How AM and FM Works
Amplitude modulation (AM) and frequency modulation (FM) are methods of creating and sending information using an electromagnetic wave. We will be discussing the amplitude modulation and frequency modulation of a radio wave, like one received by a car stereo. Let us begin with the basics of how waves work.
Waves and the Electromagnetic Spectrum
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A wave is a disturbance that travels through a medium. Waves are often visualized two dimensionally along a time axis. Figure 1illustrates the amplitude and wavelength of a wave with respect to time. The frequency of a wave is the number of times a full wavelength occurs per second.
A radio waveis part of the electromagnetic spectrum. This is the range of wavelengths over which electromagnetic radiation extends. Figure 2below shows this spectrum and the different types of waves that it contains. As we will see later, waves can be manipulated to carry information.
Figure 1 - A basic sine wave. The x-axis represents time.
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Figure 2 - The electromagnetic spectrum. Waves increase in frequency from left to right.
Amplitude Modulation
In order to send information using a wave, we must start with a carrier wave. This is a basic sine wave at a low frequency. The AM radio in cars uses carrier waves with frequencies from 540-1610 kHz (thousands of cycles per second). The carrier wave is the frequency a user selects on his/her radio. In order for a radio station to broadcast audio, the carrier wave must be manipulated. In the case of amplitude modulation, the amplitude of the carrier wave is changed to follow the recorded audio wave. This can be seen in Figure 3. An oscillator produces the carrier wave, and themodulator changes the wave's amplitude, as seen in Figure 4. A transmitter sends the final wave through an antenna, shown in Figure 5.
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Figure 3 - Amplitude modulation of a basic sine wave. The final signal has the frequency of the carrier signal and the amplitude of the information (audio) signal.
Figure 4 - Modulator transforming the audio from a microphone and the carrier wave into the final transmitted signal.
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Figure 5 - A transmission tower, like that used by an AM radio station.
When an AM radio station broadcasts its signal, the wave is transmitted in all directions. Although we cannot see them, we are constantly bombarded by radio transmissions across the spectrum. Depending on the broadcast power, the signal can travel hundreds, sometimes thousands, of miles. As the wave travels, interference can occur. Static and noise can be produced from natural occurences, such as lightning, as well as man made structures, such as telephone wires. Figure 6shows the effect interference has on a wave.
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Figure 6 - An amplitude modulated wave after some interference.
The sharp peaks are characteristic of static.
Interference only changes the amplitude of a wave. This can pose many problems to AM communication. When enough interference has occured, the amplitude of the transmitted signal can be beyond recognition.
The AM signal is picked up by an antenna, such as that on a car, and sent to the receiver. This is composed of a de-modulator and an amplifier. The demodulator does the exact opposite of the modulator; it separates the amplitude of the signal from the carrier wave. The amplifier sends this de-modulated signal (the original audio) to a speaker through which the audio is played.
Frequency Modulation
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Because of the problems interference causes in AM communication, there is another type of modulation. Frequency modulation has a similar setup as amplitude modulation, but it changes the frequency of the carrier wave instead of the amplitude. The frequency of the carrier wave is called the rest frequency. Typical rest frequencies range from 88-108 MHz (millions of cycles per second). When the amplitude of the audio signal goes above or below zero, the frequency of the carrier wave is increased or decreased, respectively. Figure 7 shows a simple audio signal changing a carrier wave by amplitude modulation and frequency modulation. A modulator changes the frequency of the carrier wave.
Figure 7 - Amplitude modulation and frequency modulation of the same signal. Notice how the amplitude of the FM signal remains constant while the frequency changes.
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Broadcasting an FM signal is more complicated than broadcasting an AM signal. Because the frequency of the signal is changed throughout, there is no single frequency to which a receiver can tune. To deal with this, bandwidth and guard bands are used. Bandwidth is the amount of frequency within the electromagnetic spectrum a signal will deviate, which is typically 75 kHz. Guard bands, typically 20 kHz, separate these frequencies and prevent cross-signal intereference. Figure 8 illustrates how two separate broadcast frequencies, each with their own bandwith, are separated by a guard band. The user tunes to the rest frequency of the broadcast signal. The receiver accounts for the bandwidth of the rest frequency.
Figure 8 - Two separate frequency bandwidths, with rest
frequencies F1 and F2 , separated by a guard band.
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Frequency modulated waves are also subject to interference; however, recall that interference only affects the amplitude of the signal. The frequency of the signal is unaltered. This is where a limiter plays a role in an FM receiver. The limiter clips distorted parts of the signal and keeps the rest, as seen in Figure 9. Once the signal is cleaned up by the limiter, the remainder of the process is similarto amplitude modulation; the audio wave is extracted from the carrier wave by the de-modulator, and the remaining signal is run through an amplifier which sends the audio to a speaker.
Figure 9 - The limiter cleaning up the incoming signal. The amplitude has been changed with static and noise, but it is trimmed away to leave a clean output.
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Conclusion
Amplitude modulation and frequency modulation are just the building blocks to sending information with waves. If you ever listen to an AM or FM radio, it is evident that distortion limits the quality of these signals. AM and FM are just two types of modulation. As the demand for better quality increases, new innovations in data transmission are available. These methods, including quadrature amplitude modulation, are much more in-depth, and they allow for a wider volume of information to be sent.
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