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E L E C T R I C A L
P R I N C I P L E S
83
Square
wave
Input
Anode
Potential
TubeT,
Grid
PDtentiol
Tube Ti
Tube T2
+
40
Anode
Potential Te
-30
Grid
Tube T2
T2
Figure
81.
Oscillogram of Overall Trigger Operation
quite square, and at the grids the peak negative
dip is only -1j volts.
As soon as the maximum
negative
is reached, both g i d s start to
rise in potential.
As previously shown in Figure
79, the grid of T I rises much faster than that of
T2 and reaches the conducting point of -8 volts
first. As soon as T I starts to conduct, the poten-
tial at its anode starts to drop, forcing the grid of
T2 down and holding T2 non-conductive.
With
,the circuit constants shown in Figure 74, after a
time interval of approximately 3 to 5 microseconds,
the charges on
all
capacitors will have been equal-
ized and the circuit will be as before except that
T I is now conducting instead of T2. The dotted
lines indicate what the rise in the grid potentials
might look like
if
the tubes could be held non-
conducting by some external means. I t is impor-
tant to note that, alrhough thc triggering action is
very fast, a definite time interval is required, hence
a peaked pulse of extremely short duration (say
1
microsecond) may nor trigger the circuit.
Figure 81 shows a sketch of both grid and anode
potentials adapted from patterns taken directly
from an oscilloscope. The potential graphs repre-
sent the po~entials when the trigger is triggered
or reversed continually
by
a square wave input.
Note that the shape of the grid potential is the
same as shown in Figure 80.
So far nothing has been said abour the ability
of this trigger circuit to distinguish between posi-
tive and negative pulses.
The constants of this
trigger are such thar it is considerably more sensi-
tive to negative pulses than it is to positive pulses.
Therefore, if the input pulse is kept within reason-
able limits, the rrigger will respond only to the
negative pulses of a square wave (Figure 8 1 )
.
For
example, a -20 volt shift in potential will cut off
the conducting tube, enabling the trigger to trans-
fer; but a +20 volt shift will not bring the grid
of the nun-conducting tube up to the conducting
point and thus cannot make the tube start to con-
duct. The only action of a +2O volt pulse on the
conducting tube is to drive the grid slightly posi-
tive. Therefore, the trigger will transfer only on
a negative pulse (or shift in potential), and the
trigger can be made to distinguish between nega-
tive and positive pulses. The limits within which
the trigger will respond only to negative pulses for
the circuit constants given is approximately 20 to
80 volts. Thar is, at least -20 volts are required to
trigger, but around 80 volts the trigger responds
to positive pulses as well as negative.
For this
reason the triggers in this unit are operated by 50
volr pulses, or roughly at the middle of the range.
Figure 81 shows why the trigger is not reversed
on a positive pulse which is theoretically large
enough to bring the grid of the non-conducting
tube up to the conducting point.
Norice that at
point a2 the grid of the non-conducting tube T I
actually appears to go negative although the square
wave inpur is shifting in a positive direction. This
is because the positive pulse acting on the grid of
the conducting tube T2 drives the anode potential
of T2 down almost 20 volts as shown at poinr a3.
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