LA1781M_09 Datasheet, PDF (21/48 Pages) Sanyo Semicon Device – For Car Radios Single-Chip Tuner IC

Usage Notes
LA1781M
1. Notes on VCC and Ground
Pin 40
Pin 25
VCC for the FM IF, AM, NC, MPX, and MRC blocks
Ground for the FM IF and AM blocks
Pin 14
Ground for the NC, MPX, and MRC blocks
Pin 61
*Pin 6
Pin 3
VCC for the FM front end, AM first mixer, and first oscillator blocks
VCC for the FM front end and AGC blocks, and the AM/FM switching pin
Ground for the FM front end, first mixer, and first oscillator blocks
2. Notes on AM Coil Connection
The VCC used for the first oscillator coil connected to pin 7 must be at the same potential as pin 61.
Connect to the IFT connected with pin 45, and to the MIX coil connected with pin 54. VCC must be at the same
potential as pin 40.
3. AM/FM Switching
Pin 6 is also used as the FM front end and RF AGC VCC
V6pin
8
Pin 6 voltage
8
OPEN
Mode
FM
AM
3.3
AM
FM
AM
Fig. 1
4. Notes on the FM Front End
Notes on interference rejection characteristics
• Intermodulation characteristics
The LA1781M applies two high-band AGC functions to prevent IM (the generation of intermodulation). These are
the narrow AGC (pin 58 : mixer input detection type) and the wide AGC (for the pin 55 input), and this results in the
antenna frequency characteristics shown in figure 2. The levels at which the AGC functions turn on are determined by
the capacitors attached at pins 55 and 58.
Δf -- AGC Sensitivity
When Δf = 0, 98.1MHz
110
100
90
The wide AGC
sensitivity when
pin 39 is 5V.
80
70
The narrow AGC
60
sensitivity when
pin 39 is at ground.
50
--5 --4 --3 --2 --1 0 1 2 3 4 5
Δf -- MHz
Fig. 2
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