參數(shù)資料
型號(hào)： LA1781M
元件分類(lèi)： 接收器
英文描述： AM/FM, AUDIO SINGLE CHIP RECEIVER, PQFP64
封裝： 14 X 14 MM, QIP-64
文件頁(yè)數(shù)： 31/48頁(yè)
文件大?。?/td> 508K
代理商： LA1781M
LA1781M
No.6038-37/48
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.
Fig. 41
7. Noise Canceler Block
The noise canceler input (pin 30) has an input impedance of about 50k
Ω. 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 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 50dB
μ) with pin 8 (the noise sensitivity setting pin), and
then set the AGC level for a weak field (20 to 30dB
μ) 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 60dB
μ, 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 1k
Ω 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.
Fig.44
Fig. 43
AM
detecter
42
+
--
50k
Ω
50k
Ω
10k
Ω
10k
Ω
10k
Ω
C42
VCC
To pin 31
31
30
+
--
IF output
Noise canceler input
1k
Ω
1
μF
2200pF
H1 W1
2.5OU
--2.5OU
--19.00ms
981.00
μs
IF audio output
f = 10kHz,180kHz dev
Detector Output -- Frequency
--50
--30
--20
--10
0
10
--40
20
35 70.01
23
0.1
7
52 3
1.0
7
52 3
10
7
52
Frequency -- Hz
Detector
output
-
dB
fr = 100kHz
fm = 10kHz 30%mod
0.1
μF
0.1
μF
0.022
μF
0.047
μF
80%mod
C31pin
= 6800pF
With no
C31 used.
30%mod
Using SEP 450H
C42pin =
Fig. 42
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