LM2889 National Semiconductor, LM2889 Datasheet - Page 9

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

Pin Description
Pin 11 Video Input Pin 11 when configured as shown is
the RF modulator video input In this application video is
coupled directly from the video clamp Alternatively video
could be DC-coupled directly to pin 11 if it is already within
the DC common-mode input range of the RF modulator (see
curves) In any case the video sync tip at pin 11 must have
a constant DC level independent of video content Because
of circuit symmetry pins 10 and 11 may be interchanged
Pin 12 RF Supply Pin 12 is the RF supply with C12 and
C7 serving as RF decouple capacitors Increasing C12 or
C7 may result in less effective RF decoupling while de-
creasing them may cause supply interaction It is important
that C7 be grounded at the RF output ground
Pin 13 Sound Tank Pin 13 is the collector output of the
sound oscillator L3 and C13 determine the oscillating fre-
quency by the relationship f
L3 or C13 will lower the operating frequency while decreas-
ing them will raise the frequency L3 and C13 also help
define the Q of the tank on which FM modulator de-
viation level depends As C13 increases Q increases and
frequency deviation decreases Likewise decreasing C13
increases deviation The other factor concerning Q is the
(Continued)
O
e
FIGURE 4 Printed Circuit Board and Component Diagram
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S
L3C13 Increasing
(Component Side 1X)
9
external resistance across the tank The series combination
R11
resistive network will decrease Q and increase deviation It
should be noted that because the level of phase modulation
of the 4 5 MHz signal remains constant variation in Q will
not effect distortion of the frequency modulation process if
the audio at pin 1 is left constant The amplitude of the
sound subcarrier is directly proportional to Q so increasing
the unloaded Q or either of the resistors mentioned above
will increase the sound subcarrier amplitude For proper op-
eration of the frequency modulator the sound subcarrier
amplitude should be greater than 2 Vp-p
Pin 14 Sound Supply Pin 14 is the sound supply and C14
is an RF decouple capacitor Decreasing C14 may result in
increased supply interaction
Printed Circuit Layout
Printed circuit board layout is critical in preventing RF feed-
through The location of RF bypass capacitors on supply is
very important Figure 4 shows an example of a properly
layed out circuit board It is recommended that this layout be
used
a
R12 usually dominates the tank Q Decreasing this
TL H 5079 – 10