Introduction to the Oscilloscope
(Tektronix TAS 250)
This write-up describes the operation of the oscilloscope, an instrument you are likely to
be using for one or more of your labs. The goal for today is for you to become very
comfortable with using the oscilloscope as a measuring device. We suggest below a
procedure that will introduce you to the basic features of the oscilloscopes you will be
using in the lab. You are encouraged to explore all of the settings on the oscilloscope;
don't hesitate to ask your TA about any knobs or buttons that aren't clear.
The oscilloscope (scope, for short) is particularly useful when we are dealing with
repetitive electrical signals, since it can literally draw a picture of the time behavior of a
signal. To accomplish this, the controls of a scope all act to affect a beam of electrons
directed at a phosphorescent screen, where the beam excites atoms in the phosphor. As a
result of the excitation, the atoms give off visible light, so that the electron beam can be
thought of as a pen writing on the screen. The scope controls act to change either the
brightness of the beam or the way in which the beam is directed to different areas of the
front screen in order to produce a visible pattern.
In the operations described below, the horizontal deflections of the beam are made
proportional to time. An external signal is applied in the vertical direction. As a result, a
pattern is traced on the screen in which the signal is displayed as a function of time. If the
external voltage is DC, a horizontal straight line would be traced out whose vertical
position depends on the voltage applied. For sine or square wave voltages, the
corresponding shape is traced on the screen, and the amplitude and period of the signal
can be measured, for example.
Equipment
A sketch of the control panel of the Tektronix TAS 250 oscilloscope is included at the
end of the writeup (Fig. 1), and the various knobs and dials are numbered there. Within
the text, there is generally a corresponding control indicator that refers to that sketch.
(C5), for example, refers to the power switch, which you will find so numbered on the
sketch.
For a signal source, we will use an electronic oscillator, the BK Model 3050 Signal
Generator. You can control three characteristics of the signal:
1. The waveform can be either sinusoidal or a “square wave,” depending on the
position of the waveform switch. Both waveforms are periodic. The square
waveform has a constant amplitude in each half cycle, positive for half a cycle and
then negative for half a cycle.
2. The frequency of the signal is controlled by the frequency knob in combination with
the frequency range switch. The latter multiplies the frequency shown on the
circular dial. For example, if the circular dial reads 45 Hz and the frequency range
is set at X100, the signal will have a frequency of 4,500 Hz.
3. The amplitude of the signal is controlled by the output level knob and output
attenuator switch. The amplitude of the signal increases as the output level knob is
rotated clockwise. The output attenuator switch causes the signal to be attenuated
by factors of 10 (at -20 db) and 100 (at -40 db) from its value at 0 dB. (The labeling
reflects a logarithmic scale widely used for electronic instruments.) Unlike the
frequency control, the output control numbers are purely relative (not volts).
Procedures
Studying the Oscilloscope: The steps described in Subsections (a) – (g) below are
designed to introduce the different features of the scope one after another. By all means,
make use of the help of the lab instructor at any step, before going on to the next, if there
is something about what you are doing that you do not understand.
(a) Survey of the Control Panel: Do not turn the scope on until you have studied the
controls on the face of the scope. Disconnect any signal cables from either of the two
input terminals (marked CH 1 and CH 2) at the bottom of the control panel (C-11).
Locate the intensity knob (C-1), below the scope screen, and turn it completely counterclockwise. This will insure that the beam does not damage the screen when the scope is
later turned on. (We will begin with an undeflected electron beam striking a single point
on the screen, and there is the danger of permanently damaging the screen.)
Locate the large knob marked SEC/DIV (C15). This is the knob that controls how fast
the beam sweeps across the screen horizontally. The actual setting of this knob will be
displayed on the oscilloscope screen. For example, if SEC/DIV is set at 2 ms, the beam
will move to the right by one division in two milliseconds. (A division refers to the
spacing of the gridlines on the oscilloscope screen.) Setting the SEC/DIV knob at
5 μs causes the beam to move one division horizontally in five microseconds, a much
faster rate of horizontal travel of the beam. As you turn the SEC/DIV knob control
clockwise, the beam sweeps faster and faster until you reach the maximum speed of 0.1
μs per division.
Now find two similar knobs marked VOLTS/DIV (C-8). These are the knobs that control
how far the beam will be deflected in the vertical direction for a given input voltage. As
with the SEC/DIV knob, the actual setting of the VOLTS/DIV knob will be displayed on
the screen. As the VOLTS/DIV knob is rotated clockwise, a signal of the same strength
causes an ever increasing vertical deflection. Suppose, for example, that a signal with a
strength of one volt were being applied. With the knob at 1 V, this would cause a vertical
deflection of 1 division. With the knob turned clockwise to the 0.2 V position, the same
signal would cause a deflection of 5 divisions. The maximum sensitivity is 1 mV, where
a 1 millivolt signal would cause a deflection of 1 division.
The scope panel is divided into two similar sets of controls, marked CH 1 and CH 2,
corresponding to the two signal inputs. This allows us to observe two different signals.
For example, a voltage and a current can be studied simultaneously. The MODE switch
(C-7) selects which channel (or both) is displayed.
The scope, by the way, has an enormous tolerance for the strength of the signals being
presented. The beam might be driven far off the screen by large signals, but no damage
to the scope is caused by giving it large signals. (There is a limit, of course – about 400V
for this scope.)
The vertical position knobs (C6) control the vertical position of the beam for the two
channels. The horizontal position knob (C12) performs the same function for the
horizontal position. Set all the position knobs so that the small dot on the knob is in the
“12 o'clock” position.
In addition to the controls described above, there are a number of other knobs and buttons
on the control panel, many of which are described below. Note that many of the buttons
have both an “in” and an “out” position. When the button is in the “in” position, that
feature is enabled. Push one of the GND buttons (C10) and note how it cycles between
the “in” and “out” positions. When the GND button is in the “in” position, the vertical
input of the scope is connected to ground potential (i.e., a potential of 0 volts).
The variable vertical scale knobs (C9) should be turned all the way clockwise until they
click into the “cal” position. Similarly, the SWP UNCAL button (C16) should be in the
“out” position. Note that the vertical and horizontal deflections are UNCALIBRATED
unless these controls are set properly! It is also possible to get erroneous readings if a
vertical channel is accidentally set in the X10 mode. If you see a P10 symbol to the left of
the VOLTS/DIV reading, press the probe X1/X10 button (C-23) to make it go away.
Before proceeding further, make sure the chop, invert, GND, X10 Mag, SWP UNCAL,
and set to 50% buttons are in the “out” position.
(b) Using Power, Intensity, Focus and Position Control: First, make sure that the
intensity knob (C1) is turned all the way clockwise. Turn the scope on by pushing in the
power button (C-5). After a few seconds, you should see some numbers at the bottom of
the screen showing the VOLTS/DIV and SEC/DIV settings. Adjust the readout knob (C3)
so that these labels are clearly displayed. Adjust the focus knob (C2) so that the individual
dots in the labels are sharp and in focus. The scale illumination knob (C4) controls the
illumination of the gridlines on the display. (If you don't see any numbers at the bottom of
the screen, simultaneously press the Probe X1/X10 (C-23) and Cursor on/off (C-24)
buttons to turn on the readout display.)
The button marked “x-y” (C-14), should be set to the “on” (depressed) position. In x-y
mode, the horizontal deflection of the beam is controlled by the signal applied to CH 1,
not by internal time controls of the scope itself. Set the mode switch (C-7) to the CH 2
position so that channel 2 controls the vertical deflection.
Turn the intensity knob (C-1) clockwise until the beam appears as a stationary spot. The
spot should be near the center of the screen.
Next, study the effect of the horizontal (C-12) and vertical (C-6) position controls
(marked with corresponding arrows) on the location of the beam spot. Then leave the
spot at center screen.
(c) Vertical Controls: At the bottom of the CH 2 section is a cable connector (C-11). A
connection should be made to the signal generator, BK Model 3050. The signal
generator can provide either sine or square waves to deflect the beam vertically. What
you want is either a sine or square signal with an amplitude of about one volt, whose
frequency can be varied.
On the scope, turn the CH 2 VOLTS/DIV knob (C-8) until it reads 1V on the screen. The
CH 2 input selector (C10) should be set to AC (“AC/DC” and GND buttons in the “out”
position). Since there is no input to deflect the beam horizontally at this point, the trace
should be a vertical line. The beam bobs up and down from center with a frequency set
by the signal generator.
With a constant amplitude from the signal generator, turn the VOLTS/DIV knob (C-8)
from 1V to .5V. The length of the vertical line should double. Try several settings of the
“vertical sensitivity” (i.e., the VOLTS/DIV knob), using values larger and smaller than
the original 1V setting. See if you understand the lengths and intensities of the traces you
obtain.
Notice that as you increase the amplitude on the screen, the trace becomes fainter. The
energy that the electron beam deposits is spread over a longer length on the screen, and
so less energy is given to the phosphor at any one point. You may have to adjust the
intensity of the beam, increasing it for larger signals, decreasing it for smaller signals, to
maintain a visible line that is not too bright, not too dim.
(d) Horizontal Controls: The most important feature of the scope is its ability to display
signals as a function of time. To see a signal displayed in time, set the “x-y” button in the
off (out) position, set the trigger mode (C18) to trigger automatically by pushing the
button marked “Auto,” set the trigger coupling switch (C19) to “AC,” and set the trigger
source switch (C20) to CH 2. The trigger controls will be described below. If the signal
you see is not stationary, adjust the trigger level knob (C17) until the waveform appears
stationary. Turn the TIME/DIV knob (C-15) to different positions, and notice how, for a
fixed frequency at the vertical input, the sine wave signal is expanded and contracted
horizontally by your choice of the TIME/DIV knob.
(e) Trigger Controls: In order to understand the “horizontal sweep” and triggering
controls, consider that there are two decisions to be made. One is the sweep speed, which
is controlled by the TIME/DIV knob. The second is the trigger condition, which requires
that the signal must exceed a preset minimum value before the sweep will be generated at
all.
The trigger level knob (C17) sets the voltage that will cause the oscilloscope to begin a
horizontal sweep. Begin with the trigger level knob in the “12 o'clock” position. As you
turn the trigger level knob clockwise, the starting voltage at the leftmost edge of the
oscilloscope should increase. Similarly, turning the trigger level knob counter-clockwise
will cause the starting voltage to decrease. To put it another way, the oscilloscope waits
until it sees the input signal cross the voltage set by the trigger level knob, and then
immediately starts a horizontal sweep. When the trigger level is set so far clockwise or
counter-clockwise that the input signal never crosses the trigger level, then the
oscilloscope signal will automatically start at random times and the signal will not be
stable.
With the trigger source switch (C-20), you can select whether to trigger on the signal
from CH 1, CH 2, or an external trigger signal on the EXT TRIG input (C22). The
trigger slope button (C21) determines whether you trigger on a rising or falling signal.
Try both positions of the trigger slope button to see how this affects triggering.
(f) Using the Measurement Cursors: The measurement cursors are quite useful for
making voltage and time measurements. You can turn on the cursors by pressing the
cursor on/off button (C-24). This will turn on either a pair of vertical lines (for making
time measurements) or a pair of horizontal lines (for making voltage measurements).
Pressing the ΔV/ΔT button (C-25) toggles you between voltage and time measurement
cursors. The position of the cursors can be adjusted by turning the cursor position knob
(C-27). The cursor that will be moved is marked either by a small circle or a small
diamond. Pressing the cursor select button (C-26) will change which cursor is being
adjusted. (In tracking mode, both cursors are selected and they move together, keeping a
constant separation.)
Use the voltage and time measurement cursors to measure the amplitude and period of
your signal. Compare these results with the readings you obtain by counting the number
of divisions on the screen, taking into account the voltage/division and time/division
settings. Also compare your measured frequency with the signal generator setting.