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 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 7. FM discriminator

Figure 8. IF AGC circuits

Typical AGC filter 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

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 820k

Ω to 1MΩ resistor

between pin 13 and ground.

Pin

Component

Single time

constant

Double time constant

3

C1

330pF

330pF

330pF

R1

–

2.2k

Ω

1.8k

Ω

C2

–

0.47

µF

0.1

µF

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