LA1781M_09 Datasheet, PDF (28/48 Pages) Sanyo Semicon Device – For Car Radios Single-Chip Tuner IC

LA1781M
• Transient response characteristics during automatic tuning
The transient characteristics for SD and IF count buffer on/off operation are determined by the time constants of the
RC circuits attached to the following pins.
(1) Muting time constant : pin 33
(2) S-meter time constant : pin 24
(3) AFC time constant : pin 34
There are two points that require consideration when using fast tuning.
(1) The SD time constant due to the S-meter time constant
Since the current I24 (pin 24) varies with the field strength, the time constant also changes. There is no hysteresis in
the comparator.
If C24 is made smaller and the pin 24 voltage is used for the keyed AGC pin 23, C23 must be chosen so that AGC
during keyed AGC operation does not become unstable.
S-meter
I24 ₂₄
SD comparator
R24
C24
Fig. 15
(2) The SD time constant due to the pin 33 muting voltage time constant
The changes in volume due to field fluctuation during weak field reception can be made smoother by setting the
attack and release times during soft muting operation.
Mute
drive
10kΩ
50kΩ
Mute
amp
Muting time constants
Attack : 10kΩ × C33
Release : 50kΩ × C33
Attack
Release
33
C33
Fig. 16
SD Sensitivity Adjustment Fig. 17
50
40
30
20
10
0
6
10
14
18
22
26
30
34
Resistance between the pin and ground -- kΩ
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