LA1781M_09 Datasheet, PDF (37/48 Pages) Sanyo Semicon Device – For Car Radios Single-Chip Tuner IC

LA1781M
• AM low band cut adjustment method
The AM low band frequency characteristics can be adjusted with C42, which is inserted between pin 42 and VCC.
Since the detector is designed with VCC as the reference, C42 must be connected to VCC.
AM
detecter
VCC
C42
42
50kΩ
50kΩ
10kΩ
+
--
10kΩ
10kΩ
To pin 31
Detector Output -- Frequency Fig. 42
20
0.1μF
10
80%mod
With no
C31 used.
0
30%mod
--10
0.022μF _C31pin
0.047μF
= 6800pF
--20
0.1μF
C42pin = ^{Using SEP 450H}
--30
Fig. 41
--40
fr = 100kHz
--50 ^{fm = 10kHz 30%mod}
3 5 70.01 2 3 5 7 _0.1 2 3 5 7 _1.0 2 3 5 7 ₁₀ 2
Frequency -- Hz
7. Noise Canceler Block
• The noise canceler input (pin 30) has an input impedance of about 50kΩ. Check the low band frequency
characteristics carefully when determining the value of the coupling capacitor used. Note that fC will be about 3Hz
when a 1μF capacitor is used in the application.
• Pins 8 and 9 are used to set the noise detector sensitivity and the noise AGC. It is advisable to first set the noise
sensitivity for a medium field (an antenna input of about 50dBμ) with pin 8 (the noise sensitivity setting pin), and
then set the AGC level for a weak field (20 to 30dBμ) with pin 9 (the AGC adjustment pin). If the noise sensitivity is
increased, the AGC will become more effective but, inversely, the weak field sensitivity will be reduced.
Noise canceler 10kHz overmodulation malfunction may be a problem. In particular, when an overmodulated signal is
input, the noise canceler may, in rare cases, malfunction. This is due to the fact that the IF detector output has a
waveform of the type shown in figure 43 due to the bands of the IF ceramic filters as shown below. (Here, the antenna
input is 60dBμ, the ceramic filters are 150kHz × 1 and 180kHz × 2, f = 10kHz, 180kHz dev.) The noise canceler
reacts to the spikes (whiskers) generated due to this overmodulation, which results in distortion to the audio output.
(The spike components due to overmodulation occur due to the bands of the ceramic filters in the tuner.) The
following describes a method for resolving this problem. This incorrect operation due to overmodulation is prevented
by removing the spike components due to this overmodulation with a low-pass filter consisting of a 1kΩ resistor and
a 2200pF capacitor shown in figure 44. However, note that the FM separation characteristics in the high band and the
AM frequency characteristics will change.
H1 W1
2.5OU
IF audio output
f = 10kHz,180kHz dev
IF output _1kΩ
31
2200pF
Noise canceler input
30
1μF
Fig.44
--2.5OU
--19.00ms
Fig. 43
981.00μs
No.6038-37/48