The Basic circuit on the left shows an unspecified signal A.C. Waveforms
I. Basic AC Waveforms
The Basic circuit on the left shows an unspecified signal
source which is not a battery. Inside the circle for the signal
source Vs there should appear the "~" sign which stands for
an A.C. (Alternating Current) signal instead of a D.C signal.
D.C. stands for direct current, i.e. the signal is always a fixed value. In contrast an A.C. signal has a maximum
value and a minimum value and it alternates between these two values during some period of time.
The graph at the side shows a D.C voltage source with a
value of 5 volts which does not change with time, but
remains constant.
A.C. signal sources can be made up of different types of Waveforms. The most common waveforms are the
Sinusoidal waveform, the Square waveform and the Triangle waveform. There is other kinds of A.C
waveforms which will be encountered as one becomes more involved with electronics.
An example of each of these A.C. waveforms is shown below :
1. The Sinusoidal Waveform. (Horizontal time is in milliseconds, vertical axis -5 to +5 volts)
2. The Square Waveform. ( Horizontal time is in microseconds, vertical axis -2.5 to +2.5V)
3. The Triangle Waveform. ( Horizontal time is in microseconds, vertical axis -2.5 to 2.5V)
Of course the Waveforms shown are all "ideal" and in the case of the Square waveform the rise and fall times
are not zero, meaning it takes a finite amount of time for the voltage to rise to the maximum value and
subsequently to go to the minimum value. These rise and fall times are normally very small and for the "ideal"
case we will ignore them. In the case of the Sinusoidal and Triangle waveforms other anomalies can appear
such as noise (which are unwanted signals "riding" on either waveform) and even waveforms that are not
completely symmetrical. One will find these problems as he/she progresses in the study of electronics and
signals.
The most common of the waveforms is the Sinusoidal type and hence we will be dealing most of the time
with these type of waveform in our studies of A.C. electronics.
II. Period and Frequency of Sinusoidal Waveforms.
The common thing with all of the AC Waveforms is that they are periodic which means that they repeat
themselves after a period of time T. Said in a different way the period of a signal is the time T that it takes
for the signal to traverse a whole cycle, after which the signal repeats itself.
We can see the meaning of period and cycle better if we take a look at the sinusoidal waveform below:
At the start the waveform is at 0V. After 0.25ms we reach a peak at 1V. After 0.5ms we are back at
0 volts. We have just completed what is called "the positive alternation".
From 0.5ms to 0.75ms the waveform goes to a minimum value of -1V. Then the waveform goes back up
again until reaches 0V at 1ms. We have just completed "the negative alternation". The time after we have traversed one positive and one negative "alternations" or one whole cycle of the waveform is called the
Period T of the signal. In this example, the period T = 1ms. After this period we start a new cycle of the
signal with exactly the same waveform as the first cycle. Hence, we call the sinusoidal signal a periodic
signal.
It is important to know well the characteristics of a Sine or Cosine function to understand better what these functions represent for us in Electronics, hence if you need a review please go to the School's Library to review some Math. In particular I recommend the AC Waveforms Tutorial at : www.sweethaven.com/acee/forms/toc01.htm
In brief, it is important to note that if the horizontal axis was changed from time to degrees the following will correspond to the data-points discussed already: The first peak will occur at an angle of 90 degrees;
the first positive alternation will occur at an angle of 180 degrees. The first minimum point at -1V will happen
when the angle is 270 degrees. The first negative alternation and the first cycle will occur at 360 degrees.
Since at 360 degrees we complete a whole circle, it is only obvious that the function will repeat itself going around one more circle.
What about Frequency? Once we have understood what the Period and the Cycle of a function
represent the concept of Frequency shall follow up more easily. Frequency is defined to be as the number
of cycles per unit time of a function expressed as f = cycles/time. When time is given in seconds, frequency has the standard unit of #cycles/second or Hertz = Hz. Taking the example we were discussing, we have 1cycle in a complete period of 1ms. If we wanted to increase the frequency this will mean that we want to include more cycles in that same period of 1ms. Initially the frequency is: f = 1cycle/1ms = 1MHz (1MegaHertz). (If you still need help with Units review DC Basics under DC Electronics.) Suppose we want to include three cycles in 1ms, then the frequency would be f = 3cycles/1ms or 3MHz.
Finally, notice what happens to the period of the signal if we include 3cycles in 1ms! Initially the Period
T was equal to 1ms, now we cover 3 cycles in 1ms and the Period has been reduced by a factor of 3
such that T(new) = 1/3 T(original) = 0.33ms. In other words, if we increase the frequency by 3 we will
reduce the Period of the signal by the same factor of 3 and:
Hence the key relationship between Period T and Frequency F is:
T = 1/f or f = 1/T
The Period and Frequency of a waveform are the reciprocal of each other!!
Note: This relationships holds for ANY periodic function and not just sinusoidal functions!
III. Magnitude and Phase of a Sinusoidal Waveform
For AC sinusoidal waveforms, besides having to
deal with frequency and Period, we need to be able
to have a measurement for the varying Magnitude,
and also account for the fact that as we traverse
through an AC circuit the Phase of the waveforms will be different than the Phase of the Input waveform.
To illustrate both factors we shall take a look at the circuit above which will also give us some idea of what is
coming up in later lessons.
First, we realize that the input is a 60Hz frequency waveform, given by a description V1: -170/170V.
The numbers -170V and 170V stand for the minimum and maximum voltages of the sinusoidal waveform.
This waveform is shown in the graph below.
Magnitude: Clearly then, one of the ways of specifying the Magnitude of the Signal is by stating
its maximum or peak voltage, in this case the Peak Voltage or Vp = 170V.
A second way of specifying the Signal's Magnitude is by giving its Amplitude which is the voltage from
its maximum of 170V to its minimum of -170V, this is also called the Peak-to-Peak Voltage or
Vp-p which in this case is two times 170V or Vp-p = 340V.
The third and most common way of giving the Magnitude of the Waveform is by using a traditional
measure called RMS or Root-Mean-Square value. The RMS value is obtained by considering the
average power of the Signal and deriving it will take a lengthy explanation. The RMS value is
equivalent to the Peak value of the signal divided by a factor of √2, in brief: Vrms = Vp/√2.
Thus, for our example Vrms = Vp/1.4142 = 0.7071*Vp = 0.7071*170V = 120Vrms, which is the
RMS voltage value of an AC signal coming directly from an outlet of our home or building!!!
Note that the RMS value is smaller than the Peak Value, since it is an average of the signal's Power!
Phase: Finally, a last consideration for our AC signal Measurements is the concept of the Phase
relationship between different signals. As a reminder, a standard Sine wave as we discussed earlier
on this same page start at 0V at an angle of 0 degrees. It goes to a Maximum or Vp at 90 degrees,
goes back to 0V at 180 degrees, then reaches a Minimum or -Vp at 270 degrees and finally completes
a whole period or cycle at 360 degrees. Thus, when thinking that the x-Axis is given by a time (t) basis,
get used to also relating the different portions of the waveform to an x-Axis that would be given in degrees.
In the circuit that we are using for illustration the waveform at the output looks as shown below:
Compare this Waveform to the previous Input Waveform. Note that the output signal is smaller in
Magnitude and also note that the output signal has its first Maximum or Peak Value later than the
input signal. Therefore, there is a time lapse between the input and output signals, or equivalently
there is a Phase Difference or Phase lapse between both signals. The input signal reaches its first Peak
voltage at about 5ms. The output signal reaches its first Peak at about 7ms. The time lapse between
both signals is then 2ms
Since the frequency of the input and the output need to stay the same for building a comparison between
both, the period is T = 1/f = 1/60Hz = 0.01667 seconds = 16.67ms. We know that one cycle is given in
360 degrees, so for a whole cycle or 360 degrees the equivalent period is 16.67ms.
We need to answer, what is the Phase Difference for 2ms?
Using a little Math, the Phase Difference between both signals is Phase Diff = (2/16.67) * 360 degrees
Phase Diff = 43.19 degrees. It's just so easy!---Just kidding, if you didn't get the first time, you may
want to review this first lesson on AC Electronics.
Well, and how about the difference in Magnitude between the input and output signals? And what if
we were to change the frequency? These and many more questions will be answered in the following
AC Electronics lessons.