LA1235
No. 873- 12/13
Cain distribution of application circuit
If IC anone is operated without front end, the tuning meter deflects toward plus side at the time of no input. This
phenomenon is caused by the fact that the noise component to be applied to the quadrature multiplication circuit is
not syummetric with respect to 10.7MHz but is shifted toward lower frequency side because the frequency
characteristic of IF amplifier attenuates at high frequencies and the phase shift circuit is of low-pass type. If the formt
end is attached and the noise which passes through the narrowband filter of IF stage and spreads symmetrically with
respect to 10.7MHz is stronger than the noise generated inside the IC, the tuning meter reads 0. As the gain of the
front end is decreased, input limiting sensitivity and usable sensitivity worsen abruptly. This phenomenon is caused
by the fact that since the tuning meter is set to 0 at the time of no input the tuning point of the quadrature circuit must
be shifted toward lower frequency side than 10.7MHz and the demodulation output waveform is deformed
asymmetrically at an input in the vicinity of usable sensitivity. However, if the gain of the front end is too increased,
the signal meter poiter does not return to zero point at the time of no signal.
Sample Application Circuit
11
8
9
10
50k
Ω
To voltage drive
type tuning meter
AF output
(To MPX IC)
To AM tuner
RF VCC
(Equivalent
to LB1450)
235S01
DS442
(Sumida)
0.047
μF
0.047
μF
220
μF
2.2
μF
2.2
μF
33pF
2.2
μH
VCC
5.1k
Ω
4.7k
Ω
3.3k
Ω
+
7
6
5
To voltage drive type
S meter
Front end gain distribution
3.3k
Ω
68k
Ω
10k
Ω
Muting
SW
15
16
12
56k
Ω
30k
Ω
1
μF
1
μF
0.022
μF
0.047
μ
F
+
14
13
1
0.047
μF
0.047
μF
0.047
μF
330
Ω
2
3
4
F.E
LA1235
10.7 MX
40 to 45dB
(Murata)
4dB