Section IV Model 120B
Paragraphs 4-23 to 4-32
cutoff, V2 conducts. As the input voltage becomes 5-8.) Sawtooth generator V205A is a Miller in-
more positive, it eventually reaches a predetermined tegrator which produces a linear sawtooth waveform.
level (a) at which the circuit changes state; that is, When switch diode CR201 ceases conduction, the in-
V1 conducts and V2 is cutoff. If the input voltage then tegrating capacitor charges through the integrating
goes negative, the common cathode potential decreases resistance, producing negative-going a potential at
and the grid of V2 becomes positive. When the input the control-grid of V205A. The amplified signal at
reaches a second predetermined (b), level V2 conducts the plate of V205A is coupled back through sawtooth
and the circuit switches back to its initial state. The cathode follower V205B to the integrating capacitor,
output of the circuit is a voltage step, either positive completing the negative feedback required by this
or negative depending upon the slope of the input Miller type integrator.
signal.
4-28. The sawtooth signal at the cathode of V205B is
also applied to SWEEP MAGNIFIER switch S102.
4-23. The input voltage levels at which a Schmitt When ,9102 is in the X5 position, the sawtooth applied
trigger circuit switches are called the hysteresis to the horizontal amplifier has a peak amplitude five
limits. Note that the circuit (figure 4-4) does not
switch unless the input crosses both limits. times greater than that applied when S102 is in the X1
position; hence, the angle of slope is increased five
times and the sweep time is one-fifth of that selected
4-24. Trigger generator V201 has some additional by the HORIZONTAL DISPLAY switch.
features: the TRIGGER LEVEL control adjusts the
bias on V2OlA and thus determines the level which the
input trigger must reach to change state; in addition, 4-29. The sawtooth signal at the cathode of V203B is
when the TRIGGER LEVEL control is at its extreme applied to the control grid of V202A. When the slope
limit, switch S202 is placed AUTO. in This opens the of the signalreaches the upper hysteresis limit of the
shunt across resistor R207 and capacitors C205 and start-stop trigger, the trigger changes state; hence,
C206, and the trigger generator becomes an astable the unblanking gate output of V203A is terminated.
multivibrator. This provides triggers to the stop- The switch diode CR20l and clamp V204 again conduct,
start trigger even though no sync is applied to the respectively, halting the charging of the integrating
trigger generator. When a sync signal above approxi- capacitor and terminating the sawtooth waveform.
mately 50 cps is applied, however, the trigger gen- The hold-off capacitor in the cathode of V203B charges
erator then synchronizes with the sync signal. The during the rising slope of the signal output of the hold-
step output of the trigger generator is taken from the off cathode follower but does not immediately dis-
plate of V201B andapplied to a differentiating network. charge when the sawtooth input to the hold-off cathode
follower terminates. This prevents the start-stop
trigger from again being triggered by a negative spike
4-25. DIFFERENTIATING NETWORK. (See figure
5-8.) The differentiating network is composed of the until the sweep circuits have fully recovered. A dif-
series combination of capacitor C207, inductor L201, ferent value of hold-off capacity is selected for each
setting of the HORIZONTAL DISPLAY switch.
and resistor R210. The r-c time constant of this net-
work is extremely small compared with the width of
the trigger generator output pulses; consequently, the 4-30. HORIZONTAL AMPLIFIER.
signal developed across inductor L201 and resistor
R210 consists of short negative and positive spikes 4-31. The grid of VlOlA (figure 5-7) receives either
that are coincident with the leading and trailing edges the sweep output from the horizontal sweep circuits
of the trigger generator output pulses. These spikes or from an external sweep source, depending on the
are coupled through capacitor C208 to the control grid setting of the HORIZONTAL DISPLAY switch. When
of V202A, one half of the start-stop trigger. external sweep is being used (EXT SENSITIVITY posi-
tions), any one of three horizontal sweep sensitivities
can be selected: .1 VOLTS/CM, 1 VOLTS/CM, or 10
4-26. START-STOPTRIGGER V202, CLAMP V203A, VOLTS/CM. The AC-DC switch allows selection of
AND SWITCH DIODE CR201. -(See figure 5-8. ) The
start-stop trigger is a Schmitt trigger circuit. A direct coupling, or ac coupling through capacitor
C101.
typical Schmitt trigger is described in paragraph 4-22.
V202 is triggered by a negative spike pulse from the 4-32. The horizontal amplifier consists of two dif-
differentiating network. One output step waveform is ferential amplifiers (V101 and V102) in cascade. When
taken from the plate of V202B and simultaneously ap-
plies to switch diode CR201 and the grid of clamp the HORIZONTAL DISPLAY switch is set to one of the
V203A. The waveform applied to V203A cuts off EXT SENSITIVITY positions, the resistance between
the cathodes of amplifier VlOl consists of the hori-
V203A, and thus disables the clamping action of this zontal VERNIER control (R275B) in series with the
tube. The waveform applied to switch diode CR201 horizontal gain control (R104); otherwise, resistor
reverse-biases the diode, and thus it ceases conduc- R107 is between the cathodes. The horizontal POSI-
tion. Another step output is taken from the plate of
V202A and applied through cathode follower V204 to TION control (potentiometer R109) between the plates
the high-voltage power supply; this serves as the crt of VlOl controls the horizontal position of the crt
trace. The second stage (V102) is neutralized by
unblanking gate. plate-to-grid cross neutralization. The output of the
horizontal amplifier is taken from the plates of V102
4-27. SAWTOOTH GENERATOR V205A AND SAW- and applied directly to the horizontal deflection plates
TOOTH CATHODE FOLLOWER V205B. (See figure of the crt. |
ACEC
HEWLETT-PACKARD
