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feedback from the staircase generator to the staircase in-
verter still determines the horizontal position of the crt beam
at the time of comparison, and the fast ramp still determines
the equivalent time/cm of the display. Thus, the crt display
always represents equivalent time, regardless of the source
of scanning voltage.
The TIME POSITION and FINE controls can adjust the
time at which the comparison pulse causes the vertical
system to take a sample by raising or lowering the dc level
of the staircase feedback signal to which the fast ramp must
run. The effect of these controls is to shift the "time win-
dow" of the observed crt display.
TRIGGER CIRCUIT
The schematic diagram of the trigger circuit shows the
input switching to the circuit, the trigger recognition circuit,
the trigger output circuit and the trigger holdoff. Fig. 3-6 is
a block diagram of the trigger circuit.
Trigger Source Switch
The TRIGGER SOURCE switch (SW400) is used to select
the triggering signal either from the trigger takeoff at the ver-
tical input or from the EXT TRIG input connector. The sig-
nal voltage received from the trigger takeoff is approxi-
mately 1/7th the amplitude of the vertical input amplitude.
The trigger input that is not used is terminated in 50 ohms
by R401 and R402 or by R405 and R406. In addition to se-
lecting the source, the TRIGGER SOURCE switch also se-
lects the polarity of the input signal that will cause a trigger
recognition, by sending the signal either directly to the
trigger circuit (with the switch at one of the + positions)
Fig. 3-6. Block diagram of the Trigger circuit.
or by connecting the signal through inverting transformer
T410 (with the switch at one of the - positions). A low-
frequency termination and a high-frequency ground for the
primary of T410 are provided by R412, R413, C412 and C413.
The values of these components match the frequency re-
sponse characteristics of the transformer.
circuit input when the TRIGGER SOURCE switch is set to
FREE RUN position. The FREE RUN position of the switch
also changes the operation of the trigger circuit so the dis-
play will free run regardless of the triggering controls.
Trigger Recognition and Output
triggering signals from 10cps up to 1 Gc or more. Below
approximately 100kc, pulse triggering is required because
of the sinewave rolloff produced by the capacitive input
coupling through C415. Above the maximum repetition rate
of the holdoff circuit, countdown occurs and trigger recogni-
tion takes place on the first input signal excursion of the
correct slope following the holdoff period. On triggering
signals above about 10Mc, the circuit attempts to syn-
chronize with the input signal during the small interval of
time between the end of the holdoff period and the time
that the circuit is ready to be triggered.
figuration, provides isolation between the trigger input and
recognition diode D430 to minimize trigger kickout when
the tunnel diode switches. Part of the bias current for D430
is provided through Q424 and the INT TRIG LEVEL adjust-
ment, R420. The remainder of the bias is supplied from
current-switch transistor Q464, which also provides bias for
Circuit Description – Type 1S1
Both triggering sources are disconnected from the trigger
The trigger circuit operates in a triggered mode on input
Input transistor Q424, connected in a common-base con-
3-9
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