LA1787M Datasheet, PDF (41/54 Pages) Sanyo Semicon Device – Single-Chip Tuner IC for Car Radios

LA1787M
• AM low band cut adjustment method
The AM low band frequency characteristics can be adjusted with C42, which is inserted between pin 42 and V_CC.
Since the detector is designed with V_CC as the reference, C42 must be connected to V_CC.
VCC
C42
Detector Output — Frequency Fig.42
10
0.022 µF (41pin)
With no used (31pin)
0
AM
detector
42
50 kΩ
50 kΩ
10 kΩ
+
–
10 kΩ
Fig. 41
10 kΩ
To pin 31
A12087
0.022 µF (41pin)
–10
With no used (31pin)
–20 0.043 µF (41pin)
% with no used (31pin)
–30
0.022 µF (41pin)
8200 pF (31pin)
fr = 1000 kHz
fm = 1 kHz, 30%
–40
0.001
2
3
5 70.01
2
3
5 7 _0.1
2 3 5 7 _1.0
Frequency — Hz
2 3 5 7 ₁₀
4. Noise Canceler Block
• The noise canceler input (pin 30) has an input impedance of about 50 kΩ. Check the low band frequency
characteristics carefully when determining the value of the coupling capacitor used. Note that f_C will be about 3 Hz
when a 1 µF 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 50 dBµ) with pin 8 (the noise sensitivity setting pin), and
then set the AGC level for a weak field (20 to 30 dBµ) 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 10 kHz 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 60 dBµ, the ceramic filters are 150 kHz × 1 and 180 kHz × 2, f = 10 kHz, 180 kHz 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 1 kΩ resistor and a 2200 pF 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 = 10 kHz,180 kHz dev
IF output _{1 kΩ}
31
2200 pF
Noise canceler input
30
1 µF
Fig. 44
A12089
–2.5OU
–19.00 µs
Fig. 43
981.00 µs
A12088
No. 6655-41/54