AM

Modulation is the process of varying a higher frequency carrier way to transmit information. Amplitude modulation (AM) is an analog modulation technique where the carrier signal amplitude is altered by a message signal. AM effectively generates two symmetrical side bands, and suppresses the carrier.

The beginning stages of radio can be traced back to the late 1880s, when Heinrich Hertz confirmed Maxwell’s electromagnetic theory by demonstrating the production and reception of radio (electromagnetic) waves. In 1901, Guglielmo Marconi transmitted transatlantic wireless signals. We often forget that these transmissions were, albeit crudely, modulated. A single transmitted carrier does not convey any information. It is only when this transmitted carrier is somehow changed—modulated—that useful information is transferred. The most elementary type of amplitude modulation is to turn the carrier on and off—changing the amplitude from 0 to maximum. In principle, you can vary the on and off times and send Morse code. However, it is much more efficient, and easier to hear, if the carrier is first tone modulated and this tone is turned on and off.

The analysis of any modulation technique must show the technique's relative complexity, modulated spectrum and bandwidth, and noise performance. Analog amplitude modulation requires a relatively simple and inexpensive technology for both transmission and reception. Likewise, it does not require much bandwidth. However, on the down side, AM has very poor noise performance. The poor noise performance results because most natural and man-made interference (static) is also an amplitude modulated signal. Thus, it is impossible with current technologies to design a receiver that receives the desired AM signal and not most of the undesired AM interference.

The fundamental equation for sine wave amplitude modulation is

where
the first term is the carrier
the second term is the lower sideband
the third term is the upper sideband
c is the carrier
s is the signal and
m is the modulation index (how much the modulated amplitude varies around its unmodulated value).

Figure 1. Two Signal Cycles of an Amplitude Modulated Wave

In Figure 1, the carrier frequency is five times the signal sine wave signal frequency.

Figure 2. An AM Carrier Spectrum

In Figure 2, the (normalized) frequency is at 8 Hz and the two side bands are 4 Hz from the carrier. Thus the (normalized) spectrum goes from 4 Hz to 12 Hz.

Figure 3. An AM Sawtooth Spectrum.

In Figure 3, the signal wave shape was changed to a square wave and therefore, this spectrum contains many more frequencies. The specific harmonic content of a square wave can be obtained using the fast Fourier transform. Incidentally, the wave shape of some stringed instruments sometimes resembles a square wave.

Refer to the RF & Communications Resources page for additional information about RF terminology, fundamentals, and National Instruments RF products.

Additional References

Helpful Web Sites:

• Interactive demonstration of AM modulation and a view of associated time and spectrum waveforms (Select Signal 2 in demo)

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