Part 1: AC Voltages and Current: Conveying Information
This chapter began by discussing the idea that AC voltages are important, in part, because they represent a way to provide information to a circuit so that the circuit can transform this input to the desired output. A circuit that operates on fixed DC voltages cannot do this because nothing in the circuit changes. Even if you don’t realize it, you probably have some experience using AC voltages to control an electrical system. When you dial a phone, you can hear different tones associated with the buttons you press. These tones are pairs of sine waves at different frequencies. They are how you communicate with the electrical equipment at the telephone company so you connect to the desired number or navigate the automated system that answers the call.
The following figure shows three hypothetical voltage waveforms that may occur in a circuit. These are just three of a countless number of waveform shapes you will find in different circuits.
Figure 1. Three different AC waveforms.
The first of the three figures depicts a very slowly changing voltage. One potential source for a waveform of this sort is the speed control trigger of an electrical drill that is used to set the speed of the drill’s motor. The frequency content of a waveform of this type will be very low. This is because the time over which the waveform changes is very long. In cases like this, when operating at very low frequencies that approach DC, the circuit analysis is simplified because most of the effects that occur from operating at higher frequencies can be ignored. Don’t worry about the details of this for now, it will become apparent in later chapters. As we discussed in the previous section, this particular waveform has a DC offset which keeps its polarity positive.
The second waveform shown is recognizable as a sine wave whose amplitude is changing. This waveform carries its information in two ways, its unchanging frequency and its changing amplitude. This type of wave shape is fairly common. When you listen to an AM radio station, the waveform used to transmit the signal is similar to this. In this case, you tune in the station by selecting the frequency, and the audio you listen to is based on the changing amplitude. This waveform is common enough it is given the name amplitude modulation (or AM) after the fact that the amplitude of the waveform is changed, or modulated, to carry information.
The third waveform is also recognizable as a sine wave. In this case, it is the frequency of the waveform, rather than its amplitude, that is changing. Like the AM waveform, this is also a very common waveform and likewise has a common use related to radio. In this case it is FM, or frequency modulation, radio signals where this type of waveform can be found. The station is selected by looking at the range of frequencies assigned to the radio station, and the audio signal is based on how far the frequency deviates from the center frequency in the range. Although both examples used for the sine wave modulation schemes (AM and FM) were based on radio, these types of waveforms are used in a variety of different applications.