The AM wave essentially carries two copies of the modulation pattern — one in each transmission sideband. As a result, it occupies a Transmission Bandwidth, 
. i.e. it takes up twice as much bandwidth as the original information. This represents one of the main disadvantages of AM modulation. The number of independent transmissions we can make will be limited by the range of frequencies available and how much bandwidth each transmission requires. The double sideband nature of AM halves the number of independent signals we can send using a given range of transmission frequencies.
Some transmission systems adopt a modified form of AM called Single Sideband Modulation (SSB). This is essentially an AM wave with one sideband suppressed or filtered before transmission. SSB modulated transmissions are called either Upper Sideband (USB) or Lower Sideband (LSB) depending on which sideband is used to carry the modulation information pattern. By using SSB we can double the number of transmissions which will ‘fit’ into a given transmission band. However, SSB transmitters and receivers are more complicated (and expensive!) than the simple AM circuits described earlier. Hence they tend only to be used for specialised purposes like aircraft or ship communications.
From the above arguments we can see that normal AM is prone to signal distortion and is wasteful of transmission bandspace. Another feature of AM is that it is also wasteful of power. To see why, consider what happens when the carrier is modulated by a single sinewave
where 
. This means that, at times, 
and the carrier becomes phase inverted. This process is illustrated in figure 9.5.
An envelope detector can't know that the carrier phase has been altered, hence it just responds to the +ve peaks of the inverted modulated carrier. This means it produces a distorted output whenever 
. A number of more complex/expensive techniques can be used to avoid this problem. In effect, better AM demodulators sense the carrier phase and take this into account. However most AM receiver systems just use the ‘cheap and cheerful’ envelope detection technique. For this reason it is usually necessary to ensure that the modulation is always limited so that 
— i.e., that 
— to avoid this problem.
For simple sinewave modulation this means that we require 
. This means that a sinewave modulated AM wave will have the form
Now the mean carrier power transmitted will be
where R is the impedance of the medium or system which the wave is being propagated along/through. Similarly, the combined mean power of the two sideband components will be
Since we can expect 
we can say that the ratio of the sideband power to the total power transmitted, 
, will be
i.e., the largest acceptable amount of modulation means that only one third of the total transmitted power appears in the modulation sideband components of the AM wave. Put another way, this means that at least 2/3rds of the transmitted power is devoted to sending a steady carrier wave. This power is virtually wasted. It only serves two purposes: 1) to let envelope detectors work correctly; 2) to provide a steady signal which is maintained when there is no actual modulation, confirming the existence of the transmitter!
In situations like domestic broadcasting one transmitter may serve ten million receivers. Under these circumstances the cheapness of the ten million envelope detectors can justify the waste of transmitter power. For specialised applications it may however be more sensible to use a more power efficient modulation system and a more expensive receiver. Some transmission systems therefore use a suppressed carrier method. This is similar to conventional AM or SSB, but with the carrier filtered away. The resulting transmissions are very power efficient, but require complex receivers which can be relatively difficult to tune. A particular problem of suppressed carrier systems arises when no modulation is being sent. Then there will be no sideband components. Nor will there be any carrier wave since it has been suppressed. Hence the suppressed carrier wave consists of nothing at all when there is no modulation! This certainly saves energy, but it can make it difficult to tune in a receiver correctly...
AM also suffers from being prone to interference effects. The AM demodulator essentially measures the power or amplitude of the signals presented to it. Any other power — e.g. pulses radiated by electric drills, Tornado radars, etc — can also be detected. This is why AM radio tends to suffer from buzzes, crackles, whistles, etc. Other forms of modulation can avoid this sensitivity and are less prone to unwanted interference.
Summary
You should now know that it is possible to communicate information by Modulating a Carrier wave. That Amplitude Modulation uses variations in the carrier size to convey information. That one way of AM modulating a wave is to use a Square Law device such as an FET. That these amplitude modulations can be demodulated using an Envelope detector. You should also see why this form of detector suffers from Ripple and Negative peak clipping distortions. That the magnitude of the modulation must be limited to avoid distortions arising due to carrier phase inversion. That AM is wasteful of transmission bandwidth and transmitter power. That it is inherently sensitive to interference. That single sideband and suppressed carrier methods exist and are less wasteful of transmission bandwidth and power. That the drawback of these ‘improved’ forms of AM is that the receiver/demodulator becomes more complicated (expensive) and difficult to use.
Content and pages maintained by: Jim Lesurf (jcgl@st-and.ac.uk)
using
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University of St. Andrews, St Andrews, Fife KY16 9SS, Scotland.
