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參數資料
| 型號： | LA7577N |
| 元件分類： | 接收器 |
| 英文描述： | FM, AUDIO SINGLE CHIP RECEIVER, PDIP24 |
| 封裝： | DIP-24 |
| 文件頁數： | 16/16頁 |
| 文件大小： | 354K |
| 代理商： | LA7577N |
LA7577N
No. 4037—9/16
FM Discriminator (Pin 2)
The quadrature detector frequency at which the 90
° phase
shift occurs is determined by the tuned circuit connected
to pin 2 as shown in Figure 7.
The detector bandwidth characteristics are determined
largely by the coil Q and damping resistance. The damp-
ing resistor should be chosen for the desired output level
and bandwidth characteristics.
FM muting is achieved by holding point A, in Figure 7, at
≤1V DC.
IF AGC (Pins 3 and 13)
The IF signal is peak detected and averaged by the lters
connected to pins 13 and 3, which are the 1st AGC and
2nd AGC, respectively, as shown in Figure 8. The IF AGC
audio component of the input signal to the video IF stage
is rst removed by an audio trap.
Figure 7. FM discriminator
Figure 8. IF AGC circuits
Typical AGC lter time constants
Mute switch (IS-15 switch)
The black-noise canceler can be disabled by pulling pin 13
to 1V or lower. An external AGC source can then be
applied to pin 3 to drive the AGC circuit. This mode of
operation is designed for use with an IS-15 (EIA standard)
switch.
Ghosting problems
Reected signals which have a phase different from that of
the main signal can cause distortion of the horizontal sync
pulse, as shown in Figure 9. As a result, the same charge-
to-discharge current ratio of the IF AGC cannot be main-
tained. If the phase difference is large, the video signal can
also be distorted as shown in Figure 10. Distortion can be
minimized by connecting a 820k
Ω to 1MΩ resistor
between pin 13 and ground.
Pin
Component
Single time
constant
Double time constant
3
C1
330pF
R1
–
2.2k
Ω
1.8k
Ω
C2
–
0.47
μF0.1μF
13
C3
0.47
μF
0.068
μF
0.047
μF
R2
820k
Ω
820k
Ω
820k
Ω
Figure 9. Horizontal sync pulse distortion
Figure 10. Video signal distortion
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