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參數(shù)資料
| 型號： | LA1784M |
| 元件分類： | 接收器 |
| 英文描述： | AM/FM, AUDIO SINGLE CHIP RECEIVER, PQFP64 |
| 封裝： | QIP-64 |
| 文件頁數(shù)： | 32/50頁 |
| 文件大?。?/td> | 568K |
| 代理商： | LA1784M |
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No. 6039-38/50
LA1784M
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
detector
42
+
–
50 k
50 k
10 k
10 k
10 k
C42
VCC
To pin 31
A12087
–50
–40
–30
–20
–10
0
10
20
3
5 70.01 2 3
5 7 0.1
2 3
5 7 1.0
2
3
5 7 10
2
0.1
F
0.1
F
0.047
F
0.022
F
30%mod
80%mod
With no
C31 used.
Using SEP 450H
C31pin
= 6800 pF
C42pin=
fr = 100 kHz
fm = 10 kHz 30%mod
Detector
output
—
dB
Frequency — Hz
Detector Output — Frequency Fig.42
Fig. 41
31
30
+
–
IF output
Noise canceler input
1 k
1
F
2200 pF
A12089
H1 W1
2.5OU
–2.5OU
–19.00
s
981.00
s
IF audio output
f = 10 kHz,180 kHz dev
A12088
Fig. 43
Fig. 44
7. Noise Canceler Block
The noise canceler input (pin 30) has an input impedance of about 50 k
. Check the low band frequency
characteristics carefully when determining the value of the coupling capacitor used. Note that fC will be about 3 Hz
when a 1 F 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 dB) with pin 8 (the noise sensitivity setting pin), and
then set the AGC level for a weak field (20 to 30 dB) 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 dB, 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 k
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.
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