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| 型號： | LA1781M |
| 元件分類： | 接收器 |
| 英文描述： | AM/FM, AUDIO SINGLE CHIP RECEIVER, PQFP64 |
| 封裝： | 14 X 14 MM, QIP-64 |
| 文件頁數(shù)： | 15/48頁 |
| 文件大小： | 508K |
| 代理商： | LA1781M |
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LA1781M
No.6038-22/48
Notes on second-channel attenuation suppression
Keyed AGC (3D AGC) is a technique for achieving good characteristics for both intermodulation and secondchannel
attenuation at the same time. When the desired signal is faint or nonexistent, the high-band AGC level will be
essentially 0, and as a result automatic tuning may malfunction and blocking oscillation may occur in the presence of
strong interfering stations. Keyed AGC helps resolve these problems.
This 3D AGC technique uses information that has the following three frequency characteristics and is a unique
Sanyo-developed system for determining the high-band AGC level.
RF and ANT circuit information : Mixer input AGC
Mixer circuit information : Mixer output AGC
CF selectivity information : S-meter output
3D AGC Features
Feature
Merit
Only the narrow AGC sensitivity (operation at
Δf < 1.5MHz) is controlled
by the field strength of the desired station.
Effective in resolving second-channel attenuation problems.
The narrow AGC sensitivity is controlled by a voltage (V23) that is under
0.5V.
Allows effective resolution of second-channel attenuation problems
without under 0.5 V. degrading three-signal characteristics.
The wide AGC can operate even when V23 = 0 (when the desired station
is not present).
Seek operations may stop incorrectly due to the occurrence of
intermodulation.
It is possible to prevent the occurrence of intermodulation in the RF
tuning circuit and antenna in the presence of strong interfering stations,
and blocking oscillation due to AGC operation can be prevented.
The narrow and wide AGC sensitivities can be set independently.
(See figure 3 and 4.)
Settings can be optimized for the field conditions.
The system has two AGC systems : narrow and wide AGC.
(See figure 5.)
Since the narrow AGC operates for the desired station and adjacent
stations, the wide AGC sensitivity can be lowered and AGC malfunction
due to local oscillator signal can be prevented.
39
5V
Δf -- AGC on Level (ANT input)
W-AGC, N-AGC -- f
Δf -- AGC on Level (ANT input)
50
70
80
90
100
110
60
50
70
80
90
100
110
60
--5
--4
--3
--2
--1
5
4
3
2
1
0
Δf -- MHz
7 1.0
10
7
5
3
2
100
7
5
3
23
25
Frequency, f -- MHz
--5
--4
--3
--2
--1
5
4
3
2
1
0
Δf -- MHz
Narrow
AGC
on
level
-
dB
μ
140
120
110
100
90
80
130
70
Pin
59
narrow
AGC
and
pin
55
wide
AGC
input
levels
-
dB
μ
Wide
AGC
on
level
-
dB
μ
keyed AGC
Pin 58 capacitor :
10pF
Pin 58 capacitor :
47pF
AGC input level frequency
characteristics such that
VRFAGC (pin 2) falls under 2V.
Wide AGC on level
frequency characteristics
Narrow
AGC on level
frequency
characteristics
Pin 55 capacitor : 3pF
Pin 55 capacitor : 10pF
Fig. 3
Fig. 5
Fig. 4
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