Chapter 1
Tuning in to Radio
In This Chapter
Realising how radio waves work
Using transmitters and receivers
Discovering the differences between AM and FM
The American cable TV channel Music Television, better known as MTV, launched on 1 August 1981. Appropriately, the first music video it played, at one minute past midnight, was ‘Video Killed the Radio Star’ by the Buggles. The song laments how the golden age of radio would be lost to the rise of television. Ironically, most teenagers today get their music-video fix by searching YouTube instead of switching on the television in the hope of catching their favourite stars, whereas radio is still doing fine. Video didn’t kill the radio star after all.
In the 1930s and 1940s, people had only one use for radio: broadcast audio signals. Today, audio broadcast over radio is as commonplace as ever, but the list of other types of information being broadcast by radio technology has skyrocketed:
Broadcast television: Nothing more than the combination of audio and video broadcast over radio.
Mobile phones: Extends the telephone networks to places that phone cables can’t reach.
Wireless networking: Replaces bulky computer network cables with data transmitted over radio.
Traditional audio radio and television: Received a technology overhaul with digital television and digital radio.
Other popular uses for radio technology: Includes radar, GPS navigation systems and wireless Bluetooth devices.
In this chapter, you discover the basic concepts of radio: what it is, how it works and how it was discovered. This chapter provides the important foundation for this minibook’s Chapters 2 and 3, in which you find out how to build circuits that receive and play radio signals broadcast in the AM radio band.
Throughout this chapter, we sometimes refer to radio as if it’s only for the broadcast of sound. Just keep in mind that radio is also used for broadcasting video and digital data as well as other types of information.
Rolling with Radio Waves
Most people think of radio as the wireless broadcast of sound, often music and speech. But the term is much broader than that; the broadcast of sound is just one application of the extremely useful electrical phenomenon called radio.
Radio takes advantage of one of the most interesting of all electrical phenomena: electromagnetic radiation (often abbreviated EMR): a type of energy that moves in waves at the speed of light, through the air and even in the vacuum of space.
EMR waves can oscillate at any imaginable frequency. The rate of the oscillation is measured in cycles per second, also known as hertz (abbreviated Hz). The term hertz doesn’t refer to the car rental company! Instead, it honours the great German physicist Heinrich Hertz, who was the first person to build a device that was able to create and detect radio waves.
Radio is simply a specific range of frequencies of EMR waves. The low end of this range is just a few cycles per second and the upper end is about 300 billion cycles per second (also known as gigahertz, abbreviated GHz). That’s a pretty big range, but EMR waves with much higher frequencies exist as well, and are in fact commonplace. EMR waves with frequencies higher than radio waves go by various names, including infrared (see Chapter 3 of this minibook), ultraviolet, X-rays, gamma rays and – most importantly – visible light.
That’s right; what people call light is exactly the same thing as what they call radio, but at higher frequencies. The frequency of visible light is measured in billions of hertz, also called terahertz and abbreviated THz. The low end of visible light (red) is around 405 THz and the upper end (violet) is around 790 THz.
Disputed paternity: Who’s the Daddy?
The history of radio technology is plagued by controversy over the question of who actually invented it. The answer most often given is Italian inventor Guglielmo Marconi, but many others made important discoveries that give them good claim to the title.
Here’s a rundown of the contenders for the title of The Father of Radio:
Marconi: He was the first person to demonstrate radio successfully and exploit it commercially. In 1901, Marconi sent a message via radio across the Atlantic from England to Canada. The message was faint, consisted of nothing but the letter ‘S’ and reception of the message wasn’t independently confirmed. Nevertheless, Marconi’s accomplishment was astonishing, and he made many important contributions to the technology and business of radio.
Tesla: In 1943, the US Supreme Court ruled that many of Marconi’s important radio patents were invalid because Nikola Tesla had already described the devices covered by Marconi’s patents. Tesla was a brilliant engineer who’s best known for being the champion of alternating current over direct current for power distribution. He publicly demonstrated wireless communication devices as early as 1893. Tesla believed that wireless technology would be used not only for communication, but also for power distribution.
Lodge: In England, Sir Oliver Lodge was building wireless telegraph systems in the mid-1890s.
Popov: In Russia, Alexander Stepanovich Popov was demonstrating wireless telegraph transmissions around the same time as Lodge.
Bose: In India, Sir Jagadish Chandra Bose was also demonstrating wireless telegraph transmissions in the early 1890s. Whether these demonstrations occurred before, after or at the same time as other demonstrations by Lodge, Popov and others is under dispute.
Many others: The list of names of others who did important research and made important discoveries in the last decades of the nineteenth century is long: Heinrich Hertz, Edouard Branly, Roberto de Moura, Ernest Rutherford, Ferdinand Braun, Julio Baviera, and Reginald Fessenden are just a few of the many individuals who made important contributions.
Therefore no one person has a clear-cut claim to being the first to invent radio. Work was being done all around the world and discoveries were being made almost every day. Perhaps the best answer is that no one ‘invented’ radio, because it’s a natural phenomenon. Radio was discovered, not invented.
What was invented were ways to exploit the phenomenon of radio by building devices to generate radio waves and modulate them to add information, as well as devices that could receive radio waves and extract the information that was added.
So here’s an interesting thought. Radio stations broadcast on a specific frequency. For example, London’s Capital radio station has broadcast on 95.8 MHz for many years. Plenty of other radio stations operate in the area, but only Capital broadcasts at 95.8 MHz – or only Capital is supposed to. Pirate stations sometimes cause interference in certain areas. The term channel is often used to refer to a radio station broadcasting at a particular frequency.
Purple is the colour you perceive when you see light whose frequency is right around 680 THz. Many other colours exist, of course, but only the colour purple is at 680 THz. So in a way, colour is the same thing as channel. If EMR waves are vibrating at 95.8 MHz, they’re Capital radio. If those same EMR waves vibrate millions of times faster, at 680 THz, they’re the colour purple.
An important concept that’s related to frequency is the idea of wavelength. The term wavelength refers to the distance between the crests of each cycle of EMR at a particular frequency. Because EMR waves travel at the speed of light, you can calculate the wavelength of a given frequency by dividing the distance that light travels in a single second by the number of cycles per second.
Light is pretty fast: it scoots along at 186,282 miles per second – or, to be precise about it, 299,792,458 metres per second (m/s). Thus, the wavelength of an EMR wave oscillating at 100 kHz is about 1.86 miles: 186,282 divided by 100,000 (or 2997924.58 m/s).
The higher the frequency, the shorter the wavelength. The wavelength of most AM broadcast radio stations is a small fraction of a mile. The wavelength of visible light is a very small fraction of a centimetre.
Sound waves aren’t a type of EMR. Sound waves are created when particles of matter bump against each other. Thus, you need matter – such as air or water – to transmit sound waves. Radio waves don’t require particles of matter to travel. In fact, radio waves travel best in the vacuum of space, where no matter is present to get in the way.
Transmitting and Receiving Radio Waves
Many natural sources of radio waves exist. But in the later part of the 19th century, scientists figured out how to generate radio waves using electric currents. In a nutshell, if you pass an alternating current (AC) into a length of wire, radio waves at the same frequency as the AC are generated.
Radio communication requires two components:
Transmitters generate radio waves.
Receivers detect radio waves.
The following two sections describe the basic operation of radio transmitters and receivers.
Making waves with radio transmitters
A radio transmitter consists of several elements that work together to generate radio waves that contain useful information such as audio, video or digital data. Figure 1-1 shows these components, as follows:
Power supply: Provides the necessary electrical power to operate the transmitter.
Oscillator: Creates AC at the frequency on which the transmitter is going to transmit. The oscillator usually generates a sine wave, which is referred to as a carrier wave..
Modulator: Adds useful information to the carrier wave. This information can be added in two main ways:
• Amplitude Modulation (AM) makes slight increases or decreases to the intensity of the carrier wave.
• Frequency Modulation (FM) makes slight increases or decreases to the frequency of the carrier wave.
For more information about AM and FM, see the later sections ‘Approaching AM Radio’ and ‘Finding out about FM Radio’.
A third method of adding information to a radio signal is simply turning the signal on and off in a pattern that represents the information. For example, radio signals can send Morse code in this way.
Amplifier: Amplifies the modulated carrier wave to increase its power. The more powerful the amplifier, the more powerful the broadcast.
Antenna: Converts the amplified signal to radio waves.
Figure 1-1: The basic components of a radio transmitter.
Catching the waves: Radio receivers
A radio receiver is the opposite of a radio transmitter. It uses an antenna to capture radio waves, processes those waves to extract only those waves that are at the desired frequency, extracts the audio signals that were added to those waves, amplifies the audio signals and finally plays them on a speaker. Figure 1-2 shows these components. Here’s how each one works:
Antenna: Captures the radio waves. Typically, the antenna is simply a length of wire. When this wire is exposed to radio waves, the waves induce a very small AC in the antenna.
RF amplifier: A sensitive amplifier that amplifies the very weak radio frequency (RF) signal from the antenna so that the signal can be processed by the tuner.
Tuner: A circuit that can extract signals of a particular frequency from a mix of signals of different frequencies. On its own, the antenna captures radio waves of all frequencies and sends them to the RF amplifier, which dutifully amplifies them all. Unless you want to listen to every radio channel at the same time, you need a circuit that can pick out just the signals for the channel you want to hear. That’s the role of the tuner.
The tuner usually employs the combination of an inductor (for example, a coil) and a capacitor to form a circuit that resonates at a particular frequency. This frequency, called the resonant frequency, is determined by the values chosen for the coil and the capacitor. This type of circuit tends to block any AC signals at a frequency above or below the resonant frequency.
You can adjust the resonant frequency by varying the amount of inductance in the coil or the capacitance of the capacitor. In simple radio receiver circuits (such as the one you read about in Chapter 2 of this minibook), the tuning is adjusted by varying the number of turns of wire in the coil. More sophisticated tuners use a variable capacitor (also called a tuning capacitor) to vary the frequency.
Detector: Responsible for separating the audio information from the carrier wave. For AM signals, you can do this separation with a diode that rectifies the AC signal and a low pass filter. What’s left is the audio AC signal with a DC offset that you can feed to an audio amplifier circuit through a series capacitor to remove the DC signal component. For FM signals, the detector circuit is more complicated.
Audio amplifier: Amplifies the weak signal that comes from the detector so that you can hear it. You can achieve this amplification by using a simple transistor amplifier circuit as we describe in Book II, Chapter 6. You can also use an op-amp IC, which is the subject of Book III, Chapter 3.
Figure 1-2: The basic components of a radio receiver.
Of course, loads of variations exist to this basic radio receiver design. Many receivers include additional filtering and tuning circuits to better lock on to the intended frequency – or to produce better-quality audio output – and exclude other signals. Still, most receiver circuits contain these basic elements.
Approaching AM Radio
The original method of encoding sound information on radio waves is called amplitude modulation (AM), which was developed in the first few decades of the 20th century. AM is a relatively simple way of adding audio information to a carrier wave so that sounds can be transmitted.
One of the straightforward forms of AM modulators simply runs the power supply for an oscillator circuit through an audio transformer that’s coupled to a microphone or other sound source. Figure 1-3 shows this arrangement.
Figure 1-3: The basic AM modulator circuit.
The circuit in Figure 1-3 uses a 1 MHz crystal oscillator, which is often used to generate the clock frequencies for microprocessor circuits. 1 MHz is perfect for a simple AM transmitter circuit because 1 MHz falls right in the middle of the band that’s used for AM radio transmissions.
Although you can’t buy a crystal oscillator very easily at the shops, you can get one on the Internet easily enough. Search for ‘1 MHz crystal oscillator’ and you can find several component distributors to sell you one for a few pounds.
The crystal oscillator is contained in a metal can that has three pins, one each for:
Ground
Supply voltage (typically 9 VDC)
Oscillator output
By running the supply voltage through the secondary coil of a transformer whose primary coil is connected to an audio input source such as a microphone, the actual voltage supplied to the oscillator fluctuates based on the variations in the input signal. Because crystal oscillator is contained in a metal can that has three pinscrystal oscillators are very stable, these voltage variations don’t affect the frequency generated by the oscillator, but they do affect the voltage of the oscillator output. Thus, the audio input signal is reflected as voltage changes in the oscillator’s output signal.
A better AM modulation circuit uses a transistor as shown in Figure 1-4. In this circuit, the carrier-wave generated by an oscillator that isn’t shown in the circuit is applied to the base of a transistor. Then, the audio input is applied to the transistor’s emitter through a transformer. The AM signal is taken from the transistor’s collector.
This circuit works by the transistor amplifying the input from the oscillator through the emitter-collector circuit. As the audio input varies, however, it induces a small current in the secondary coil of the transformer. This, in turn, affects the amount of current that flows through the collector-emitter circuit. In this way, the intensity of the output varies with the audio input.
Figure 1-5 shows how a carrier wave is combined with an audio signal to produce an AM radio waveform. As you can see, the carrier wave is a constant frequency and amplitude. In other words, each cycle of the sine wave is of the same intensity. The current of the audio wave varies, however. When the modulator circuit combines the two, the result is a signal with a steady frequency, but the intensity of each cycle of the sine wave varies depending on the intensity of the audio signal.
Figure 1-4: Using a transistor for amplitude modulation.
Figure 1-5: How the carrier wave and the audio signal are combined to produce an AM waveform.
Finding out about FM Radio
The AM radio system that we describe in the preceding section is relatively simple but has several weaknesses. The main drawback is that getting an AM radio receiver to distinguish between a signal broadcast by a radio transmitter and spurious signals at the same frequency (generated by other sources) is difficult, if not impossible.
The most obvious example is lightning. When lightning strikes, it generates a brief but powerful burst of electromagnetic radiation with a very large spectrum of frequencies. The noise generated by a lightning strike includes just about the entire range of frequencies used by AM radio. If you’re listening to an AM radio station when the lightning strikes, the sudden burst of radio energy on the frequency you’re listening to is interpreted as sound. Thus, when lightning strikes, you can hear a crackle on the radio.
Signals that interfere with an intentional broadcast are called static, which is the main drawback of AM radio. To counteract it, a better method of superimposing information on a radio wave, called frequency modulation (FM), was developed in 1933. (See the sidebar ‘The tragic genius behind FM radio’ for the fascinating and sad story about the inventor of FM radio.)
In FM, the intensity of the carrier wave isn’t varied. Instead, the exact frequency of the carrier wave is varied in sync with the audio signal. When the audio signal is higher, the frequency of the broadcast signal goes up a little. When the audio signal is lower, the frequency slows down a bit.
Figure 1-6 shows how this variation appears in a graph:
At the top of the figure is the carrier wave that clocks the specific frequency of the broadcast station.
In the middle is the audio signal that’s to be superimposed on the carrier wave.
At the bottom is the resulting modulated signal.
Figure 1-6: How the carrier wave and the audio signal are combined to produce an FM waveform.
The frequency variations in a frequency-modulated signal are all within a small proportion of the carrier-wave frequency. Typically, the frequency stays within 100 kHz of the base frequency.
FM modulators usually employ a type of electronic component called a varactor, which is a type of diode that has an unusual characteristic: it has capacitance like a capacitor, and its capacitance increases when voltage is applied across the diode. In essence, a varactor is a voltage-controlled variable capacitor.
The schematic symbol for a varactor, shown in the margin, looks like a cross between a diode and a capacitor.
You can use varactors in oscillator circuits to create an oscillator that vibrates faster when voltage increases. This ability makes them ideal as FM radio modulators. As the voltage of the audio input increases, the capacitance of the varactor increases and thus the frequency of the oscillator increases. When the voltage decreases, the capacitance of the varactor decreases and so does the oscillator’s frequency. Figure 1-7 shows a sample of an FM modulator circuit that uses a varactor.
Figure 1-7: An FM modulator circuit that uses a varactor.