Amplitude modulation (AM) is a modulation technique in which the amplitude of a high frequency sine wave (usually at a radio frequency) is varied in direct proportion to that of a modulating signal. The modulating signal carries the required information and often consists of audio data, as in the case of AM radio broadcasts or two-way radio communications. The high frequency sine wave (the carrier) is modulated by adding the modulating signal to it in a mixer. A simplified AM radio transmitter system is shown below.
A simplified AM radio transmitter system
A simple form of amplitude modulation was originally used to modulate audio voice signals onto a low-voltage direct current (dc) carrier on a telephone circuit. A microphone in the telephone handset acts as a transducer, and uses the sound waves produced by the human voice to vary the current passing through the circuit. At the other end of the telephone line, a second transducer (in the form of a small loudspeaker mounted in the remote handset) uses the varying voltage to produce sound waves that are close enough to the original speech patterns to be recognisable as the voice of the caller. Although the human voice is composed of frequencies ranging from 300 to approximately 20,000 hertz, the public switched telephone system limits the frequencies used to between 300 and 3,400 hertz, giving a total bandwidth of 3,100 hertz. This bandwidth is perfectly adequate for purely voice transmission, since the higher frequencies in the human voice (i.e. those above 3,100 hertz) are not really needed for recognisable speech reproduction. The use of a limited bandwidth also makes the telephone system much simpler from an engineering perspective.
Whereas telephone signals can be transmitted at audio frequencies, the same is not really a practical proposition for radio transmissions. The main reason for this is that the optimum length of a radio antenna is a half or a quarter of a wavelength. Since a typical audio frequency of 3,000 hertz has a wavelength of approximately 100 kilometres, the antenna would need to have a length of 25 kilometres to be effective - not a realistic proposition. By comparison, a radio frequency of 100 megahertz would have a wavelength of approximately 3 metres, and could use an antenna 80 centimetres long. It becomes necessary, therefore, to use a radio frequency carrier signal in order to transmit audio signals, which are used to modulate the carrier waveform.
A typical amplitude modulated signal
Modulating a carrier wave by adding another, lower frequency signal results in a signal that has most of its power concentrated in the carrier, with the rest shared between two sidebands, one above the carrier in frequency and one below it. The highest frequency in the modulating signal is typically less than ten percent of that of the carrier. The process of creating these sideband frequencies by adding another signal to the carrier is known as heterodyning. In the simplest case, the carrier can be modulated by adding another single-frequency sine wave signal to it, changing the carrier's shape (or envelope) as illustrated above. The sideband frequencies account for approximately 33% of the transmitted power. If a more complex modulating signal (such as an audio signal) is used to modulate the carrier, the sidebands account for only about 20-25% of the total transmitted power.
Consider, for example, a 100 kHz carrier that is modulated by a steady audio signal (or tone) of 5 kHz. When these signals are added, two sidebands are produced. One sideband has a frequency equal to the sum of the carrier and the modulating signal (100 kHz + 5 kHz = 105 kHz), while the other sideband has a frequency equal to the difference between the carrier and the modulating signal (100 kHz - 5 kHz = 95 kHz). The two sidebands are 5 kHz equidistant from the carrier (one above it and one below it), giving a total bandwidth for the modulated signal of 10 kHz (105 kHz - 95 kHz). The resulting frequency spectrum is illustrated below.
A 100 kHz carrier modulated by a 5kHz audio tone
Of course, most audio signals (speech and music, for example) are far more complex than a single-frequency audio tone, and are composed of many different frequencies. When a carrier is modulated with a more complex audio signal, therefore, all of the frequencies present in the audio signal are represented in the resulting output signal. In this case, the total bandwidth is the difference between the sum and the difference values of the carrier and the highest frequency component of the modulating signal. To simplify things, the modulated signal bandwidth will be twice that of the modulating signal. For a modulating audio signal with frequency components ranging from 0 - 6 kHz, therefore, the bandwidth of the modulated signal for a 100 kHz carrier will be 106 kHz - 94 kHz = 12 kHz. This produces a more complex frequency spectrum, which might look something like that shown below.
A 100 kHz carrier modulated by an audio signal (frequencies up to 6 kHz)
The bandwidth of each sideband is equal to that of the modulating signal, and the two sidebands are mirror images of each other, each carrying the same information as the original audio signal. This type of basic amplitude modulation, which results in two sidebands and a carrier, is usually referred to as double sideband amplitude modulation (DSB-AM). It is a very inefficient form of modulation in terms of its power usage, because at least two thirds of the transmitted power is concentrated in the carrier signal, with the remaining power being evenly split between the two sidebands. Since the sidebands contain identical information, only one sideband is actually needed to carry the transmitted audio information. The other sideband is redundant, and the carrier signal contains no useful information. DSB-AM is also therefore spectrally inefficient, because fewer stations can make use of a given transmission band. The main benefit of DSB-AM is that, because of its relative simplicity, receiving equipment is cheaper to produce.
The process of demodulation for DSB-AM is relatively straightforward. The radio frequency carrier can be removed from the signal using a simple diode detector consisting of a diode, a resistor, and a capacitor. The incoming signal is rectified by the diode, which allows only half of the alternating waveform to pass through it. The capacitor removes the remaining radio frequency signal components to provide a smooth output, and the resistor allows the capacitor to discharge. An AM receiver can thus be produced relatively cheaply, since there is no requirement for specialised components. The basic circuit diode detector circuit is shown below.
A basic diode detector circuit
Because the modulating signal is added to the carrier, the instantaneous amplitude of the modulated signal will depend on the instantaneous amplitude of the modulating data. The modulation index is a measure of the degree to which the modulating signal increases the maximum amplitude of the carrier signal. If the carrier's amplitude is made to vary between 50% above and 50% below its un-modulated value, it is said to have a modulation index of 0.5. If the amplitude is made to vary by 100% above and below its un-modulated value, it has a modulation index of 1.0. A modulation index of 1.0 for the A3E transmission mode will give a maximum transmitter power efficiency of 33%. Increasing the modulation index would result in greater power efficiency, but would result in distortion at the receiver.
The power efficiency of the transmitter can be increased by removing (suppressing) the carrier from the AM signal to create a reduced-carrier transmission, or double-sideband suppressed-carrier (DSBSC). DSBSC is three times more power-efficient than DSB-AM. A similar scheme, in which the carrier is only partially suppressed, is called double-sideband reduced-carrier (DSBRC). Both schemes require the carrier to be regenerated by a local oscillator in the receiver in order that demodulation can be achieved using standard demodulation techniques. In addition to transmitter efficiency, spectral efficiency can be achieved by completely suppressing both the carrier and one of the sidebands, although the complexity of both the transmitter and the receiver is increased significantly. The ITU designations for the various amplitude modulation schemes are shown in the table below.
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The carrier frequencies used in some applications are very high (radar frequencies, for example, range from 3MHz up to 300 GHz). At very high frequencies, many standard electronic components cannot function properly. A superheterodyne receiver is one that reduces the frequency of an incoming signal by adding a lower frequency to it using a mixer (a process known as superheterodyning) to reduce the frequency of the AM signal, which is centred on the carrier frequency, to some lower frequency called the intermediate frequency (IF) prior to processing. The intermediate frequency obtained is the difference (or beat) frequency between the incoming AM signal's carrier frequency and that of the local oscillator. The receiver will use a tuner to select the required carrier frequency, and to adjust the frequency of the receiver's local oscillator so that the intermediate frequency will always have the same value (the tuner and the local oscillator or therefore tightly coupled). This both simplifies the design of the receiver and reduces its cost, since the majority of its components will be required only to operate at a single intermediate frequency rather than over a range of frequencies. A simple superheterodyne receiver system is shown below.
A superheterodyne receiver
The band-pass filter in the tuner filters out all signals except the selected carrier frequency. The receiver bandwidth is usually some fraction of the carrier frequency. A receiver bandwidth of 2%, for example, means that any signals between 2% above and 2% below the carrier frequency are allowed to pass through the filter. For a carrier frequency of 850 kHz, this would mean that all signals between 833 kHz and 867 kHz are accepted by the receiver. If the same fraction is applied to the intermediate frequency, then for a fixed IF of 452 kHz, only signals that are within the range 443 kHz to 461 kHz will pass. The local oscillator is set to 398 kHz to reduce the 850 kHz carrier to 452 kHz (the beat frequency).
Any adjacent signals are also superheterodyned, but remain at the same margin above and below the original signal. If the incoming signal includes interference at 863 kHz, a conventional 2% receiver will allow the interference to pass, since the interference falls within the range 833 kHz to 867 kHz. If the signal is superheterodyned using a local oscillator frequency of 398 kHz, the interfering signal will be shifted down to a beat frequency of 465 kHz. If the resulting IF frequency is also limited to a bandwidth of 2%, any frequencies below 443 kHz or above 461 kHz will be filtered out. This means that the interference at 465 kHz will be eliminated from the signal (i.e. it has been suppressed). It is apparent, therefore, that the superheterodyne receiver is more selective. The term used to describe the process of narrowing the receiver bandwidth in this way is arithmetic selectivity.
In order to increase both the power efficiency and spectral efficiency of the transmitter, it is necessary to remove both the carrier and one of the sidebands from the transmitted AM signal. A simplified single sideband AM transmitter is shown below.
A single sideband AM transmitter system
The receiver must restore the carrier signal before demodulation can take place by creating its own carrier signal using a local oscillator and adding it to the received SSB AM signal in a mixer. A suitable receiver system might look something like that shown below.
A single sideband AM receiver
A simple form of AM, often used for digital communications is on-off keying, in which binary data is represented as the presence or absence of the carrier wave. This method is often used at radio frequencies to transmit Morse code.
A simple amplitude modulated digital signal