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| 型号： | LA1784M |
| 元件分类： | 接收器 |
| 英文描述： | AM/FM, AUDIO SINGLE CHIP RECEIVER, PQFP64 |
| 封装： | QIP-64 |
| 文件页数： | 16/50页 |
| 文件大小： | 568K |
| 代理商： | LA1784M |
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No. 6039-23/50
LA1784M
Notes on second-channel attenuation suppression
Keyed AGC (3D AGC) is a technique for achieving good characteristics for both intermodulation and second-
channel 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
–4
–5
–2
–3
–1
0
50
70
80
90
100
110
60
1
2
3
4
5
Pin 58 capacitor:
10 pF
Narrow
AGC
on
level
—
dB
f — MHz
f — AGC on Level (ANT input)
Pin 58 capacitor:
47 pF
39
keyed AGC
Fig.3
140
130
120
110
100
90
80
70
7 1.0
2
3
5
7 10
2
3
5
7 100
2
3
5
Wide AGC on level frequency characteristics
Narrow AGC on level
frequency characteristics
AGC input level frequency
characteristics such that
VRFAGC (pin 2) falls under 2 V.
Pin
59
narrow
AGC
and
pin
55
wide
AGC
input
levels
—
dB
Frequency, f — MHz
W-AGC, N-AGC — f
Fig.5
–4
–5
–2
–3
–1
0
50
60
70
80
90
100
110
1
2
3
4
5
Pin 55 capacitor: 3 pF
Pin 55 capacitor: 10 pF
Wide
AGC
on
level
—
dB
f — MHz
f — AGC on Level (ANT input)
39
5V
keyed AGC
Fig.4
3D AGC Features
Feature
Merit
Only the narrow AGC sensitivity (operation at
f < 1.5 MHz) is
Effective in resolving second-channel attenuation problems.
controlled by the field strength of the desired station.
The narrow AGC sensitivity is controlled by a voltage (V23) that is
Allows effective resolution of second-channel attenuation problems without
under 0.5 V.
degrading three-signal characteristics.
Seek operations may stop incorrectly due to the occurrence of
The wide AGC can operate even when V23 = 0 (when the desired
intermodulation.
station is not present).
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.
Settings can be optimized for the field conditions.
(See figure 3 and 4.)
The system has two AGC systems: narrow and wide AGC.
Since the narrow AGC operates for the desired station and adjacent
(See figure 5.)
stations, the wide AGC sensitivity can be lowered and AGC malfunction
due to local oscillator signal can be prevented.
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