LA1787M Datasheet, PDF (26/54 Pages) Sanyo Semicon Device – Single-Chip Tuner IC for Car Radios

LA1787M
• 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
• 3D AGC Features
Feature
Only the narrow AGC sensitivity (operation at ∆f < 1.5 MHz) is
controlled by the field strength of the desired station.
The narrow AGC sensitivity is controlled by a voltage (V₂₃) that is
under 0.5 V.
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
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.
∆f — AGC on Level (ANT input) Fig.3
∆f — AGC on Level (ANT input) Fig.4
Pin 55 capacitor: 3 pF
110
110
100
100
90
80
70 ^{keyed AGC}
39
60
Pin 58 capacitor:
10 pF
Pin 58 capacitor:
47 pF
50 _{–5 –4 –3 –2 –1 0 1 2 3 4 5}
∆f — MHz
W-AGC, N-AGC — f Fig.5
70
90
80
keyed AGC
70
39
60
5V
Pin 55 capacitor: 10 pF
50 _{–5 –4 –3 –2 –1 0 1 2 3 4 5}
∆f — MHz
80
Wide AGC ^{on level frequency characteristics}
90
Narrow _{AGC on level frequency characteristics}
100
110
120
130 ^{AGC input level frequency}
characteristics such that
140 ^{VRFAGC (pin 2) falls under 2 V.}
7 _1.0 2 3 5 7 ₁₀ 2 3 5 7 ₁₀₀
Frequency, f — MHz
23 5
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