How It Works: Radios
AM or FM, radios are the most common wireless communications device on the face of the planet. In fact, way back in its early history, the term for radio was "wireless," which now denotes an entire industry! We take them for granted, these little radios, finding them playing in cars, homes and offices, tagging along with joggers, tradesmen and teenagers after school.
In fact, the lowly radio we see so much of today is the heart of virtually every "wireless" device we see for sale today. Cellular phones, pagers, televisions (yes, televisions) wireless LANs, most everything "wireless" (with the exception of infrared devices) uses radio to get from here to there. The funny thing is: most every kind of radio transmitting device uses some form or variation of either AM or FM to do its job.
Let's have a look at how radios work! What Are Radio Waves? Radio waves are part of a general class of waves known as electromagnetic waves. In essence, they are electrical and magnetic energy which travels through space in the form of a wave. They are different from sound waves (which are pressure waves that travel through air or water, as an example) or ocean waves (similar to sound waves in water, but much lower in frequency and a LOT bigger). The wave part is similar, but the energy involved is electrical and magnetic, not mechanical.
Electromagnetic waves show up as many things: At certain frequencies, they show up as radio waves. At much higher frequencies, we call them infrared light. Still higher frequencies make up the spectrum known as visible light. This goes on up into ultraviolet light, and x-rays, things that radio engineers rarely have to worry about. For our discussions, we'll leave light to the physicists, and concentrate on radio waves.
Radio waves have two important characteristics that change. One is the amplitude, or strength of the wave. This is similar to how high the waves are coming into shore from the ocean. The bigger wave has a higher amplitude. The other thing is frequency. Frequency is how often the wave occurs at any point. The faster the wave repeats itself, the higher the frequency. Frequency is measured by the number of times in a second that the wave repeats itself. Old timers remember when frequency was described in units of cycles per second. In more recent times we have taken to using the simplified term of hertz (named after the guy who discovered radio waves). Metric prefixes are often used, so that 1000 hertz is a kilohertz, one million hertz is a megahertz, and so on.
The important thing in any communications system is to be able to send information from one place to another. This means we have to find a way to impress that information on the radio wave in such a way that it can be recovered at the other end. This process is known as modulation. In order to modulate a radio wave, we have to change either or both of the two basic characteristics of the wave: the amplitude or the frequency.
If we change the amplitude, or strength, of the signal in a way corresponding to the information we are trying to send, we are using amplitude modulation, or AM. The earliest means of radio communications was by Morse code, and the code key would turn the transmitter on and off. The amplitude went from nothing to full power whenever the key was pressed, a basic form of AM.
Modern AM transmitters vary the signal level smoothly in direct proportion to the sound they are transmitting. Positive peaks of the sound produce maximum radio energy, and negative peaks of the sound produce minimum energy.
The main disadvantage of AM is that most natural and man made radio noise is AM in nature, and AM receivers have no means of rejecting that noise. Also, weak signals are (because of their lower amplitude) quieter than strong ones, which requires the receiver to have circuits to compensate for the signal level differences.
In an attempt to overcome these problems, a man named Edwin H. Armstrong invented a system that would overcome the difficulties of amplitude noise. Instead of modulating the strength (or amplitude) of the transmitted signal, or carrier, he modulated the frequency. Though many engineers at that time said that FM was not practical, Armstrong proved them all wrong, and FM today is the mainstay of the broadcast radio services.
In a frequency modulated system, the frequency of the carrier is varied according to the modulating signal. For example, positive peaks would produce a higher frequency, while negative peaks would produce a lower frequency. At the receiving end, a limiting circuit removes all amplitude variations from the signal, and a discriminator circuit converts the frequency variations back to the original signal.
In this way, the effects of amplitude noise are minimized. Since the recovered audio is dependent only on the frequency, and not the strength, no compensation for different signal levels is required, as is the case with AM receivers.
There are many types of modulation, but all are variations of AM or FM. AM variations include Single Sideband (SSB), Double Sideband (DSB), and Vestigial Sideband (VSB, commonly used in television). FM variations include Phase Shift Keying (PSK), Minimum Shift Keying (MSK) and others. There are hybrid systems as well, which combine both kinds of modulation. In the interest of saving download time and Web Page space, we will not cover these forms at length.
Many FM broadcast stations now transmit subcarriers on their signals. Subcarriers are signals that are modulated on the carrier, just like the normal audio signals, except that they are too high in frequency to be heard. Normal audio signals range in frequency from 20 to 20,000 hertz. Most subcarriers start at 56,000 hertz (56 KHz). These subcarriers are themselves modulated, sometimes with audio signals, such as background music, but more and more these days with various forms of data.
Some of the information carried on these subcarrier data services include stock quotes, weather reports, news, sports and even paging signals. There is no limit to the variety of data that can be sent on a subcarrier signal, and broadcasters are finding new things to send all the time!
Even the FM stereo which most of us are used to relies on a subcarrier. In an FM stereo system, the main audio, which is monophonic, stops at around 18 KHz. At 19 KHz is a pilot signal (a subcarrier) and then there is a special stereo subcarrier centered around 38 KHz. The receiver uses this special subcarrier in combination with the monaural signal to produce the left and right channel audio signals. The 19 KHz pilot signal is used as a reference so that the receiver can properly recover the 38 KHz stereo subcarrier.
Well, yes and no... Digital radio systems still rely on the same modulation process that analog radios do. Instead of modulating the analog audio signal onto the carrier, digital radios modulate a digital signal onto the carrier. But, if you look at the signal at the most basic level, even digital radios still use some form of AM, FM or combination of both.
So there you have it! Radio, the backbone of the wireless industry. It forms the basis of all of the mobile and portable communications systems we use today. Whether it's music, data, television, phone calls or whatever, if it's wireless, chances are it's radio!