LM1295 National Semiconductor, LM1295 Datasheet - Page 7

• Manufacturer Part Number: LM1295
• Description: DC Controlled Geometry Correction System for Continuous Sync Monitors
• Manufacturer: National Semiconductor

Circuit Description
VERTICAL OSCILLATOR
The vertical oscillator is an injection-locked ramp generator
with automatic level control. The automatic level control
maintains the oscillator output ramp height with changes in
input frequency. The oscillator requires negative-going TTL
level vertical sync pulses, wider than 200 ns, to lock. In the
absence of vertical sync, the oscillator runs at free-run fre-
quency. The vertical output amplitude is controlled by a volt-
age between 0V and 4V on the V Height input with a range
of about 1.8 to 1, and by a voltage between 3V and 4V on the
Vertical Dynamic Height input with a range of about 1.3 to 1.
The control bandwidth of the V Height input is low due to the
automatic level control, but that of the Vertical Dynamic
Height is greater than 125 kHz. The oscillator has a circuit,
requiring an external capacitor, 2f Cap, that prevents the os-
cillator from locking at twice the vertical sync frequency. The
oscillator ramp voltage is converted into differential currents
superimposed on DC currents of about 315 µA for each out-
put. The voltage to current conversion gain is inversely pro-
portional to the value of the resistor connected between the
Rvert pins (21 and 22). Differential current outputs are pro-
vided instead of voltage to avoid ground noise. The ramp
voltage goes to the parabolic function generator and to two
multipliers used as voltage controlled amplifiers, one for hori-
zontal trapezoid correction and the other for horizontal paral-
lelogram correction.
PARABOLIC FUNCTION GENERATOR
The parabolic function generator makes a parabolic wave-
form from the vertical ramp. Its output goes to three multipli-
ers used as voltage controlled amplifiers, one each for V Dyn
Focus, E-W Pin, and H Bow.
Block Diagram
(Continued)
FIGURE 3.
7
VOLTAGE CONTROLLED AMPLIFIERS
The V Dyn Focus voltage controlled amplifier is controlled by
the V Dyn Focus Cntl input. Its output goes to an op amp
whose output is V Dyn Focus. The voltage controlled ampli-
fier has zero gain at approx. 2V input, maximum positive
gain at 4V, and maximum negative gain at 0V. The E-W Pin,
H Bow, H Trap and H Para voltage controlled amplifiers are
identical to the V Dyn Focus stage, each adjusted by its cor-
responding Cntl input. The bandwidth of the Cntl inputs is
greater than 125 kHz. The E-W Pin and H Bow amplifiers
have the parabolic waveform as their input, and the H Trap
and H Para amplifiers have the vertical ramp as their input.
The parabolic waveform and the ramp amplitudes track the
vertical output amplitude so the correction amplitudes follow
accordingly. The outputs of the E-W Pin amplifier (parabola)
and the H Trap amplifier (ramp) are summed together in an
op amp summing circuit, with H Trap weight
Pin. The output of the summing amplifier is H Dyn Width,
used for correcting E-W pincushion and horizontal trapezoid
distortion. The outputs of the H Bow amplifier (parabola) and
the H Para amplifier (ramp) are summed together similarly,
with H Bow and H Para equally weighted. The output of the
summing amplifier is H Dyn Cntr, used for correcting hori-
zontal bow and horizontal parallelogram distortion. All three
op amp outputs are identical structures and are typical low
output impedance type op amp outputs, capable of sinking
or sourcing 5 mA minimum.
DS012324-3
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that of E-W