LA1787M Datasheet, PDF (32/54 Pages) Sanyo Semicon Device – Single-Chip Tuner IC for Car Radios

LA1787M
• 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 a smaller value is used for C24, you must select a value for C such that the AGC does not become unstable
when the pin 24 voltage is used for keyed AGC.
S-meter
10k
24
C24
Fig. 15
A12080
(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
Attack
50kΩ
Release
33
Mute
amp
Muting time constants
Attack: 10 kΩ × C33
Release: 50 kΩ × C33
C33
Fig. 16
A11766
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
No. 6655-32/54