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LA1787M
4. Changes to applications
Component values that change from LA1781M/LA1784M applications
(Since the total AM gain has changed in the LA1787M)
? AM SD adjustment resistor (pin 55): Because Vsm is higher.
? AM level adjustment resistor (pin 31): Since the post-detection audio amplifier gain is higher than in the LA1781M
and LA1784M, the output level is also higher. This resistor must be changed
to match the set value.
? AM mixer coil (pin 54), IFT coil (pin 45) damp resistor: Since the IF block gain is increased, the mixer (pin 54) and
IFT (pin 45) coil damping must be adjusted.
? Separation adjustment resistor (pin 19): Since an internal 4 k ? resistor has been added to the pin 19 input circuit to
improve the separation temperature characteristics, the value of the external
resistor must be reduced from that used with the LA1780M, LA1781M, and
LA1784M. (See the following page.)
Composite Sign
5k ?
DECODER
Composite Sign
5k ?
DECODER
Added 4 k ? resistor
19
30k ?
0.047 μ F
LA1781M/1784M
A13602
19
30k ?
0.047 μ F
LA1787M
A13603
Functions
1. Notes on the FM Front End
Notes on interference rejection characteristics
? Intermodulation characteristics
The LA1787M 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.
? f — AGC Sensitivity
When ? f = 0, 98.1 MHz
110
100
90
The wide AGC
sensitivity when
pin 39 is 5 V.
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
? f — MHz
Fig. 2
No. 6655-25/54
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