Model 120B Section IV
Paragraphs 4-16 to 4-22
the negative-going output is applied to switch diode
CR201.
4-19. Switch diode CR201 normally clamps the input
GRlD OF
V201A to sawtooth integrator V205A to a low negative volt-
age, preventing sawtooth integrator V205A from gen-
erating a sawtooth waveform. When the negative-
I
I going step signal from the start-stop trigger is applied
-1 1 to the switch diode, however, the diode is reverse
PLATE OF biased and allows the sawtooth integrator to generate
V201B a sawtooth waveform.
I 4-20. The sawtooth output of V205B is simultaneously
I
I applied to the horizontal amplifier to serve as the
JUNCTION I
horizontal sweep signal and the hold-off cathode fol-
OF C207
' lower V203B. The output of the hold-off cathode fol-
AND C208
lower is applied to the input of the start-stop trigger
I
to terininate the sweep. The values of R and C selected
I
by the HORIZONTAL DISPLAY switch determines the
- I
PLATE OF slope of the sweepand therefore the time of the sweep.
V202B Clamp V204 ensures that the sawtooth always starts
at the same level. Figure 4-3 shows the relative
I I
I I timing of the sweep generator waveforms.
I I
I I
I
4-21. TRIGGER GENERATOR V201. (See figure
GRID OF 5-8.) The sync signal selected by the TRIGGER
V203B
SOURCE switch is applied through capacitor C202 and
(SEE NOTE)
I resistor R201 to the input control grid of V201, which
I
I
I I is a Schmitt trigger circuit when the TRIGGER LEVEL
I I
I I control is not in AUTO. The Schmitt trigger is a
form of bi-stable multivibrator, and is used to obtain
pulses with rapid rise and fall times.
GRlD OF
V202A
4-22. Figure 4-4 illustrates a simplified Schmitt
trigger and typical input and output waveforms. If in-
LENGTH OF SAWTOOTH DEPENDS
itially the input voltage (figure 4-4) is such that V1 is
ON RC TIME CONSTANT
SELECTEDBY
HORIZONTAL DISPLAY^
+ DC
SWITCH LD-S- 530
Figure 4-3. Sweep Generator Waveforms
V4 reaches the firing potential, V4 fires and the volt- 4 b (w OUTWT
age at the junction of V4 and V5 drops, deionizing V5.
C14 starts charging again, and the cycle repeats. A v I v2
60-mv square wave is developed across R54, and
INPUT
this signal is applied through the SENSITIVITY switch
to the vertical amplifier.
4-16. SWEEP GENERATOR.
4-17. BLOCK-DIAGRAM DESCRIPTION. (See figure
4-2.) The sync signal from the TRIGGER SOURCE -
-
- - -
switch is applied to trigger generator V201. The trig-
ger generator produces a rectangular wave with a
repetition frequency equal to that of the input sync
signal. This rectangular wave is applied to a differ-
entiating network which converts it to positive and
negative spikes that are coincident with the leading
and trailing edges of the rectangular wave. These
spikes are then applied to the input of start-stop trig-
ger V202.
4-18. The negative spike switches the start-stop I
ob a b LO 5-53!
trigger, which produces two step outputs. The posi-
- - -
tive-going output is applied through V204 to the high-
voltage power supply as the crt unblanking gate, and Figure 4-4. Typical Schmitt Trigger |
ACEC
