LA7577N Просмотр технического описания (PDF) - SANYO -> Panasonic
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LA7577N Datasheet PDF : 16 Pages
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LA7577N
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.
Typical AGC filter time constants
Pin
Component
Single time
constant
Double time constant
C1
330pF
330pF
330pF
3
R1
–
2.2kΩ
1.8kΩ
C2
–
0.47µF
0.1µF
C3
0.47µF
0.068µF
0.047µF
13
R2
820kΩ
820kΩ
820kΩ
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
Reflected 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 820kΩ to 1MΩ resistor
between pin 13 and ground.
Figure 7. FM discriminator
IF AGC (Pins 3 and 13)
The IF signal is peak detected and averaged by the filters
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 first removed by an audio trap.
Figure 9. Horizontal sync pulse distortion
Figure 10. Video signal distortion
Figure 8. IF AGC circuits
No. 4037—9/16
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