Tektronix 535A, Instruction Manual

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Operating Instructions — Type 535A
will work equally well. For applications of this type, the
triggering mode used is purely a matter of choice. The pri­
mary thing to consider in choosing the triggering mode is
whether or not it allows you to obtain the display you
want.
To determine the best mode of operation for a particular
application, it is usually best to try each triggering mode
in the application. The Automatic mode should be tried
first since this triggering mode provides stable triggering in
most applications without the necessity of setting the
STABILITY or TRIGGERING LEVEL controls. If the Auto­
matic mode does not provide the desired display, it will
then be necessary for you to try one or more of the other
triggering modes.
Automatic Triggering Mode
The automatic mode is most frequently used because of
its ease of operation. This mode is useful in obtaining stable
triggering from waveforms with frequencies of from approxi­
mately 60 cycles to 2 megacycles. The principal advant­
age of this type of operation is that it is not necessary to
adjust either the STABILITY or TRIGGERING LEVEL controls
to obtain a stable display. This permits you to observe a
large number of waveforms with different shapes and
amplitudes without adjusting any of the triggering controls.
In the absence of a triggering signal, the sweep continues
to run to provide a convenient reference trace on the oscillo­
scope screen.
The automatic triggering mode is selected by placing the
TRIGGERING MODE switch in the AUTO, position. The trig­
gering source and slope is then selected and the input signal
is applied to the oscilloscope. No other control adjustments
are required. Since the TRIGGERING LEVEL control has no
effect on the display when automatic triggering is used, it
is impossible to select the point on the triggering waveform
where the sweep is triggered. Each sweep is instead trig­
gered at the average voltage point of the waveform.
DC
Triggering Mode
The DC triggering mode is selected by placing the TRIG­
GERING MODE switch in the DC position. This mode per­
mits excellent triggering on all types of waveforms in the
frequency range from dc to approximately 5 mc. The DC
mode also permits the selection of the point on the trig­
gering waveform where the sweep is triggered through the
use of the TRIGGERING LEVEL CONTROL.
To use the DC triggering mode you must set the STABIL­
ITY and TRIGGERING LEVEL controls after first selecting
the triggering signal source and slope. The method used to
set the STABILITY and TRIGGERING LEVEL controls is the
same for the DC mode, AC mode, and AC Low Frequency
Reject mode. The STABILITY control must be set properly
before the TRIGGERING LEVEL control can be adjusted.
The STABILITY control has a PRESET position at the fully
counterclockwise setting of the control. This position permits
proper triggering in most applications without necessitating
additional adjustment of the STABILITY control. If it be­
comes impossible or difficult for you to obtain proper trig­
gering with the STABILITY control at PRESET, you must then
adjust the control. This is done with the TRIGGERING LEVEL
control fully counterclockwise. The STABILITY control is
rotated clockwise from the PRESET position until a trace
appears on the screen. The proper setting for the control
is then obtained by rotating the knob slowly counterclock­
wise until the trace just disappears.
The TRIGGERING LEVEL control determines the point on
the triggering waveform where triggering of the sweep
occurs. This control is set to provide the display you want
while at the same time causing stable triggering. Rotating
the control clockwise causes the sweep to trigger at more
positive points on the waveform while rotating the control
counterclockwise causes the sweep to trigger at more nega­
tive points. If the displayed waveform is vertically centered
under the graticule setting the TRIGGERING LEVEL control
at 0 will cause the sweep to start at approximately the mid­
voltage point of the waveform. In the DC mode, rotating the
vertical position control will change the point on the trig­
gering waveform where the sweep is started.
The + and — markings for the TRIGGERING LEVEL con­
trol should not be confused with similar markings for the
TRIGGER SLOPE control. The markings on the TRIGGER
SLOPE control indicate only whether triggering occurs on
the rising or falling portion of the triggering waveform. The
markings for the TRIGGERING LEVEL control indicate
whether triggering occurs on the upper or lower portion
of the waveform.
AC Triggering
Mode
Selection of the AC triggering mode is made by placing
the TRIGGERING MODE switch in the AC position. This
mode provides useful triggering in the frequency range of
approximately 15 cycles to 5 mc. These frequency limits
vary slightly depending upon the shape and amplitude of
the triggering waveform. In the AC mode, triggering is
unaffected by the dc components of the triggering signal
or by the vertical positioning of the trace. The triggering
level can be selected to provide the desired display using
the STABILITY and TRIGGERING LEVEL controls. These two
controls are set as described for the DC triggering mode.
AC Low Frequency Reject Triggering Mode
The AC Low Frequency Reject triggering mode is used
when the TRIGGERING MODE switch is in the AC LF REJECT
position. This mode is similar to the AC mode except that
low-frequency waveforms are rejected by the triggering cir­
cuit. This triggering mode works well with high-frequency
waveforms, but it may be difficult to obtain stable triggering
with frequencies below approximately 1000 cycles.
Occasionally you may be required to obtain stable trig­
gering from a fairly high-frequency waveform that is mixed
with a great deal of low-frequency noise or line-frequency
pickup. In such cases the additional noise and pickup can
make it very difficult to obtain a stable display. If this
occurs, you can select the AC Low Frequency Reject trig­
gering mode, thereby eliminating the effects of the low fre­
quency noise and pickup. The low frequencies are blocked
from
the triggering circuit while the high-frequency trigger-
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