The graph which appears on the display screen is a result
of an electron beam moving across a phosphor coating the inside of the
Cathode-ray tube (CRT). The Cathode-ray tube is a vacuum tube in which
electrons are accelerated and deflated under the influence of the electron
field. This leaves an imprint of the path of the beam. Grid lines horizontal
and vertical on the faceplate(Graticule) are used as guide lines for measurement.
The Graticule is used to measure the amplitude on the Y-axis, and the period
on the x-axis. You can use these lines to measure such things as amplitude
and frequency, among other things.
TRACE INTENSITY
The trace intensity control is located
to the right hand side of the display. It adjusts how bright the trace
will be. Turn the knob in a clockwise direction and the brightness is increased,
turn the knob anti-clockwise for reduction in intensity.
TRACE FOCUS
This control is located above the intensity
control, and adjusts the clarity of the trace on the display. Turn the
control to the left to reduce clarity, and turn the control to the left
to increase clarity.
TRACE VERTICAL CENTER ADJUST
There's two controls (one for each
input channel) located above the volts/div control for their respective
channels. To move trace towards the bottom of the screen, turn knob to
the left. Turn knob to the right to move the trace towards the top.
HORIZONTAL TRACE ADJUST
Located to the right of the time base
control, this control is simply used to adjust the trace to the right side
on the display, or to the left side on the display. By turning the knob
in a clockwise direction the trace will move towards the right, and counter
clockwise to adjust the trace towards the left.
TRIGGER SOURCES
Trigger sources are grouped into two
categories. The first is when the signal is an internal signal, and the
second type is when the signal is an external signal. The trigger circuit
operates the same whether it's an external or an internal triggering source.
The switches 'source' and 'int' determine the trigger source. When the
source switch is switched over to 'int' the internal triggering sources
are turned on. When on 'norm' and the signal is applied the sweep is then
activated. When there's no signal, the base line trace will disappear The
P-P Auto-TV Line mode is needed for a waveform with a repetition rate of
at least 20Hz, the scope will then trigger off this waveform. When the
signal is lower than 20Hz the 'Sweep free' mode is activated. The range
of the trigger level is determined by the peak-to-peak range of the trigger
signal. The TV Field can be activated when both the P-P Auto and Norm buttons
are pushed. This allows triggering on television field signal. By pressing
the spring-return button marked 'SGL SWP RESET' the scope will then be
armed for a single-sweep display. For each signal that is triggered, only
one sweep will be displayed. By pressing the 'SGL SWP RESET' again, another
sweep will be displayed on the screen. This control is very useful for
signals, which cause unstable displays, or signals that are non-repetitive.
TIME BASE SELECTIONS
The oscilloscope has a time base usually
referred to as the main time base and it is probably the most helpful.
A lot of the oscilloscopes have 'delayed time base', a time base sweep
that begins only after a pre-determined time from the start of the main
time base sweep. The delayed time base requires the setting of a delay
time, and possibly the use of delayed trigger modes.
TRIGGER CONTROLS
The trigger controls let you stabilize
repeating waveforms and capture single-shot waveforms. The trigger makes
repeating waveforms appear static on the oscilloscope display. Imagine
the jumble on the screen that would result if each sweep started at a different
place on the signal.
TRIGGER LEVEL AND SLOPE
For edge triggering, the trigger level
and slope controls provide the basic trigger point. You select the slope
and voltage level that you want to trigger at. When the trigger signal
matches your settings, the oscilloscope generates a trigger. The slope
control determines whether the trigger point is on the rising or the falling
edge of a signal. A rising edge is a positive slope and a falling edge
is a negative slope. The level control determines where on the edge the
trigger point occurs.
TRIGGER MODES
The trigger mode determines whether
or not to draw a waveform, if the oscilloscope does not detect a trigger.
Common trigger modes include normal and auto. In normal mode the oscilloscope
only sweeps if the input signal reaches the set trigger point; otherwise
(on an analog oscilloscope) the screen is blank. Normal mode can be annoying,
since you may not see the signal at first, if the level control is not
adjusted correctly. Auto mode causes the oscilloscope to sweep, even without
a trigger. If no signal is present, a timer in the oscilloscope triggers
the sweep. This ensures that the display will not disappear if the signal
drops to small voltages. It is also the best mode to use if you are looking
at many signals and do not want to bother setting the trigger each time.
In our lab, you will probably use both modes: normal mode because it is
more flexible, and auto mode because it requires less alteration. Some
oscilloscopes also include special modes for single sweeps, triggering
on video signals, or automatically setting the trigger level.
TRIGGER COUPLING
as you can select either AC or DC coupling
for the vertical system, you can choose the kind of coupling for the trigger
signal. Besides AC and DC coupling, your oscilloscope may also have high
frequency rejection, low frequency rejection, and noise rejection trigger
coupling. These special settings are useful for eliminating noise from
the trigger signal to prevent incorrect triggering.
TRIGGER HOLDOFF
Sometimes getting an oscilloscope to
trigger on the correct part of a signal requires great skill. Many oscilloscopes
have special features to make this task easier. Trigger holdoff is an adjustable
period of time during which the oscilloscope cannot trigger. This feature
is useful when you are triggering on complex waveform shapes, so that the
oscilloscope only triggers on the first trigger point it sees.
We hope that you have enough knowledge now to complete
the lab and then answer the questions. Good luck.
