Amplitude Modulation AM

In AM transmission, the carrier signal is modulated so that its amplitude varies with the changing amplitudes of the modulating signal. The frequency and phase of the carrier remain the same; only the amplitude changes to follow variations in the information. Figure 4.4-3 shows how this concept works. The modulating signal becomes an envelope to the carrier.

The total bandwidth required for AM can be determined from the bandwidth of the audio signal: BWt=2*BWm.

Amplitude Modulation
Fig. 4.4-3 Amplitude modulation

AM Bandwidth

The bandwidth of an AM signal is equal to twice the bandwidth of the modulating signal and covers a range centered on the carrier frequency (see Figure 4.4-4). The shaded portion of the graph is the frequency spectrum of the signal.

Amplitude Modulation Bandwidth

The bandwidth of an audio signal (speech and music) is usually 5 kHz. Therefore, an AM radio station needs a minimum bandwidth of 10 kHz.

AM stations are allowed carrier frequencies anywhere between 530 and 1700 kHz (1.7 MHz). However, each station's carrier frequency must be separated from those on either side of it by at least 10 kHz (one AM bandwidth) to avoid interference. If one station uses a carrier frequency of 1100 kHz, the next station's carrier frequency cannot be lower than 1110 kHz (see Figure 4.4-5).

fc = Carrier frequency of (he station fc fc fc fc fc fc fc fc fc fc fc fc fc fc

Fig. 4.4-5 AM band allocation

Fig. 4.4-5 AM band allocation

Example 4.4-1

We have an audio signal with a bandwidth of 4 kHz. What is the bandwidth needed if we encode the signal using AM?

Solution

An AM signal requires twice the bandwidth of the original signal: BW=2(4) kHz=8 kHz

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Responses

  • Leon
    Why in amplitude modulation need twice bandwidth?
    8 years ago

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