC adjustment resistor
0.022 μ F
30 ?
VCC
620 ?
1MH
100 μ H
100 μ H
FC18
+
Fig. 35
57
62
A12084
No. 6655-38/54