In wave motion, the altering of wave amplitude in a systematic way, leaving frequency unchanged. In AM radio, an electrical signal is used to modulate the amplitude of the broadcast carrier radio wave. A radio receiver reproduces the signal from the modulated wave by demodulation.
Amplitude modulation (AM) is a technique used in electronic communication, most commonly for transmitting information via a carrier wave wirelessly. and phase modulation, used often in remote controls, in which the phase is varied.)
In the mid-1870s, a form of amplitude modulation—initially called "undulatory currents"—was the first method to successfully produce quality audio over telephone lines.
Forms of amplitude modulation
As originally developed for the electric telephone, amplitude modulation was used to add audio information to the low-powered direct current flowing from a telephone transmitter to a receiver.
In contrast to the telephone, in radio communication what is modulated is a continuous wave radio signal (carrier wave) produced by a radio transmitter. In its basic form, amplitude modulation produces a signal with power concentrated at the carrier frequency and in two adjacent sidebands. Amplitude modulation that results in two sidebands and a carrier is often called double sideband amplitude modulation (DSB-AM). At least two-thirds of the power is concentrated in the carrier signal, which carries no useful information (beyond the fact that a signal is present);
To increase transmitter efficiency, the carrier can be removed (suppressed) from the AM signal.
Even greater efficiency is achieved—at the expense of increased transmitter and receiver complexity—by completely suppressing both the carrier and one of the sidebands.
A simple form of AM often used for digital communications is on-off keying, a type of amplitude-shift keying by which binary data is represented as the presence or absence of a carrier wave.
In 1982, the International Telecommunications Union (ITU) designated the various types of amplitude modulation as follows:
| Designation | Description |
|---|---|
| A3E | double sideband full carrier - the basic AM modulation scheme |
| R3E | single sideband reduced carrier |
| H3E | single sideband full carrier |
| J3E | single sideband suppressed carrier |
| B8E | independent sideband emission |
| C3F | vestigial sideband |
| Lincompex | linked compressor and expander |
Example
Suppose we wish to modulate a simple sine wave on a carrier wave.
The equation for the simple sine wave of frequency ωm (the signal we wish to broadcast) is
m(t) = Msin(ωmt + φ),with φ its phase offset relative to c(t).
Amplitude modulation is performed simply by adding m(t) to C. The amplitude-modulated signal is then
y(t) = (C + Msin(ωmt + φ))sin(ωct)The formula for y(t) above may be written
The broadcast signal consists of the carrier wave plus two sinusoidal waves each with a frequency slightly different from ωc, known as sidebands.
A more general example
Consider a general modulating signal m(t), which can now be anything at all.
Or, in complex form:
Taking Fourier Transforms, we get:
,where δ(x) is the Dirac delta function — a unit impulse at x — and capital functions indicate Fourier Transforms.
This makes clear the two sidebands that this modulation method yields, as well as the carrier signals that go with them.
As already mentioned, if multiple signals are to be transmitted in this way (by frequency division multiplexing), then their carrier signals must be sufficiently separated that their spectra do not overlap. This analysis also shows that the transmission bandwidth of AM is twice the signal's original (baseband) bandwidth — since both the positive and negative sidebands are 'copied' up to the carrier frequency, but only the positive sideband is present originally.
An analysis of the power consumption of AM reveals that DSB-AM with its carrier has an efficiency of about 33% — very poor.
Modulation index
As with other modulation indices, in AM, this quantity, also called modulation depth, indicates by how much the modulated variable varies around its 'original' level.
So if h = 0.5, the carrier amplitude varies by 50% above and below its unmodulated level, and for h = 1.0 it varies by 100%.
Variations of modulated signal with percentage modulation are shown below.
Amplitude modulator designs
Circuits
A wide range of different circuits have been used for AM, but one of the simplest circuits uses anode or collector modulation applied via a transformer.
Modulation circuit designs can be broadly divided into low and high level.
Low level
Here a small audio stage is used to modulate a low power stage, the output of this stage is then amplified using a linear RF amplifier.
AdvantagesThe advantage of using a linear RF amplifier is that the smaller early stages can be modulated, which only requires a small audio amplifier to drive the modulator.
DisadvantagesThe great disadvantage of this system is that the amplifer chain is less efficient, because it has to be linear to preserve the modulation.
An approach which marries the advantages of low-level modulation with the efficiency of a Class C power amplifier chain is to arrange a feedback system to compensate for the substantial distortion of the AM envelope. A simple detector at the transmitter output (which can be little more than a loosely coupled diode) recovers the audio signal, and this is used as negative feedback to the audio modulator stage.
High level
AdvantagesOne advantage of using class C amplifiers in a broadcast AM transmitter is that only the final stage needs to be modulated, and that all the earlier stages can be driven at a constant level.
DisadvantagesA large audio amplifier will be needed for the modulation stage, at least equal to the power of the transmitter output itself.
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