Dalhousie University
Faculty of Engineering
ECED 2000 Electric Circuits
Laboratory 4 Introduction to the Oscilloscope
Objective:
To learn the basic operation of the Oscilloscope as a measurement
instrument.
Introduction: The oscilloscope is a voltage measurement device that allows you to see
signal waveforms. The ‘scope has five basic functional areas:
1) The display,
3) Vertical controls,
2) Horizontal (or Time Base)
4) Trigger controls and
controls,
5) Measurement controls
1) The Display System
The display system is where the waveform is displayed. Many modern scopes have a flat
screen technology but many of the controls are based on the old Cathode Ray Tube
(CRT) technology. The “trace” moves from left to right at a speed determined by the time
base controls and repeats over and over as determined by the trigger controls. If the
waveform is the same, trace after trace, the display shows a stable waveform as a picture
of the incoming signal. The display usually shows a graticule that is ten divisions wide by
either eight or ten divisions high. The display can be adjusted for intensity and the
graticule can be hidden but there are not many adjustments to be made here.
2) Time Base or Horizontal Controls
The time base controls the length of time being shown across the screen. When the time
base is set for 1 ms/division each division along the horizontal axis represents a time of 1
ms, while the entire screen width is 10 ms. Decreasing the time base sensitivity to 2
ms/div allows you to see twice as much of the input signal. The time base typically can
be set for times per division following a 1, 2, 5, 10 etc sequence. The time base can be
expanded for a closer look at some particular detail (Main/Delayed mode) or it can be
slowed down to function as a chart recorder (Roll mode). The time base can be given
over to one of the input channels so that X-Y plots (X/Y mode) can be shown.
3) Vertical Sensitivity Controls
The vertical controls determine how big the waveform looks. The units are volts per
division (V/Div) and can be adjusted through the same 1, 2, 5, 10 sequence as the time
base. A 10 V peak to peak sine wave displayed with a 10V/Div setting will span one
vertical division. If the sensitivity is increased to 5V/Div the display will span 2 divisions
etc (hence a “bigger” picture of the signal).
4) Trigger Controls
The trigger controls are the settings that determine what event initiates the display. The
trigger controls look for a particular voltage level (LEVEL) and slope ( _or _ or in other
words a +ve slope or a –ve slope). When this condition is detected the “beam” starts its
journey across the screen at a speed determined by the time base. As it reaches the right
hand side of the screen it quickly zips back to the left side and waits for another trigger
event. When that occurs the trace is again initiated. If the signal is stable and the trigger
controls are constant, the trace will look the same each time and gives the appearance of a
constant display of the signal. The trigger can be adjusted for “Single Sweep” mode
which only triggers once so that one time only transients can be captured on the display.
5) Measurement Controls
The measurement controls allow the ‘scope to be used to measure specific voltage (peak
to peak, RMS, average, etc) and or time (delay time, rise time, frequency etc) features.
Cursors can be evoked to aid in time or voltage measurements and drop down menus
allow dozens of common electronic circuit measurements.
The Oscilloscope is an incredibly versatile measurement device and has become the
“right arm” of many engineers of all stripes.
Special Note: The white “AUTO SCALE” button near the center of the scope is like the
“get out of jail free” card in Monopoly. If you don’t know what to do or can’t find your
signal often pressing the AUTO SCALE button will result in a reasonable signal display.
It does not always result in the correct display but at least it is something. Novices often
use this button but as you become more familiar with the scope you will (or should) find
yourself using this button less and less.
Student Name: ________________________________ Banner ID: B00
Pre-Lab (Submit this page to your TA the day of the Lab):
Answer the following questions to prepare you for the Introduction to Oscilloscopes Lab.
1) Consider a voltage signal described by the sine wave v(t) = 2 sin(2,000πt) V.
a. What is the peak to peak amplitude of this signal?
b. What is the frequency in rad/s?
What is the frequency in Hz?
c. What is the period of the signal, i.e. how long is one cycle in ms?
d. How many cycles would fit in a 10ms window?
2) Given the following signal, and given that the trigger controls are set to a zero volt
level and a positive slope ( _ ), determine places where a trigger signal will be
generated by circling all of the points that fit this criteria.
If the trigger controls are changed to zero level and negative slope ( _ ), identify the
“trigger points” by drawing a square around them.
3) The following screen shows a sine
wave signal. The horizontal and vertical
sensitivities are shown above the display.
If the little green triangle at the top of the
screen identifies the trigger event,
determine:
a. The trigger settings for slope and level
b. The frequency of the waveform
c. The peak to peak amplitude
4) If the vertical sensitivity is adjusted to
2V/Div, draw the waveform that the
scope will display if the signal is the
same as in part 3.
5) With the vertical sensitivity set back
to 1 V/Div, the time base is adjusted
to 200 s/Div. Draw the trace that the
‘scope will display if the signal is the
same as part 3.
.
Procedure:
1) Turn the oscilloscope ON and connect Channel 1 to the function
generator on your bench by means of the BNC cable. Turn the
function generator ON, key in 2V p-p (peak to peak), key in 1.00 kHz
and “enable” the output (if applicable). Hit the AUTO SCALE button
and observe the ‘scope display. Adjust the horizontal (time base) and
vertical sensitivity and record the changes these new settings make.
Go to the trigger controls and adjust the trigger level, what does this
do to the wave form display? Toggle between +ve and –ve slope. How does this
affect the display?
2) Key in a 1V DC offset to the output of the function generator and observe that the
waveform shifts in the vertical direction but the trigger point stays the same. Now
press the channel 1 button in the vertical control section and change the INPUT
COUPLING from DC to AC. What does this do to the display? Remove the offset
when you finish this part.
3) Connect the following R-C circuit (the
AC supply is your function generator
output) where the input voltage is
measured on Channel 1 (with a BNCBNC connector) and the capacitor
voltage is measured on channel 2 (with a
X10 probe, make sure the ground clip is
on the ground side of the circuit). Any
AC voltage will do but your 2V p-p,
1kHz signal is already available. Use the
frequency and the time delay between
the two waveforms to determine the
phase shift between the two signals using the ratio:
Time delay/period = phase shift/360°
4) Access the drop down measurement menu to get automatic readouts of Vp-p,
frequency and phase shift. Are they correct?
5) Obtain the 555 timer circuit from the office and use the ‘scope to determine the
output voltage and frequency of the square wave output (Pin 3).
This timer circuit must be powered from a DC power supply (Vcc on pin 8 is
anywhere between 5 – 10 V). Remember Red is +ve (Vcc) and Black is –ve (or
ground). Use the MAIN/DELAY button (in the time base control area) to observe one
of the voltage transitions in greater detail. Sketch this waveform when the transition is
magnified by 100 times. Use the Measurement options to measure the “Rise Time” of
the waveform.
6) Use of X10 Probes: Connect the following circuit using two 100 Ohm resistors (any
two resistors less than 1k will do)
and measure the output voltage with
a direct connection to the scope (i.e.
a X1 probe). The supply can either
be a 10V DC supply or 2V p-p sine
wave. Verify that this circuit is a
50% voltage divider. Now change to
two 470 k resistors and repeat the
measurement with a direct
connection (X1). The ‘scope places a
significant “load” on the voltage
divider (what is 470k in parallel with
1M?). Now switch to a X10 probe and observe the effect. Is the measurement closer
to being correct. How could you improve it even more?
Probes are useful for three specific tasks:
a) When you are trying to measure voltages across high resistance elements (>100K)
b) When you are making High voltage measurements (>80V).
c) When you are making high frequency measurements (> 10 kHZ).
Report
Write a one page report outlining what you learned about the oscilloscope today and
complete the attached oscilloscope work sheet. Try to answer all the questions in the
body of the procedure.
Student Names & Banner #s:
______________________________________________
______________________________________________
Date: ___________________
DALHOUSIE UNIVERSITY
Department of Electrical & Computer Engineering
ECED 2000 Electric Circuits – Lab 4
Oscilloscope Worksheet
(Due with your report a week after)
Fall 2014
1. When measuring AC voltage signals, what do the two axes of the oscilloscope screen
represent? What are the four basic function groups of an oscilloscope?
2. When the vertical (“Y”) axis of an oscilloscope is shorted (GND position), the result
should be a straight line in the middle of the screen as shown in (a). Determine the DC
polarity of the voltage source in (b).
(a)
(b)
3. For the two cases shown below, what display control(s) need to be adjusted on the
oscilloscope in order to show the signal properly?
4. Assuming the vertical sensitivity control is set to 0.5 volts per division, and the time
base control is set to 2.5 ms per division, calculate the amplitude of this sine wave (in
volts peak, volts peak-to-peak, and volts RMS) as well as its frequency. Redraw the
waveform if the time base controls are adjusted to 1 ms per division?
5. The waveform shown below is the input signal to channel 1 of a standard oscilloscope.
If the trigger controls are set to +ve slope, trigger level = 0 V, identify the trigger events
that will initiate a display sweep. If the scope is in the process of sweeping across the
screen and a trigger event happens, what does the scope do?