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HB 01-26-09 Oscilloscope (1) Lab 5 4
screen. When the ramp voltage across the horizontal deflection plates is V
L
, the electron
beam is at the left of the screen. When the ramp voltage is zero the beam is in the horizontal
center of the screen. When the ramp voltage is V
R
the beam is at the right of the screen.
We assume that |V
L
|=|V
R
|. How quickly the electron beam is swept across the screen is
controlled by the TIME BASE switch.
In the most usual mode of operation, the voltage on the horizontal deflection plates is
held at V
L
and the electron beam is blanked (shut off). If a “trigger” pulse is applied to the
circuit (source of the trigger pulse discussed in a moment) the electron beam is turned on
and the time base oscillator applies the ramp to the horizontal deflection plates. The input
voltage is applied to the vertical deflection plates. See Fig. 4. The electron beam is swept
across the screen from left to right at a constant speed. This takes a time P. At the same
time the vertical deflection of the electron beam is proportional to the input voltage. A spot
of light is observed to move across the screen if the sweep speed is slow enough. This spot
of light traces the input voltage for a time P. If the sweep speed is fast the trace will be too
faint to see unless many sweeps executing the same trace are made. When the ramp reaches
V
R
the electron beam and the light spot reach the right side of the screen. The sweep voltage
is then very quickly returned to V
L
. During this process the electron beam is shut off or
“blanked” so that a return trace is not seen. For a stationary trace to be properly observed
the trigger pulses must all occur at similar points of the input voltage waveform. Those
similar points are taken to be the same voltage and the same slope of the input voltage.
The trigger pulses, the pulses that start the sweep voltage, can be derived from one of the
following signals. The only one of these signals that is relevant to this lab is the first one.
1. the input voltage, which is the voltage whose waveform you are interested in examin-
ing. This is the voltage, which after suitable amplification or attenuation by the scope
electronics, is applied to the vertical deflection plates.
2. the line voltage (110 V AC wall outlet). This may be a good choice if you know that
your input signal is “synchronized” with the line voltage. The line voltage as a source
of trigger pulses is not used in this lab.
3. some other “external” voltage that is not the line voltage but that you think is syn-
chronized with your input signal. This capability is not used in this experiment.
We consider the case where the trigger pulse is derived from the input voltage, which is the
voltage that is to be observed. The operator of the scope, by using a scope control usually
marked “trigger level”, chooses a value of the input voltage at which the trigger pulse is
to occur. We call this voltage V
T
. This can be a positive or negative voltage, but for the
trigger pulse to occur V
T
must be between between the maximum and minimum values of
the input voltage. If it is not, the trigger pulse will not occur. The scope operator can also
choose whether the trigger pulse occurs when the slope of the input voltage (with respect
to time) is positive or negative. Look at Fig. 4, which shows a sinusoidal input voltage
and the horizontal sweep voltage as a function of time. The trigger voltage V
T
has been
chosen as positive. The triggering slope has also been chosen as positive. Fig. 4 shows that
whenever the input voltage has a positive slope and reaches the value V
T
, a trigger pulse is
produced and the ramp voltage produced by the scope sweeps the electron beam horizontally
across the screen at a rate determined by the TIME BASE. In Fig. 4 the time it takes for
the electron beam to go from the left side of the screen to the right side of the screen is
designated by “P.” It does this repetitively, always starting at the “same” point of the input
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