LM2889 National Semiconductor, LM2889 Datasheet - Page 8

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

Applications Information
When the signal inputs are exactly balanced ideally there is
no RF carrier at the output Circuit board layout is critical to
this measurement For optimum performance the output
and supply decoupling circuitry should be configured as
shown in Figure 3
The video clamp level is derived from a resistive divider con-
nected to supply (V
pin 10 which is biased externally should also be referenced
to supply (see Figure 2 )
Pin Description
Pin 1 Audio Input Pin 1 is the audio input to the sound
FM generator Frequency deviation is proportional to the
signal at this pin A pre-emphasis network comprised of R1
C2 and the device input impedance yields the following re-
sponse with an 80 mVrms audio input
Increasing R1 lowers the boost frequency and decreases
deviation below the boost frequency Increasing C2 only
lowers the boost frequency C1 is a coupling capacitor and
must be a low impedance compared to the sum of R1 and
the device input impedance (1 5 k )
FIGURE 3 Correct RF Supply Decoupling
RF decouple supply directly to output ground
Pre-Emphasis Network
Response
S
) To maintain good supply rejection
(Refer to Figure 2 )
(Continued)
TL H 5079– 8
TL H 5079– 9
8
Pin 2 Video Clamp The video clamp restores the DC
component to AC-coupled video The video is AC-coupled
to the clamp via C3 Decreasing C3 will cause a larger tilt
between vertical sync pulses in the clamped video wave-
form
Pin 3 Ground Although separate on the chip level all
ground terminate at pin 3
Pins 4 5 Channel 4 Oscillator Pins 4 and 5 are the col-
lector outputs of the channel 4 oscillator L1 and C5 set the
oscillator frequency defined by f
ing L1 will decrease the oscillator frequency while decreas-
ing L1 will increase the oscillator frequency Decreasing C5
will increase the oscillator frequency and lower the tank Q
causing possible drift problems R2 and R3 are the oscillator
loads which determine the oscillator amplitude and the tank
Q Increasing these resistors increases the Q and the oscil-
lator amplitude possibly overdriving the RF modulator
which will increase output RF harmonics Decreasing R2
and R3 reduces the tank Q and may cause increased drift
C4 is an RF decoupling capacitor Increasing C4 may result
in less effective decoupling at RF Decreasing C4 may intro-
duce RF to supply coupling
Pins 6 7 Channel 3 Oscillator Pins 6 and 7 are the chan-
nel 3 oscillator outputs Every component at these pins has
the same purpose and effect as those at pins 4 and 5
Pin 8 Channel 4 RF Output Pin 8 is the channel 4 RF
output and R13 is the load resistor The RF signal is AC
coupled via C15 to the output filter which is a two channel
VSB filter L5 is parallel resonant with the filter input capaci-
tance minimizing loss in the output network R14 terminated
the filter output
Pin 9 Channel 3 RF Output Pin 9 is the channel 3 RF
output with all components performing the same functions
as those in the pin 8 description
Pin 10 RF Modulator Sound Subcarrier Input Pin 10 is
one of the RF modulator inputs and may be used for video
or sound It is used as a sound subcarrier input in Figure 2
R8 R9 and R10 set the DC bias on this pin which deter-
mines the modulation depth of the RF output (see Applica-
tion Notes) R12 and C11 AC-couple the sound subcarrier
from the sound modulator to the RF modulator R12 and
R11 form a resistive divider that determines the level of
sound at pin 10 which in turn sets the picture carrier to
sound subcarrier ratio Increasing the ratio of R11 R12 will
increase the sound subcarrier at the output C10 forms an
AC ground preventing R8 R9 and R10 from having any
effects on the circuit other than setting the DC potential at
pin 10 R11 and R12 also effect the FM sound modulator
(see pin 13 description)
O
e
0 159
S
L1C5 Increas-