Instruction manual

HB 08-18-2010 3
3 Description of How a Scope Displays a Single Periodic Voltage
A periodic function in time is one that repeats itself again and again, such as a sine wave
or a square wave. A scope can display a voltage that is periodic in time in exactly the same
way. The periodic voltage is connected to the vertical input of the scope, and if the scope is
adjusted correctly, a graph of the voltage as a function of time appears as a stationary trace
on the screen of the scope.
The ramp voltage that is applied to the horizontal deflection plates of the scope is usually
supplied by the TIME BASE oscillator unit of the scope. We consider that case here. (If
the scope has “X-Y” capability the voltage to the horizontal deflection plates can also be
supplied from an external voltage source.)
Every scope has a signal generator or time base oscillator. This oscillator produces a
voltage that is a linear ramp as a function of time. Fig. 3 shows the ramp voltage for one
sweep of the electron beam. This linear ramp, when applied to the horizontal deflection
plates, sweeps the electron beam from the left side of the screen to the right side of the
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.