Tektronix 535A, Instruction Manual

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Circuit Description — Type 535A
This “sample'' of the vertical signal is also ac-coupled
through Vertical Signal Out C. F., V593B and C599, to a
front-panel binding post labeled VERT. SIG. OUT.
Delay Line
The output signal from the Vertical Amplifier is coupled
through the balanced Delay Line to the vertical deflection
plates of the crt. The function of the Delay Line is to retard
the arrival of the waveform at the deflection plates until
the crt has been unblanked and the horizontal sweep has
started.
This delay, as mentioned, insures that the very
“front” of fast vertical signals can be observed. The line
is adjusted, by means of the variable capacitors connected
across the line, for optimum transient response.
HORIZONTAL DEFLECTION SYSTEM
Time-Base Trigger
General
The Time-Base Trigger develops a pulse which will initiate
a cycle of action in the Time Base Generator. To display
signals below five megacycles, a TRIGGERING MODE switch
allows the operator to select the type of triggered operation
most suitable for the waveform to be displayed. A second
switch, the TRIGGER SLOPE switch, allows the operator to
select the “slope", either positive or negative, which will
cause triggered operation of the sweep. To display signals
above five megacycles, the Time Base Trigger is bypassed,
and the signal is applied to the Sweep-Gating Multivibrator
in the Time-Base Generator. No choice of triggering slope is
available in this mode.
Trigger-Input Amplifier
Triggering signals may be developed from several sources.
The most common source of triggering signals utilizes the
internal circuitry of the oscilloscope to sample the signal
present in the vertical amplifier. Using an internal source
of triggering signal, either triggered operation in the various
triggering modes, or synchronized operation, is available.
Triggered or synchronized operation of the time-base
circuitry may also be effected from external sources. Opera­
tion in any of the available modes is possible with external
signals.
In the +Line or —Line positions of the TRIGGER SLOPE
switch a voltage at the power line frequency is used to
develop the triggering signal.
The Trigger-Input Amplifier is polarity-inverting, cathode-
coupled amplifier. It serves two basic functions in the Time-
Base Trigger. First, it provides a source of negative-going
signal to drive the following stage. Secondly, by means of
the TRIGGERING LEVEL control, it enables the operator to
select the signal level at which triggered operation of the
Time-Base will occur.
To trigger from a negative-going signal, the grid of the
V24A section is connected to the input signal source. The
grid of the V24B section is connected to a dc bias source,
which is adjustable with the TRIGGERING LEVEL control.
This bias voltage establishes the voltage present at the plate
under no-signal conditions.
The voltage at the grid of V24A and the voltage at the
plate of V24B are in phase with each other; that is, they
both go through ac zero in the same direction at the same
time.
Thus, the V24A section acts as a cathode-follower, and
the signal voltage developed across the cathode resistor
becomes the input signal to the V24B section.
To trigger from a positive-going signal, the grid of the
V24A section is connected to the TRIGGERING LEVEL con­
trol, and the grid of the V24B section is connected to the
input signal. With this configuration, the voltage at the
plate of the V24B section will be 180 degrees out of phase
with the input-signal voltage.
In each of the cases outlined above, a negative-going
signal is produced at the plate of the V24B section of the
Trigger-Input Amplifier irrespective of the polarity of the
input signal.
Also, the amplitude of the triggering signal necessary to
cause operation of the following stage is determined by the
setting of the TRIGGERING LEVEL control.
Trigger Multivibrator
The Trigger Multivibrator is a dc-coupled multivibrator.
In the quiescent state, ready to receive a signal, the V45A
section is conducting and the plate voltage is down. Since
the
plate is dc-coupled to the grid of the V45B section, that
grid is held below cutoff. With the V45B section cut off its
plate voltage is up and no output is developed.
The negative-going portion of the signal from the Trigger-
Input Amplifier is required to drive the grid of the V45A
section down. As the V45A section grid is driven negative
the current flow through the tube is restricted, and the
voltage at the plate starts to rise.
The rise in voltage at the plate of the V45A section
carries the grid of the V45B section in the positive direction.
The cathodes of both sections are coupled together, and
follow the action of the grids. With the V45A section grid
going in a positive direction, and the cathode in a negative
direction, the V45B section starts to conduct. As the V45B
section starts to conduct the cathodes of both sections follow
the action of the V45B section grid; hence the cathode
voltage starts to rise.
As the V45A section grid goes down and its cathode goes
up it stops conducting. As the V45B section conducts, its
plate voltage drops, creating a negative step at the output.
This transition occurs rapidly, regardless of how slowly the
triode grid falls.
When the signal applied to the grid of the V45A section
goes in a positive direction the action described in the
previous paragraphs reverses itself. That is, the V45A sec-
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