Tektronix 502A, Instruction Manual

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Operating Instructions —Type 502A
gering will normally be used when you are observing a
waveform which bears a fixed time relationship to the power­
line frequency.
4. To trigger the sweep from some external waveform bear­
ing a definite time relationship to the observed waveform,
connect the external waveform to the TRIGGER INPUT con­
nector and set the TRIGGER SELECTOR switch at EXT AC
or EXT DC.
External triggering provide definite advantages over other
configurations of triggering in certain cases. With external
triggering, the triggering signal generally remains essen­
tially constant in amplitude and shape. It is thereby possible
to observe the shaping and amplification of a signal by each
stage of a circuit without resetting the triggering controls
for each observation. Also time and phase relationships be­
tween the waveforms at different points in the circuit can be
seen. If, for example, the external triggering
signal is de­
rived from the waveform at the input to a circuit, the time
relationship and phase of the waveforms at each point in
the circuit are automatically compared to the input signal by
the display presented on the face of the crt.
Automatic Mode
Automatic triggering is obtained by rotating the TRIGGER­
ING LEVEL control fully counterclockwise to the AUTO­
MATIC position. This provides a preset triggering level which
is set to allow triggering at the average voltage point of the
applied waveform. The sweep runs at approximately a 50
cycle rate when no triggering signals are applied. Auto­
matic triggering can be used with triggering signals obtained
from the LINE, UPPER, LOWER, or EXT positions of the TRIG­
GER SELECTOR switch, but for most waveforms, it is useful
only for triggering at frequencies above approximately 50
cycles. Automatic triggering saves considerable time
in
observing a series of waveforms since it is not necessary to
reset the triggering level for each observation.
DC Coupling
DC coupling is selected in the UPPER DC, LOWER DC, and
EXT DC positions of the TRIGGER SELECTOR switch. This
coupling of the triggering signal is particularly useful in
triggering from waveforms which
are not adaptable to ac
coupling, such as random pulse trains or very low frequency
waveforms. Random pulse trains pose a special problem in
ac coupling since the random occurrence of the input wave­
forms causes the average voltage level to shift. This in turn
may cause the triggering level to shift to an unstable point.
This problem is not encountered with dc coupling since the
triggering point is determined only by instantaneous voltages.
With dc coupling the triggering signal is obtained from
either the LOWER or the UPPER BEAM amplifiers, hence
varying the respective vertical position controls will change
the triggering point. For this reason, you may find it neces­
sary to readjust the TRIGGERING LEVEL controls when you
change the vertical position of the trace. If you desire to
eliminate this effect, you can use ac coupling provided the
triggering signal is otherwise suitable for this type of coupling.
With dc coupling, the dc level of external triggering signals
will also affect the triggering point. Generally, when the
triggering signal is small compared to its dc level, ac coup­
ling should be used.
AC Coupling
AC coupling is selected in the LINE, LOWER AC, UPPER
AC, and EXT AC positions of the TRIGGER SELECTOR switch.
This coupling provides stable triggering on virtually all types
of waveforms. As a general rule, however, ac coupling is
unsatisfactory for triggering with low amplitude waveforms
at frequencies below approximately 15 cycles. This figure
will vary depending upon the amplitude and shape of the
triggering waveform and should not therefore be set as an
absolute standard. Triggering at frequencies below 15 cycles
can be accomplished when higher amplitude triggering
signals are used.
With ac coupling, the triggering point depends on the
average voltage level of the triggering signals. If the
triggering signals occur at random, 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 dc coupling.
Trigger Slope
If you wish to trigger the sweep on the rising (positive
slope) portion of the triggering waveform, place the TRIGGER
SELECTOR switch at +. If you wish to trigger on the falling
(negative slope) portion of the triggering waveform, place
the TRIGGER SELECTOR switch at -.
Using the TRIGGERING LEVEL Control
In all positions except AUTOMATIC and RECURRENT, the
TRIGGERING LEVEL Control determines at which point on the
triggering signal the sweep is triggered. Using this control,
the sweep can be triggered at any point on the waveform so
long as the slope of the waveform is great enough to provide
stable triggering. With dc coupling, the sweep cannot be
triggered with any degree of stability at the top of a square
wave, for example, 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 considerable trace jitter.
You can use the same method to set the TRIGGERING
LEVEL control for either type of coupling. After selecting
the triggering slope, rotate the TRIGGERING LEVEL control
fully
counterclockwise 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 waveform. See
Figure 2-8.
Free-Running Operation
With the Type 502A, you can get a periodic, free-running
sweep, independent of any external triggering or synchroniz­
ing signal by rotating the TRIGGERING LEVEL control fully
clockwise to the RECURRENT position. This permits you to
observe the UPPER and LOWER BEAM traces without an in­
put signal.
2-5