LM2889 National Semiconductor, LM2889 Datasheet - Page 7

• Manufacturer Part Number: LM2889
• Description: LM2889 TV Video Modulator
• Manufacturer: National Semiconductor

Applications Information
SOUND FM MODULATOR
Frequency deviation is determined by the Q of the tank cir-
cuit at pin 13 and the current entering the audio input pin 1
This current is set by the input voltage V
impedance (1 5 k ) and any impedance network connect-
ed externally A signal of 60 mVrms at pin 1 will yield about
VIDEO CLAMP
When video is not available at DC levels within the RF mod-
ulator common-mode range or if the DC level of the video is
not temperature stable then it should be AC-coupled as
shown in the typical applications circuit ( Figure 2 ) The
clamp holds the horizontal sync pulses at 5 2V for V
The clamp coupling capacitor is charged during every sync
pulse and discharged when video information is present
The discharge current is approximately 20 A This current
and the amount of acceptable tilt over a line of video deter-
mines the value of the coupling capacitor C1 For most ap-
plications 1 F is sufficient
Typical Application
g
25 kHz deviation when configured as shown in Figure 2
FIGURE 2 Two Channel Video Modulator with FM Sound
IN
the device input
S
e
12V
7
RF MODULATION
Two RF channels are available with carrier frequencies up
to 100 MHz being determined by L-C tank circuits at pins
4 5 and 6 7 The signal inputs (pins 10 and 11) are com-
mon to both modulators but removing the power supply
from an RF oscillator will also disable that modulator
The offset between the two signal pins determines the level
of the RF carrier output To preserve the DC content of the
video signal amplitude modulation of the RF carrier is done
in one direction only with increasing video (toward peak
white) decreasing the carrier level This means the active
composite video signal at pin 11 must be offset with respect
to pin 10 and the sync pulse should produce the largest
offset
The largest video signal (peak white) should not be able to
suppress the carrier completely particularly if sound trans-
mission is needed This requires that pin 10 be biased
above the largest expected video signal Because peak
white level is often difficult to define a good rule to follow is
to bias pin 10 at a level which is four times the sync ampli-
tude above the sync tip level at pin 11 For example the DC
bias at pin 10 with 0 5V sync clamped to 5 2V on pin 11
should be 5 2
a
(4
c
0 5)
e
7 2V
TL H 5079– 7