Waveforms, Oscilloscopes and Electrocardiograms©2004
Experiment 6
Objective: To understand the elementary physics behind electrical waveforms, the use of
an oscilloscope and to apply our understanding of both to the operation of an
electrocardiogram.
DISCUSSION:
Waveforms:
A waveform is a representation of how alternating current (AC) varies with time.
Perhaps the most familiar AC waveform is the sine wave, which derives its name from
the fact that the current or voltage varies with the sine of the elapsed time. The sine wave
is unique in that it represents energy entirely concentrated at a single frequency. Common
uses for sine waves are in wireless technology. An ideal, unmodulated wireless signal has
a sine waveform, with a frequency usually measured in megahertz (MHz) or gigahertz
(GHz). Household utility current has a sine waveform with a frequency of 60 Hz in most
countries including the United States, although in some countries it is 50 Hz.
A waveform consists of various properties, two of which are amplitude (A) and
period (T). Take a look at the example of a sine waveform in Figure 1. In this lab, the
amplitude is in Volts (V) since we are working with AC circuits. Usually, we don’t
measure the current; we measure the voltage, mostly because measuring voltage is easier.
We also measure voltage instead of current because there is often a direct relationship so
if we measure voltage we can easily calculate the current.
The amplitude of the waveform is defined as the height of the wave from the
midpoint to a peak (or trough). The period (or frequency) is the length of time it takes the
waveform to travel from one point to an identical point in the next wave. In the example
below, the period is measured from trough to trough. The period is measured in Hertz
(Hz) which means a specific number of cycles per second.
Voltage
A
T
Time
Figure 1: A sine waveform.
Other common AC waveforms are the square wave, the ramp up and the ramp
down waves (both are also known as a saw-tooth wave), and the triangular wave.
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Oscilloscope:
A device that displays the waveform created by an electrical device, such as a
frequency generator or a specific circuit, is known as an oscilloscope. The “trace” is
generated on the screen in a specific manner so that the amplitude and period can be
measured. Analog oscilloscopes used a cathode ray system that plotted the trace on a
phosphorus screen. To do this, it used magnetic plates to control the beam, much like
non-plasma computer monitors and TVs. Digital scopes, one the other hand, sample the
signal at a very fast rate, then plot the results on a graph. We will use the PASCO
interface and software in this lab which performs as a digital scope.
The amplitude of the waveform can be read from the scope. The volts/div setting
tells us how many volts each vertical division on the graph means. So, if we have a wave
that is 4 divisions high and the volts/div is set at 2V/div, the waveform’s amplitude is 8V.
Likewise, we can read the period of the waveform from the time/div setting on the
horizontal axis. If the setting is 2 ms/div (two milliseconds per division), and the number
of divisions from trough to trough is 3, the period of the wave is 6 ms. To convert this to
frequency in Hz, take the inverse:
1
f 
(1)
T
Electrocardiograms:
According to the American Medical Association, the leading cause of death
among males and females in the United States is heart disease. The most useful test for
heart disease due to atherosclerosis (hardening of the arteries due to fatty deposits) is an
electrocardiogram (ECG). An ECG measures electrical impulses that occur during each
contraction of the heart muscle that forms a
heartbeat. In most cases, several electrodes are
attached to the chest and additional electrodes are
attached to an arm and a leg. The results are
usually displayed on an oscilloscope screen or on a
(a)
piece of chart-recorder paper.
Premature contraction
The waveform of a healthy heart produced
by an ECG has a characteristic shape (see Fig. 2).
An unhealthy heart will have an unusual waveform
or an irregular pattern to the beat. The standard
ECG is performed in a doctor's office or at the
hospital. For episodic heart abnormalities, a
miniature ECG unit that continuously monitors
heart activity, can be worn while one goes about
one's daily tasks. (This technique is called Holter
monitoring.)
(b)
Figure 2: Diagrams of electrocardiogram
traces. The characteristics of a healthy
heart are shown on the top. The waveform of a patient with an unhealthy heart
is shown on the bottom.
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To set up the EKG sensor, you will use 3 patches. The patches are pre-glued and
should be place on the inside of the arm. Peel off the backing in order to expose the glue.
One patch (electrode) should placed
just above or below the elbow of the left arm,
one on the elbow of the right arm and one on
the right wrist. The leads are connected to the
tabs on the patches. The red (positive) lead
from the sensor should be connected to the left
elbow patch. The green (negative) lead should
be connected to the right elbow patch. The
black (reference) lead should be connected to
the wrist patch. Be sure the EKG wires can
hang freely and do not twist the edge of the
patch.
Green Lead
On Elbow
Red Lead
On Elbow
Black Lead
On Wrist
EKG
EXERCISES FOR OSCILLOSCOPE:
1. Setup the Signal Interface and the frequency generator as shown in Fig. 4.
a. Plug the power brick into the back of the Interface, then into the strip on your
table.
b. Connect the white cable into the back of the Interface then into your
computer’s USB port.
c. Plug the EKG sensor into Analog Channel A.
d. Plug a voltage sensor into Analog Channel B.
e. Plug the leads of the voltage sensor in the output, black into ground, red into
wave.
f. Turn on the Interface (switch is on the back).
Figure4: PASCO Interface setup.
2. Start DataStudio. It should be located somewhere under your start menu.
3. When prompted, choose ‘Create Experiment’.
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4. Set up the software to recognize the hardware:
a. In the software, add an “EKG Sensor” to Channel A.
b. Add a “Voltage Sensor” to Channel B.
c. Under the “Output” list, choose the Output icon with the wave symbol.
d. Bring up the Signal Generator window. In version 1.9.7r8, you do this by clicking
on the output section of the interface (the part that the voltage sensor is physically
plugged into).
5. Set up the Oscilloscope
a. Notice that there is a list of Data options on the upper left (usually). Also notice
that there is a list of Display options on the bottom left (usually).
b. Click and drag the “Scope” onto “Voltage, ChB (V)”.
EXERCISES FOR WAVEFORMS:
1. Sine waveform.
a. In the signal generator window, set the waveform to Sine, the voltage to 5V and
the frequency to 100Hz.
b. Click on the “On” button to make sure the interface is creating an output.
c. Click the start button in the main DataStudio window and watch the Oscilloscope
window. You should see a sine waveform in the graph.
d. Click on the “Single Trace” button in the scope window
.
e. Change the V/div setting in the Signal Generator window to some arbitrary value.
f. What happens to the waveform?
g. Set the V/div setting so that the entire waveform can be seen.
h. What is the voltage of your wave, according to the scope?
i. Does this match the voltage that you set on the signal generator?
j. Now, change the time/div setting.
k. What happens to the waveform?
l. Set the time/div setting so that the entire waveform can be seen.
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m. What is the period of your wave, according to the scope?
n. What is the frequency of your wave (see Eq. 1)?
o. Does this match the voltage that you set on the signal generator?
p. Sketch your sine wave.
2. Square waveform.
a. In the Signal Generator window, choose “Square Wave” from the drop down
menu.
b. Repeat exercises 1d-1o for the Square waveform.
3. Other waveforms: Repeat exercise 1 for the Triangle Wave and the Ramp Up wave.
EXERCISES FOR ELECTROCARDIOGRAMS:
1. Click and drag the “Scope” icon to “Voltage, ChA(V)”.
2. Set the horizontal divisions to 200 or 500 ms/div.
3. Click on the single trace icon.
4. Wire up your partner with the EKG device.
5. Click start in DataStudio. It should stop when one trace is complete.
6. What do you see? Can you identify a repetitive signal associated with your
heartbeat?
7. Using the cursor in the scope window
, find the period of your heartbeat.
8. Find the frequency of your heartbeat.
9. If the frequency is beats per second, how many beats will you have in 10
seconds?
10. With a partner's assistance, measure your actual heart rate for a ten second period.
11. Do your heartbeat rates match?
12. Repeat the exercise for all the members of your lab group.
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