Tektronix 3B5, Инструкция по эксплуатации

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Operating Instructions— Type 3B5
triggering point may be enough so it is impossible to main­
tain a stable display. In such cases, use DC coupling.
DC. DC trigger coupling is available only when the
sweep is triggered from an external signal in the EXT posi­
tion. DC coupling can be used to provide stable triggering
with low-frequency signals which would be attenuated in
the AC position or with low-repetition rate signals. The
LEVEL control can be adjusted to provide triggering at the
desired DC level on the waveform.
Trigger Slope
The Trigger SLOPE lights indicate the portion of the trigger
signal to which the trigger circuits respond. When the minus
( — ) light is on, the display starts on the negative-going
portion of the waveform; the plus (+) light indicates that
the display starts on the positive-going portion of the wave ­
form. The trigger slope can be changed by pressing the
CHANGE SLOPE button (or by external program). Each
time the button is pressed the trigger slope changes from
( — ) to (+) or vice versa. The operation of the CHANGE
SLOPE switch in conjunction with the LEVEL control is
shown
in Fig. 2-3.
When several cycles of a signal are displayed, the trigger
slope is often unimportant. However, if only a certain
portion of a cycle is to be displayed, correct slope setting
is important to provide a display which starts at the desired
point on the input signal.
Trigger Level
The front-panel Trigger LEVEL control (or external program
level control) selects the voltage level on the triggering
waveform at which the sweep is triggered. When the LEVEL
control is set in the clockwise region, the trigger circuit
responds at a more positive point on the trigger signal.
When the LEVEL control is in the counterclockwise region,
the trigger circuits responds at a more negative point on
the trigger signal. Fig. 2-3 illustrates this effect for both
trigger slope conditions.
The NOT TRIG'D readout on the readout panel provides
a convenient indication of the condition of the trigger circuit.
If the LEVEL control is correctly adjusted with an adequate
trigger signal applied, the readout is off. However, if the
LEVEL control is misadjusted, the Trigger Function switch
incorrectly set, or the trigger signal too low in amplitude,
the NOT TRIG'D readout lights up. This readout feature
can be used for a general indication of correct triggering
and is particularly useful when setting up the trigger circuits
when a trigger signal is available without a trace displayed
on the CRT (such as single sweep operation).
To set the LEVEL control, first select the trigger source,
coupling and slope. Then set the LEVEL control to midrange
(except for DC coupling). If the display does not start at
the desired point, adjust the LEVEL control for correct trig ­
gering. With DC coupling (external trigger only), stable
triggering may be obtained at any setting of the LEVEL
control as determined by the DC level of the trigger signal.
To obtain correct DC trigger level, turn the LEVEL control
counterclockwise. Then, turn the LEVEL control clockwise
until the display is triggered at the correct DC level on the
displayed signal.
Auto
Triggering
The sweep is automatically triggered in the INT-AUTO
position of the Trigger Function switch (or when selected by
external program). Auto triggering can be used for most
applications. This trigger mode provides a stable display
when a trigger signal is available (above about 50 Hz).
Complete rotation of the LEVEL control provides a range of
adjustment between about 10% and 90% of the peak-to-
peak level of the triggering slope. The front-panel LEVEL
control also determines the auto triggering level for External
Mode operation. The trigger circuit automatically compen­
sates for differences in trigger signal amplitude to provide
this range of LEVEL control adjustment for all signals which
meet the minimum amplitude requirements. When the LEVEL
control is set for the desired triggering point at a certain
percentage of the leading edge, it triggers at this same
point even if the signal amplitude is increased (e.g., if set
to trigger at the 30% point on a one-division signal, it also
triggers at the 30% point if the amplitude is increased to
eight divisions; see Fig. 2-4).
NOTE
The auto trigger mode may not produce the
desired display of complex signals such as pulse
trains near the low-frequency limit of this mode
(about 50 hertz). The triggering point may shift
from the desired pulse to a lower amplitude pulse
which follows. The resultant display will appear
unstable. In such cases, use normal internal trig ­
gering or external triggering.
The auto trigger mode is particularly useful when a
reference trace is needed and no trigger signal is available.
Then, the Sweep Generator free runs to produce a reference
trace. The trace also free runs when the repetition rate of
the trigger signal drops below about 50 Hz. When a trigger
signal is again present, the free running condition ends
and the sweep returns to a triggered, stable display.
When a seek command is received in the INT-AC, EXT-
AC, EXT-DC positions of the Trigger Function switch, the
trigger circuits automatically switch to auto triggering if a
trigger signal is not present or if the LEVEL control is mis­
adjusted. Then, a stable auto triggered display is presented,
if possible. This feature assures that a stable display is
presented, whenever possible, in the Seek Mode of operation.
Selecting Sweep Rate
The MANUAL TIME/DIV switch selects the calibrated
sweep rate for the Sweep Generator in the Manual Mode of
operation. The VARIABLE control provides continuously
variable sweep rates between the settings of the MANUAL
TIME/DIV switch. Whenever the UNCAL readout is on,
the sweep rate is uncalibrated. The readout is off when the
VARIABLE control is set to the CAL position.
When making time measurements from the graticule, the
area between the first and ninth graticule lines provides the
most linear time measurements (see Fig. 2-5). Therefore,
the first and last division of the display should not be used
for making accurate time measurements. Position the start
of the timing area to the first graticule line and set the
MANUAL TIME/DIV switch so the end of the timing area
falls between the first and ninth graticule lines.
2-8