LM2889 National Semiconductor, LM2889 Datasheet - Page 5

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

Circuit Description
The sound carrier oscillator is formed by differential amplifi-
er Q3 Q4 operated with positive feedback from the pin 13
tank to the base of Q4 Frequency modulation is obtained
by varying the 90 degree phase shifted current of Q9 Q14’s
emitter is a virtual ground so the voltage at pin 1 deter-
mines the current R11 which ultimately modulates the col-
lector current of Q9
The video clamp is comprised of devices Q58-Q60 The
clamp voltage is set by resistors R40 R41 R49 and R50
The
rent Q59 and the above mentioned resistor string help
maintain a temperature stable clamp voltage
The channel B oscillator consists of devices Q24 and Q25
cross-coupled through level-shift zener diodes Q22 and
Q23 A current regulator consisting of devices Q17– Q21 is
used to achieve good RF stability over temperature and
Circuit Diagrams
V
BE
R42 current sets the capacitor discharge cur-
(Refer to Circuit Diagrams)
5
supply The channel B modulator consists of multiplier de-
vices Q28 – Q31 Q34 and Q35 The top quad is coupled to
the channel B tank through isolating devices Q26 and Q27
A DC potential between pins 10 and 11 offsets the lower
pair to produce an output RF carrier at pin 8 That carrier is
then modulated by both the sound subcarrier at pin 10 and
the composite video signal at pin 11 The channel A modu-
lator shares pin 10 and 11 buffers Q32 and Q33 with chan-
nel B and operates in an identical manner
The current flowing through channel B oscillator diodes
Q22 Q23 is turned around in Q36– Q38 to source current
for the channel B RF modulator In the same manner the
channel A oscillator Q54– Q57 uses turn-around Q49 – Q51
to source the channel A modulator One oscillator at a time
may be activated by its current turn-around and the other
oscillator modulator combination remains off
TL H 5079– 5