Tektronix 545A Instruction Manual page 14

Operating nstructions — Type 545A
TIME BA E A TRIGGERING MODE switch to AUTO, and
the TIME BA E A TRIGGER
the intensity until the trace is visible and connect the probe
tip to the CAL OUT connector.
BRATOR switch for 2 centimeters of displayed signal.
the TIME BA E A TIME/CM switch to display approximately
3 or 4 cycles of the Calibrator waveform and adjust the
probe compensation control to obtain flat tops on the dis ­
played Calibrator square-waves (see Figure 2-3).
First unlock the Locking
clockwise. The probe is then compensated by rotating the
probe body while watching the oscilloscope display for the
desired waveform. When compensation is completed, care ­
fully turn the Locking leeve clockwise to lock it without
disturbing the adjustment of the probe.
Horizontal Sweep
The usual oscilloscope display is a graphical presentation
of instantaneous voltage versus time. Voltage information
is presented by vertical deflection of the trace and time
information is presented by horizontal deflection. To obtain
a useful display, it is necessary for the spot formed by the
electron beam to be deflected horizontally at a known rate
so that any horizontal distance on the screen represents
a definite known period of time. The trace formed by the
deflection of the spot across the screen is known as the
horizontal sweep. ince the horizontal deflection of the
spot bears a definite relationship to time and provides the
means for making time measurements from the screen, the
horizontal sweep is also known as the time base.
The Type 545A Oscilloscope features two independent
time-base units: Time Base A and Time Base B. Either of
the two time bases may be displayed on the oscilloscope
screen at any time. Controls affecting Time Base A and
Time Base B operation are in the correspondingly labeled
blocks on the oscilloscope front panel. The HORIZONTAL
DI PLAY switch determines which time base is used and the
type of display to be presented. Time Base A is selected
when the HORIZONTAL DI PLAY switch is in position A
and Time Base B is selected when the switch is in posi ­
tion B.
Fig. 2-4. The oscilloscope plots instantaneous voltage versus time,
thereby serving both as a voltmeter and a timer.
2-4
LOPE switch to + INT. Turn up
et the AMPLITUDE CALI ­
leeve by turning it counter ­
The rate at which the spot is deflected across the screen
is accurately controlled by the setting of the appropriate
TIME/CM controls.
determine the sweep speed and the horizontal size of the
displayed waveform. The controls are set to display the
et
portion of the waveform you wish to observe.
Time Base A has 24 accurately calibrated sweep rates
ranging from .1 microsecond to 5 seconds per centimeter.
These calibrated sweep rates are obtained only when the
VARIABLE TIME/CM control is in the fully clockwise posi ­
tion. The VARIABLE TIME/CM control permits you to vary
the sweep rate continuously between .1 microsecond and
approximately 12 seconds per centimeter. All sweep rates
obtained with the VARIABLE TIME/CM control in any posi ­
tion but fully clockwise are uncalibrated.
sweep rates are indicated when the UNCALIBRATED lamp
is lit.
Time Base B has sweep rates ranging from 2 microseconds
to 1 second per centimeter in 18 steps. There is no VARI ­
ABLE TIME/CM control for Time Base B. The red LENGTH
control adjusts the sweep length between approximately 4
and 10 centimeters.
Sweep Triggering
The oscilloscope display is formed by the repetitive sweep
of the spot across the oscilloscope screen. If the sweeps are
allowed to occur at random or at a rate unrelated to the
rate of occurrence of the input waveform, the displayed
waveform will be traced out at a different point on the
screen each time the sweep runs. This will either cause the
waveform to drift across the screen or to be indistinguish ­
able.
In most cases it is desirable for a repetitive waveform to
appear stationary on the oscilloscope screen so that the
characteristics of the waveform can be examined in detail.
As a necessary condition for this type of display, the start
of the sweep must bear a definite, fixed-time relationship
to the appearance of the input waveform. This means that
the sweep must be synchronized with the input waveform.
In the Type 545A Oscilloscope this is accomplished by
starting (triggering) the sweep with the displayed wave ­
form or with another waveform bearing a definite time
relationship to the displayed waveform.
The following paragraphs outline the means for selecting
the triggering source, triggering slope, and triggering level
with specific information regarding the operation of the
controls affecting triggering.
Base A and Time Base B are virtually identical so that the
following information is applicable to both time-base units.
Triggering of the sweep is adjusted by the respective
TABILITY, TRIGGERING LEVEL, TRIGGERING MODE, and
TRIGGER LOPE controls.
Selecting the Triggering Source
In preparing the Type 545A Oscilloscope for triggered
operation of the sweep, it is first necessary to select the
triggering signal source which will provide the best display
for the particular application. The sweep can be triggered
by the displayed waveform, a line-frequency waveform,
The settings of the TIME/CM controls
Triggering controls for Time
Uncalibrated