age is down. This holds the grid of V45B below
cutoff, since the two circuits are dc-coupled.
With V45B in a state of cutoff its plate voltage
is up, hence no output is being produced.
A negative-going signal is required at the grid
of V45A to force the Trigger Multi into its other
state in which a triggering pulse can be pro
duced. However, since the signal at the grid of
V45A is an amplification of the triggering signal,
it contains both negative- and positive-going
portions.
The negative-going portion of the signal will
drive the grid of V45A in the negative direction,
and the cathodes of both tubes will follow the
grid down. At the same time the plate voltage
of V45A starts to rise, which causes the grid volt
age of V45B to rise. With the grid of V45B go
ing up and its cathode going down, V45B starts
to conduct. As V45B starts conducting its cath
ode starts going up; hence the cathode of V45A
starts going up. With the grid of V45A down
and its cathode up, V45A cuts off. And since
V45B is conducting its plate voltage drops, creat
ing a negative step in the output. This transition
occurs very rapidly, regardless of how slowly
the grid signal of V45A falls.
When the signal at the grid of V45A starts in
the positive direction, just the opposite chain of
events will occur. V45A will start conducting
again, which in turn will drive the grid of V45B
below cutoff. This will cause the voltage at the
plate of V45B to rise, which in turn will complete
the negative step-voltage output from the Trig
ger Multi circuit.
The Trigger-Input Amplifier V24 amplifies the
triggering signal that in turn is used to drive the
Trigger Multi. The amplified signal is always
taken from the plate of V24B, but the grid of
either tube (V24A or V24B) can be connected to
the input circuit. When the Slope switch SW20
is in the — position the grid of V24A is con
nected to the input circuit and the grid of V24B
is connected to a bias source adjustable by
means of the TRIGGERING LEVEL control R17.
With this configuration V24 is a cathode-coupled
amplifier, and the signal at the output plate is in
phase with the signal at the input grid. The cir
cuit operation is then as follows: With the Slope
switch in the — position, triggering of the Time-
Base Generator will occur on the falling (nega
tive-going) portion of the triggering signal. Re
calling that a negative-going signal is required
at the grid of V45A to drive the Trigger Multi
into the other state of its bistable operation, this
signal must be of the same polarity as the origi
nal signal at the input circuit.
However, when it is desired to trigger the
Time-Base Generator on the rising or positive
going portion of the triggering signal, the signal
at the grid of V45A must be opposite in polarity
to that at the input circuit. This is accomplished
by placing the Slope switch in the -|- position.
With this arrangement the grid of V24B is con
nected to the input circuit and the grid of V24A
is connected to the bias source. This eliminates
V24A from the amplifier circuit and V24B be
comes a plate-loaded amplifier. The output
waveform will therefore be opposite in polarity
to the grid waveform.
The TRIG. LEVEL CENT, control R24 determines
the division of current through both tubes, and is
adjusted so that the quiescent voltage at the
plate of V24B lies in the center of the hysteresis
of the Trigger Multi. The TRIGGERING LEVEL
control R17 is adjusted to vary the bias on the
tube to which it is connected. This in turn varies
the quiescent voltage at the plate of V24B about
the level established by the TRIG. LEVEL CENT,
control. By adjusting the TRIGGERING LEVEL
control, the operator can select the point on the
waveform at which he wishes to trigger the
Time-Base Generator.
When the Time-Base Trigger circuit is switched
into the automatic mode of triggering (TRIGGER
ING LEVEL) control turned full left, the Automatic
switch SW17 converts the Trigger Multi from a
bi-stable configuration to a recurrent (free-run
ning) configuration. (This is not to be confused
with the action of the Recurrent switch, shown
on the Time-Base Generator diagram, which
causes the Sweep-Gating Multi to free-run.) This
is accomplished by coupling the grid circuit of
V45B to the grid circuit of V45A via R32. In ad
dition, the dc-coupling between the Trigger Input
Amplifier and the Triggering Multi is removed
when the switch is in this position.
The addition of R32 to the circuit causes the
Triggering Multi to free-run in the absence of a
triggering signal. For example, assume the grid
of V45A is just being driven into cutoff. The
voltage at the plate of V45A starts to rise, carry
ing with it the voltage at the grid of V45B. Since
the two grids are coupled through R32, this
causes the voltage at the grid of V45A to start
rising. The time-constant of the R32-C31 network
is such that it takes about .01 second for the volt
age at the grid of V45A to rise exponentially
from its starting point, below cutoff, to a point
where plate current can start.
AS V45A starts to conduct its plate voltage
drops, which in turn lowers the voltage at the
grid of V45B. The voltage at the grid of V45A
CIRCUIT DESCRIPTION — TYPE 502
4-5
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