Unit 7.
AC Waveforms and the Oscilloscope
Up to now, we've been working with dc (directcurrent) circuits.
In this unit we'll study the difference between dc and ac (alternating
current). We'll also begin to use the oscilloscope, an important
instrument for making measurements on ac circuits. Another piece of equipment
that we'll start using is the function generator, which generates
ac voltages and currents.
The textbook has many chapters on ac circuits,
but they're too advanced for this course. (You'll study those chapters
in a later course, EET 155.) Although I'm not asking you to do any
reading in the book, I do give you page references below, so that you
can look up these topics in the book if you wish. Most of the material
in this Unit is discussed in Chapter 11 of the textbook.
Work through the eLesson and selftest questions below.
After completing the eLesson, take Quiz #7, perform Lab
#7, and do Homework #7.
Unit 6 Review
 This unit will build on material that you studied in Unit
6. So let's
begin by taking this selftest to review what you learned in that unit.

DC and AC
 Direct current (dc) is current that flows in one
direction only.
 A dc voltage source is a voltage source that produces direct current.
 Examples: Batteries and dc power supplies (such as the power supply
built into the trainer that you use in lab) are dc voltage
sources.
 Alternating current (ac) is current whose direction
periodically reverses.
 An ac voltage source is a voltage source that produces
alternating current.
 Examples: Electrical outlets in the walls of your home provide alternating
current. The trainer that you use in lab also contains an ac voltage
source called a function generator, which you'll start using this
week.

Waveform
 In most dc circuits, current and voltage remain constant as time
passes. But in ac circuits the voltage and current change as time
passes.
 It's possible to write down mathematical expressions that describe
how the values change in time, but a simpler and more common way is
to draw diagrams showing how the voltage or current changes in time.
 Such a plot, or graph, of a current or voltage versus time is called
a waveform.
 Examples: Below are diagrams of three of the most common waveforms
we'll deal with: a triangle wave, a square wave, and
a sine wave. Notice that each of these diagrams plots voltage
(measured in volts or millivolts) on the vertical axis, and time (measured
in microseconds or milliseconds) on the horizontal axis.
Periodic Waveform
 A periodic waveform is a waveform whose values are repeated
at regular intervals.
 All three of the waveforms shown above are periodic waveforms.
Cycle
 The plot of a periodic waveform shows a regularly repeating pattern
of values, each of which is called a cycle.
 Example: In the picture of the sine wave shown just above,
we see a little less than two full cycles. The first cycle extends
from 0 ms to 50 ms, and the second (incomplete) cycle extends from
50 ms to the edge of the chart, where it is cut off.

Period
 The time required for the values to rise and fall through one complete
cycle is called the period of the waveform.
 The symbol for period is T.
 Period is measured in units of seconds, abbreviated s.
 Example: The sine wave shown above has a period of 50 ms.

Frequency
 The frequency of a periodic waveform is the number of cycles
that occur in 1 second.
 The symbol for frequency is f.
 Frequency is measured in units of cycles per second, or Hertz, abbreviated Hz.
Relationship Between Period and Frequency
 Period and frequency are the reciprocal of each other:
and
 Example: The sine wave shown earlier has a period of 50 ms.
Therefore, its frequency is 20 Hz.

Peak Value
 The maximum value reached by an ac waveform is called its peak
value.
 If the waveform is a voltage waveform, then its peak value is also
called its peak voltage, abbreviated V_{p}.
 If the waveform is a current waveform, then its peak value is also
called its peak current , abbreviated I_{p}.
 The peak value of a waveform is sometimes also called its amplitude,
but the term “peak value” is more descriptive.
 Example:The sine wave shown earlier has
a peak value of 500 mV_{p}.
Notice that I write a "p" after the unit to show that I'm
talking about a peak value.

PeaktoPeak Value
 The peaktopeak value is
the difference between a waveform's positive peak value and its negative
peak value.
 If the waveform is a voltage waveform, then its peaktopeak value
is also called its peaktopeak voltage, abbreviated V_{pp}.
 If the waveform is a current waveform, then its peaktopeak value
is also called its peaktopeak current , abbreviated I_{pp}.
 If the waveform is symmetrical about the time axis, then the peaktopeak
value equals twice the peak value.
 Example: The sine wave shown earlier has
a peaktopeak value of 1 V_{pp}. That's the difference between
the maximum positive value (which is 500 mV) and the maximum negative
value (which is 500 mV).
Notice that I wrote a "pp" after the unit to show that I'm
talking about a peaktopeak value.

p and pp
 As mentioned in the two examples above, we write p as
the subscript of a quantity or unit to show that we're talking about
a peak value, and we write pp as the subscript
of a quantity or unit when we're talking about a peaktopeak value.
 Example: For the sine wave above, we could write
V_{p} = 500 mV_{p}
or
V_{pp} = 1 V_{pp}.
 Similarly, if we were dealing with a current waveform whose peak
value is 20 mA and whose peaktopeak value is 40 mA, we could write
I_{p} = 20 mA_{p}
or
I_{pp} = 40 mA_{pp}.
 Some other textbooks use pk (instead of p)
as the abbreviation for peak values, and pp (instead
of pp) as the abbreviation
for peaktopeak values. So you may see these other abbreviations
used from time to time.
Review of Electrical Quantities
 Period and frequency are two of the electrical quantities presented
in the table that you first
saw in Unit 1 of this course. By now you should have learned the units
of most of these electrical quantities, as well as the symbols for
the quantities and their units. If you haven't already done so, be
sure to play the ElectricalUnits
Matching Game and ElectricalSymbols
Matching Game until you've got these units and symbols memorized.
Function Generator
 The function generator, or signal generator, is an
instrument designed to produce ac waveforms, such as square waves,
triangle waves, and sine waves. Using it, you can set the peak value
and the frequency of these waveforms.
 Here is a photo of the builtin function generator on the trainers
that we use in our electronics labs. It provides the basic controls
that any function generator must have:
 an Amplitude control to set the
waveform's peak value
 Frequency and Range controls to set the waveform's
frequency
 a Function control to set the shape of the waveform.
Lab
6 will teach you how to use these controls.
 The photo below shows a professionalquality function generator made
by Tektronix. It provides all of the controls discussed above, as well
as more advanced features. You'll use this function generator in later
courses at Sinclair.
Oscilloscope
 The oscilloscope is an instrument designed to display waveforms.
Using it, you can measure period, frequency, peak values, peaktopeak
values, and other important quantities.
 Shown here is a Tektronix 2213, one of the types of oscilloscopes
that we have in Sinclair's electronics labs. At the left is the screen
on which waveforms are displayed. To the right are the knobs and switches
that you can adjust to control the waveform's appearance.
 Below are three separate photos showing how a triangle wave, a square
wave, and a sine wave look on the oscilloscope screen.
Using the Oscilloscope to Measure Voltage
 The oscilloscope displays a graph of voltage
versus time, with voltage plotted on the vertical
axis and
time plotted on the horizontal axis.
 To measure a waveform's peaktopeak voltage, you count how many
vertical divisions (squares) the waveform covers on the oscilloscope's
screen, and then you multiply this number times the setting of the
oscilloscope VOLTSPERDIVISION knob.
 This learning object will show you how to do it:

Using the Oscilloscope to Measure Period and Frequency
 Remember, the oscilloscope displays a graph of voltage versus
time,
with voltage plotted on the vertical axis and time
plotted on the horizontal axis.
 To measure a waveform's period, you count how many
horizontal divisions (squares) the waveform covers on the oscilloscope's
screen, and then you multiply this number times the setting of the
oscilloscope SECONDSPERDIVISION knob.
 Once you know the waveform's period, you can use the formula f = 1 ÷ T to find
its frequency.
 This learning object will show you how to do it:

Oscilloscope Challenge Game
 The oscilloscope is a complicated piece of equipment. You'll need
plenty of practice to learn how to use it correctly.
 To start learning this skill, take some time right now to play Oscilloscope
Challenge. In particular, work through the game's "Study" section,
which is a tutorial containing several pages of notes to help you
identify and use the oscilloscope's controls. This will be a good
preparation for Lab 6, in which you'll begin using a real oscilloscope
to make measurements.
Unit 7 Review
 This eLesson has covered several important topics related to ac
waveforms, including:
 period
 frequency
 peak values
 peaktopeak values
 function generator and oscilloscope.
 To finish the eLesson, take this selftest to check your understanding
of these topics.

Congratulations! You've completed the eLesson for this unit.
Nick Reeder
 Electronics Engineering Technology  Sinclair Community College
Send comments to nick.reeder@sinclair.edu
