
Circuit Description—Type 324
R244. If R246 is set to a point midway between the center
tap and either side, 0 V will be applied and the two tran
sistors will be conducting equal current. The collector volt
ages will be approximately equal under those circum
stances. A signal input to the Comparator Am plifier will
generate an in-phase signal at the Q239 collector and an
inverted signal at the Q231 collector. Both outputs are
made available to contacts of the (TR IG G ER SLOPE)
switch.
Triggering action occurs when the selected collector
varies approximately 0.1 V from a balanced output condi
tion. If the TR IG G E R LE VE L potentiometer is offset from
0 V , signals at the Q231 base must compensate for the
offset before causing trigger action. Through the use of the
TR IG G ER control, both T R IG G E R LE V E L and TR IG G E R
SLOPE can be manipulated to select any point along the
rising or falling slope of a signal to cause trigger action. If
two separate signals of different amplitudes are presented
simultaneously to the comparator, R246 (TR IG G ER
LEVEL) can be set to a point where only the larger of the
two signals can cause triggering action, thereby causing the
sweep to be triggered at the frequency of the larger signal.
Trigger Multivibrator
Non-Automatic Operation. The Trigger Multivibrator is
a Schmitt Trigger circuit when operated in the non
automatic mode. When Q253 is conducting, the current
through R260 and that through the voltage divider con
nected to the base of Q263 create a combination of volt
ages which prevent Q263 from conducting. When the out
put voltage from the comparator decreases, the Q253
emitter voltage decreases and the collector voltage in
creases. Q263 is thereby permitted to go into conduction.
Q263 emitter voltage rises and Q253 cuts off. The sudden
increase of Q253 collector voltage is coupled through R256
and C256, aiding Q263 conduction. The current increase
through R262 creates a negative step which is differentiated
and applied to the Sweep Generator circuit. This develops a
negative trigger which initiates a horizontal sweep. When
the Q253 base voltage returns sufficiently positive, Q253
goes back into conduction, cutting Q263 off. The circuit is
then ready for another cycle.
Automatic Operation. When the TR IG G E R control is
switched to either the + or — AUTO position, the following
circuit changes are made: switch wafer IF inserts C250 in
the Comparator Amplifier output signal path, causing the
Q253 DC base voltage to be determined by R251, R252
and R253; wafer 2F inserts C221 in the triggering signal
path, placing the Q231 base at ground potential; wafer 2R
connects the base of Q239 to ground, simultaneously in
serting one of the Trig Auto Caps into the base and collec
tor circuits of Q263. The Trig Auto Cap value is dependent
upon the position of the T IM E /D IV switch.
In A U TO T R IG G E R mode, the Schmitt trigger circuit
becomes a free-running multivibrator which will synchro
nize to a triggering signal having a frequency greater than
the multivibrator repetition rate. In the absence of trigger
ing signals from Q231, operation occurs as follows:
Assume that Q253 is conducting, Q263 is cut off, and that
C270 has no charge on it. Circuit design causes the junction
of R257 and R258 to go slightly positive, charging C270.
The voltage at the base of Q263 increases in proportion to
the voltage 'Vamp'' at the C 270-R 257-R 258 junction, until
the Q263 is turned on. This increases the voltage at the
emitter, turning Q253 off. The rise of Q253 collector volt
age is coupled through C253 and R256, aiding Q263 con
duction. The resulting current lowers the voltage at the
collector of Q263, sending a negative gate to C301 in the
Sweep Generator. The negative gate voltage is also coupled
through R264, causing C270 to discharge. As C270 dis
charges, its negative-going voltage ramp is coupled through
R257, decreasing the Q263 base voltage. The Q 263 emitter
voltage follows the base, carrying the Q 253 emitter with it.
Q253 conducts when its emitter becomes sufficiently nega
tive. The resulting change of Q253 collector voltage is
coupled through R256, cutting Q263 off. The cycle then
repeats itself.
If a signal is coupled in through C250 during A U T O
operation, it will either combine with the voltage ramp to
cause switching action, or it will override the ramp and
cause switching action by itself.
Consider the A U TO condition existing when Q 263 is cut
off. A positive-going ramp occurs at the base of Q263. If a
negative signal simultaneously appears at the base of Q253,
it will be coupled to the emitter, lowering the Q 263 emitter
voltage. The positive ramp at the base and the negative
signal at the emitter combine their effects to increase the
emitter-base forward bias, placing Q 263 in conduction. In
similar fashion when Q253 is turned off, a positive signal at
its base will work in conjunction with the negative ramp at
its emitter to turn Q253 on. It should be noted that if both
the ramp and the input signal are required to produce
switching action, the switching rate will not be much
greater than the AU TO frequency, although it will be
synchronized to the signal frequency or a sub-multiple of it.
This situation occurs when trigger inputs are less than those
specified under Trigger Sensitivity at the beginning of this
manual.
If the signal in from the comparator has a higher fre
quency than the A U TO multivibrator, and has sufficient
amplitude to override the ramp voltage, its effect alone will
cause the previously explained switching action, creating
negative gates at the frequency of the input signal.
Smaller Trig Auto Caps are substantiated when the
T IM E /D IV switch is changed from the .5 to .2 mS positions
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