Tektronix 502 series, Instruction Manual

Page: 17 / 114

In the dc mode, when the triggering signal is
obtained from either the lower or the upper
beam amplifiers, varying the respective vertical
position controls will change the triggering point.
For this reason, you may find it necessary to re­
adjust the TRIGGERING LEVEL control when you
change the vertical position of the trace. If you
desire to eliminate this effect, you can use the
ac mode providing the triggering signal is other­
wise suitable for this mode of operation. In the
dc mode, the dc level of external triggering
signals will also effect the triggering point. Gen­
erally, when the triggering signal is small com­
pared to its dc level, the ac mode should be
used.
AC Mode
The ac mode is selected in the LINE, LOWER
AC, UPPER AC, and EXT AC positions of the
TRIGGER SELECTOR switch. This mode provides
stable triggering on virtually all types of wave­
forms. As a general rule, however, the ac mode
is unsatisfactory for triggering with low ampli­
tude waveforms at frequencies below approxi­
mately 15 cycles. This figure will vary depend­
ing upon the amplitude and shape of the trig­
gering waveform and should not therefore be
set as an absolute standard. Triggering at fre­
quencies below 15 cycles can be accomplished
when higher amplitude triggering signals are
used.
In the ac mode, the triggering point depends
on the average voltage level of the triggering
signals. If the triggering signals occur at ran­
dom, the average voltage level will vary causing
the triggering point to also vary. This shift of
the triggering point may be enough so that it is
impossible to maintain a stable display. In such
cases you should use the dc mode.
How to select triggering slope.
If you wish to trigger the sweep on the rising
(positive slope) portion of the triggering wave­
form, place the TRIGGER SELECTOR switch at
+ . If you wish to trigger on the falling (nega­
tive slope) portion of the triggering waveform,
place the TRIGGERING SELECTOR switch at —.
How to set the triggering level.
In the ac and dc triggering modes, the TRIG­
GERING LEVEL control determines at which point
on the triggering signal that the sweep is trig­
gered. Using this control, the sweep can be con­
tinuously triggered at any point on the waveform
so long as the slope of the waveform is great
enough to provide stable triggering. In the DC
mode, the sweep cannot be triggered with any
degree of stability at the top of a square wave,
for example, because the time that the voltage
remains constant is comparatively long. As a
result, the sweep triggers at random points along
the top of the square wave producing consider­
able trace jitter.
You can use the same method to set the TRIG­
GERING LEVEL control for either the ac or dc
mode. After selecting the triggering slope, ro­
tate the TRIGGERING LEVEL control fully count­
erclockwise to the AUTOMATIC position. Then
rotate the TRIGGERING LEVEL control clockwise
until the sweep no longer triggers. Continue to
rotate the control in the clockwise direction until
the sweep again triggers and a stable display is
obtained. Further rotation of the control in the
clockwise direction causes the sweep to trigger
at more positive points on the triggering wave­
form. See Figure 3-4.
FREE-RUNNING OPERATION
With the Type 502, you can get a periodic,
free-running sweep, independent of any external
triggering or synchronizing signal by rotating the
TRIGGERING LEVEL control fully clockwise to the
RECURRENT position. This permits you to ob­
serve the upper- and lower-beam traces without
an input signal.
OPERATING INSTRUCTIONS — TYPE 502
3-5