Appendix on Use of Oscilloscope
Appendix: Use of Oscilloscopes
Here are notes on using an oscilloscope. Your text outlines the theory of operation, while here
we concentrate on practical application.
Screen,
showing trace
Control panel
Fig. 1 Oscilloscope in use.
An oscilloscope graphs voltage versus time for a signal applied to its input. The graph is shown
on the screen, which is the face of a cathode ray tube (CRT) when the signal (a voltage that
varies in time) is put in by means of some combination of input terminals. There are several ways
to do this, depending on how one wants to analyze the signal.
In the case shown, time-varying voltage across a capacitor is applied across the input terminals
of what is called channel 1 (CH1). This is done by means of the white wires shown. One wire is
connected to a ground socket (GND) and the other to the red side of an adapter that leads into
the CH1 input. Thus, in the case shown, what is graphed on the screen is the voltage at the CH1
input (with respect to GND) as a function of time. Voltage is the y axis or vertical axis, and time is
the x axis or horizontal axis.
Actually there are several ways to use the scope. One way is to display voltage versus time, as
shown. There are two different channels for this type of display, called CH1 and CH2. You can
choose to view either input versus time on the screen. Or, you can look at both signals at once
(dual trace). A different way to use the scope is that you can view the voltage input in CH2 as a
function not of time but rather as a function of the voltage applied to the CH2 input. This is called
X-Y mode. CH1 appears as the x coordinate on the screen, and CH2 appears as the y
coordinate.
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Appendix on Use of Oscilloscope
Intensity
Focus
Triggering controls
Toggle to X-Y mode
Control Panel for
BK PRECISION
20 MHz
Oscilloscope
Model 2120B
(Typical of most
analog
oscilloscopes)
ON
POWER
TIME BASE
CH1
CH2
CH2 input
External
triggering
(Note BNC
connectors)
Mode
Calibration
signal
switch
Signal input to CH1.
(Note banana plugs.)
Fig. 2 Control panel
The controls are grouped into several sets, each with its own region on the face of the
oscilloscope control panel. These regions are shown in Figure 2,










Channel 1 input and controls
Channel 2 input and controls
Mode selecting switch
Calibration signal source
External trigger input
Main power switch and indicator
Time base controls
Intensity and focus for screen
Triggering controls
X-Y toggle switch
With the mode switch (VERT MODE) positioned at CH1, the scope displays a graph of voltage
applied across the CH1 input as a function of time. It does this by means of a point of light called
a trace that moves (usually rapidly) across the screen, over and over again. If the signal repeats
in time, the point starts its trip across the screen (called a sweep) only when certain triggering
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Appendix on Use of Oscilloscope
criteria are met. Thus it starts at the same corresponding point in the pattern of a repeating or
periodic signal.
POWER
The push button toggles the power on (light on) or off. Note that if, when you turn the power on,
no light comes on, you should check to see that the scope is plugged in. Also, if when you turn
the power on a very bright spot appears on the screen, you should turn down the intensity
immediately to avoid harming the screen.
CH1
The signal to be studied is a voltage applied across the CH1 input, either via a coaxial cable
attached by means of a BNC (Berkley Nucleonics Corporation) connector, where you twist and
snap to connect it, or by means of two insulated wires with banana plugs. The latter method is
shown above and requires a BNC to banana plug adapter. For the banana connection, the black
connector is grounded by means of the scope case to the ground wire in the electrical outlet.
Thus the voltage at the red connection is measured relative to this ground. The ground
connection can be made in an alternative way, as shown, by using the GND socket in control
panel.
The input is coupled to the scope either as a.c. or d.c. For a.c. (the usual case) which places a
capacitor in series with the input so that any d.c. bias is blocked, and the signal voltage will
average to zero over time. When making d.c. measurements (an unusual application for an
oscilloscope) or to see a d.c. bias, the d.c. coupling mode can be used.
The position adjustment permits you to move the base line (indicating zero voltage) up or down.
Thus is useful when you want to display two signals at once by means of the dual trace mode.
Usually for simple (CH1 only vs. time) measurements you want the base line on the horizontal
axis in the middle of the screen. Notice too that the position dial can be used to choose a
different triggering signal, if you pull on it. Normally you do not want this, so keep the position dial
pushed in.
The volts/division dial controls how much voltage corresponds to each vertical division on the
screen. It has an outside part (calibrated) and an inside part (red, variable). Usually, you want to
make calibrated measurements, where the numbers around the dial actually correspond to the
divisions on the screen. For this, you only want the outer dial to be active. Thus, turn the red
inner dial all the way around to the right, keeping the outer dial fixed, until it clicks into place.
Suppose you do this, and then you center the trace and adjust the calibrated volts/division dial to
50 mV per division. Then you apply a signal to channel 1 and find the trace goes up and down
from one division (about a cm) below the axis to two divisions above. Then this means the
applied voltage is going from -.05 Volts to +.1 Volts.
CH2
This works the same as CH1 and is activated when the mode switch points to CH2.
MODE SWITCH
Thus, you can have two signals, one applied to CH1 and the other to CH2, and then switch back
and forth between them using the mode switch. It is similar to switching channels on your
television. Alternatively, you can show the two signals on the same screen (dual trace mode). In
this case, the point of light alternately sweeps out first the CH1 signal, and then the CH2 signal,
switching back and forth between the two. You can then compare the two signals. It is then
handy to displace one of the signals using the position controls.
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Appendix on Use of Oscilloscope
Finally, rather than plotting CH1 or CH2 versus time, you can plot the voltage applied to the CH2
input (the y coordinate) versus the voltage across the CH1 input (x coordinate). This is the X-Y
mode. The X-Y mode is not really selected by the mode switch, but is toggled on or off using the
mode button near the top of the panel amongst the time-base controls. When this is selected, the
time base is disabled and is therefore irrelevant.
TIME BASE
The variable-sweep dial should normally be turned all the way to the right until it clicks, so that the
numbers on the time/division dial will correspond directly to the time it takes the trace to move
across one division. This means that if you set the time/division to 50s (i.e. 50 microseconds
per division) then a signal that repeats 20,000 times a second (20 kilohertz) will make a pattern
on the screen that repeats once in each horizontal division. Changing the time/division to 20s
per division will make a display where the pattern repeats after two and a half horizontal divisions.
(Comment: Changing the sweep rate in this way often requires changing the intensity and or the
focus. Sometimes the triggering will need adjustment as well.) The time base is disabled when
the X-Y toggle button enables the X-Y mode.
FOCUS AND INTENSITY
Intensity should be adjusted to give a clearly visible trace, but not too high. Too much intensity
(often signaled by a halo around a very bright trace) can burn the phosphor on the screen. Even
a fairly low intensity trace can cause damage if the same pattern is displayed for a long enough
time. This is why computers have screen savers. Intensity often needs readjustment after the
sweep rate (time/division) is adjusted, and the focus also may need adjustment after adjusting the
intensity.
TRIGGERING
This selects the criteria that trigger the sweep, i.e. that start the trace moving across the screen.
These criteria are usually the vertical position (voltage) and the slope (derivative of voltage with
respect to time) of some signal. The signal that does the triggering may either be the signal
under study, e.g. in CH1, or it may be a signal in the other channel, or it may be an external
signal. When the display keeps bouncing around does not settle down to a nice pattern, or else
when it keeps sliding along to the right or left, it is usually the triggering you need to fiddle with.
The triggering level controls the height (that is, the voltage, plus or minus) at which the sweep can
start. Normally, it will only start when the voltage criterion is met and when the slope is positive.
By pulling on the dial, you can change the sign of the slope criterion so that the sweep triggers
when the slope is negative.
The coupling usually wants to be auto or normal. One difference is that the auto coupling will
show the trace when there is no signal, where the normal coupling will not.
The source of the triggering signal can be either CH1, CH2, an external signal (input as shown in
Fig.2) or the 60Hz a.c. power supply. The latter is handy when you suspect there is some
problem in your experimental measurements caused by interference from the a.c. line voltage.
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