LA1781M_09 Datasheet, PDF (22/48 Pages) Sanyo Semicon Device – For Car Radios Single-Chip Tuner IC

LA1781M
• 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
Only the narrow AGC sensitivity (operation at Δf < 1.5MHz) is controlled
by the field strength of the desired station.
The narrow AGC sensitivity is controlled by a voltage (V23) that is under
0.5V.
The wide AGC can operate even when V23 = 0 (when the desired station
is not present).
The narrow and wide AGC sensitivities can be set independently.
(See figure 3 and 4.)
The system has two AGC systems : narrow and wide AGC.
(See figure 5.)
Merit
• Effective in resolving second-channel attenuation problems.
• 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
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.
• Settings can be optimized for the field conditions.
• 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.
Δf -- AGC on Level (ANT input) Fig. 3
110
100
90
Pin 58 capacitor :
80
10pF
70 ^{keyed AGC}
39
60
Pin 58 capacitor :
47pF
50
--5 --4 --3 --2 --1 0 1 2 3 4 5
Δf -- MHz
W-AGC, N-AGC -- f
Fig. 5
70
80
Wide ^{AGC on level frequency characteristics}
90
100
Narrow _{AGC on level frequency characteristics}
110
120
130 AGC input level frequency
characteristics such that
140 ^{VRFAGC (pin 2) falls under 2V.}
7 _1.0
2 3 5 7 ₁₀
2 3 5 7 ₁₀₀
Frequency, f -- MHz
23 5
Δf -- AGC on Level (ANT input) Fig. 4
Pin 55 capacitor : 3pF
110
100
90
80
keyed AGC
70
39
60
5V
Pin 55 capacitor : 10pF
50
--5 --4 --3 --2 --1 0 1 2 3 4 5
Δf -- MHz
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